diff --git a/src/cmd/cgo/gcc.go b/src/cmd/cgo/gcc.go
index d3de3906b48..300ccae350b 100644
--- a/src/cmd/cgo/gcc.go
+++ b/src/cmd/cgo/gcc.go
@@ -1121,6 +1121,9 @@ func (p *Package) hasPointer(f *File, t ast.Expr, top bool) bool {
 		if t.Name == "error" {
 			return true
 		}
+		if t.Name == "any" {
+			return true
+		}
 		if goTypes[t.Name] != nil {
 			return false
 		}
diff --git a/src/cmd/cgo/internal/test/cgo_test.go b/src/cmd/cgo/internal/test/cgo_test.go
index 5393552e07a..04e06cf95ec 100644
--- a/src/cmd/cgo/internal/test/cgo_test.go
+++ b/src/cmd/cgo/internal/test/cgo_test.go
@@ -106,6 +106,7 @@ func TestSetEnv(t *testing.T)                { testSetEnv(t) }
 func TestThreadLock(t *testing.T)            { testThreadLockFunc(t) }
 func TestUnsignedInt(t *testing.T)           { testUnsignedInt(t) }
 func TestZeroArgCallback(t *testing.T)       { testZeroArgCallback(t) }
+func Test76340(t *testing.T)                 { test76340(t) }
 
 func BenchmarkCgoCall(b *testing.B)      { benchCgoCall(b) }
 func BenchmarkGoString(b *testing.B)     { benchGoString(b) }
diff --git a/src/cmd/cgo/internal/test/test.go b/src/cmd/cgo/internal/test/test.go
index e83e367174a..4dd14facb50 100644
--- a/src/cmd/cgo/internal/test/test.go
+++ b/src/cmd/cgo/internal/test/test.go
@@ -959,6 +959,18 @@ char * const issue75751p = &issue75751v;
 #define issue75751m issue75751p
 char * const volatile issue75751p2 = &issue75751v;
 #define issue75751m2 issue75751p2
+
+typedef struct { void *t; void *v; } GoInterface;
+extern int exportAny76340Param(GoInterface);
+extern GoInterface exportAny76340Return(int);
+
+int issue76340testFromC(GoInterface obj) {
+	return exportAny76340Param(obj);
+}
+
+GoInterface issue76340returnFromC(int val) {
+	return exportAny76340Return(val);
+}
 */
 import "C"
 
@@ -2407,3 +2419,22 @@ func test69086(t *testing.T) {
 func test75751() int {
 	return int(*C.issue75751m) + int(*C.issue75751m2)
 }
+
+// Issue 76340.
+func test76340(t *testing.T) {
+	var emptyInterface C.GoInterface
+	r1 := C.issue76340testFromC(emptyInterface)
+	if r1 != 0 {
+		t.Errorf("issue76340testFromC with nil interface: got %d, want 0", r1)
+	}
+
+	r2 := C.issue76340returnFromC(42)
+	if r2.t == nil && r2.v == nil {
+		t.Error("issue76340returnFromC(42) returned nil interface")
+	}
+
+	r3 := C.issue76340returnFromC(0)
+	if r3.t != nil || r3.v != nil {
+		t.Errorf("issue76340returnFromC(0) returned non-nil interface: got %v, want nil", r3)
+	}
+}
diff --git a/src/cmd/cgo/internal/test/testx.go b/src/cmd/cgo/internal/test/testx.go
index 9a63b9e1008..21ba52260ef 100644
--- a/src/cmd/cgo/internal/test/testx.go
+++ b/src/cmd/cgo/internal/test/testx.go
@@ -595,3 +595,21 @@ func test49633(t *testing.T) {
 		t.Errorf("msg = %q, want 'hello'", v.msg)
 	}
 }
+
+//export exportAny76340Param
+func exportAny76340Param(obj any) C.int {
+	if obj == nil {
+		return 0
+	}
+
+	return 1
+}
+
+//export exportAny76340Return
+func exportAny76340Return(val C.int) any {
+	if val == 0 {
+		return nil
+	}
+
+	return int(val)
+}
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 00c9e8c9a3e..dc1e5b29e59 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -1558,6 +1558,9 @@ func (p *Package) doCgoType(e ast.Expr, m map[ast.Expr]bool) *Type {
 		if t.Name == "error" {
 			return &Type{Size: 2 * p.PtrSize, Align: p.PtrSize, C: c("GoInterface")}
 		}
+		if t.Name == "any" {
+			return &Type{Size: 2 * p.PtrSize, Align: p.PtrSize, C: c("GoInterface")}
+		}
 		if r, ok := goTypes[t.Name]; ok {
 			return goTypesFixup(r)
 		}
diff --git a/src/cmd/compile/abi-internal.md b/src/cmd/compile/abi-internal.md
index eae230dc070..8de8356132a 100644
--- a/src/cmd/compile/abi-internal.md
+++ b/src/cmd/compile/abi-internal.md
@@ -833,6 +833,51 @@ The riscv64 has Zicsr extension for control and status register (CSR) and
 treated as scratch register.
 All bits in CSR are system flags and are not modified by Go.
 
+### s390x architecture
+
+The s390x architecture uses R2 – R9 for integer arguments and integer results.
+
+It uses F0 – F15 for floating-point arguments and results.
+
+Special-purpose registers used within Go generated code and Go assembly code
+are as follows:
+
+| Register | Call meaning | Return meaning | Body meaning |
+| --- | --- | --- | --- |
+| R0 | Zero value | Same | Same |
+| R1 | Scratch | Scratch | Scratch |
+| R10, R11 | used by the assembler | Same | Same |
+| R12 | Closure context pointer | Same | Same |
+| R13 | Current goroutine | Same | Same |
+| R14 | Link register | Link register | Scratch |
+| R15 | Stack pointer | Same | Same |
+
+*Rationale*: These register meanings are compatible with Go’s stack-based
+calling convention.
+
+#### Stack layout
+
+The stack pointer, R15, grows down and is aligned to 8 bytes.
+
+A function's stack frame, after the frame is created, is laid out as
+follows:
+
+    +------------------------------+
+    | ... locals ...               |
+    | ... outgoing arguments ...   |
+    | return PC                    | ← R15 points to
+    +------------------------------+ ↓ lower addresses
+
+This stack layout is used by both register-based (ABIInternal) and
+stack-based (ABI0) calling conventions.
+
+The "return PC" is loaded to the link register R14, as part of the
+s390x `BL` operation.
+
+#### Flags
+The s390x architecture maintains a single condition code (CC) field in the Program Status Word (PSW).
+Go-generated code sets and tests this condition code to control conditional branches.
+
 ## Future directions
 
 ### Spill path improvements
diff --git a/src/cmd/compile/internal/s390x/galign.go b/src/cmd/compile/internal/s390x/galign.go
index d880834c220..1fb371a52c6 100644
--- a/src/cmd/compile/internal/s390x/galign.go
+++ b/src/cmd/compile/internal/s390x/galign.go
@@ -20,4 +20,6 @@ func Init(arch *ssagen.ArchInfo) {
 	arch.SSAMarkMoves = ssaMarkMoves
 	arch.SSAGenValue = ssaGenValue
 	arch.SSAGenBlock = ssaGenBlock
+	arch.LoadRegResult = loadRegResult
+	arch.SpillArgReg = spillArgReg
 }
diff --git a/src/cmd/compile/internal/s390x/ssa.go b/src/cmd/compile/internal/s390x/ssa.go
index de00f1ef8ce..ce060597d9a 100644
--- a/src/cmd/compile/internal/s390x/ssa.go
+++ b/src/cmd/compile/internal/s390x/ssa.go
@@ -10,6 +10,7 @@ import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/logopt"
+	"cmd/compile/internal/objw"
 	"cmd/compile/internal/ssa"
 	"cmd/compile/internal/ssagen"
 	"cmd/compile/internal/types"
@@ -540,6 +541,19 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		p.From.Type = obj.TYPE_REG
 		p.From.Reg = v.Args[0].Reg()
 		ssagen.AddrAuto(&p.To, v)
+	case ssa.OpArgIntReg, ssa.OpArgFloatReg:
+		// The assembler needs to wrap the entry safepoint/stack growth code with spill/unspill
+		// The loop only runs once.
+		for _, a := range v.Block.Func.RegArgs {
+			// Pass the spill/unspill information along to the assembler, offset by size of
+			// the saved LR slot.
+			addr := ssagen.SpillSlotAddr(a, s390x.REGSP, base.Ctxt.Arch.FixedFrameSize)
+			s.FuncInfo().AddSpill(
+				obj.RegSpill{Reg: a.Reg, Addr: addr, Unspill: loadByType(a.Type), Spill: storeByType(a.Type)})
+		}
+		v.Block.Func.RegArgs = nil
+
+		ssagen.CheckArgReg(v)
 	case ssa.OpS390XLoweredGetClosurePtr:
 		// Closure pointer is R12 (already)
 		ssagen.CheckLoweredGetClosurePtr(v)
@@ -1029,3 +1043,22 @@ func ssaGenBlock(s *ssagen.State, b, next *ssa.Block) {
 		s.Br(s390x.ABR, succs[1])
 	}
 }
+
+func loadRegResult(s *ssagen.State, f *ssa.Func, t *types.Type, reg int16, n *ir.Name, off int64) *obj.Prog {
+	p := s.Prog(loadByType(t))
+	p.From.Type = obj.TYPE_MEM
+	p.From.Name = obj.NAME_AUTO
+	p.From.Sym = n.Linksym()
+	p.From.Offset = n.FrameOffset() + off
+	p.To.Type = obj.TYPE_REG
+	p.To.Reg = reg
+	return p
+}
+
+func spillArgReg(pp *objw.Progs, p *obj.Prog, f *ssa.Func, t *types.Type, reg int16, n *ir.Name, off int64) *obj.Prog {
+	p = pp.Append(p, storeByType(t), obj.TYPE_REG, reg, 0, obj.TYPE_MEM, 0, n.FrameOffset()+off)
+	p.To.Name = obj.NAME_PARAM
+	p.To.Sym = n.Linksym()
+	p.Pos = p.Pos.WithNotStmt()
+	return p
+}
diff --git a/src/cmd/compile/internal/ssa/_gen/S390XOps.go b/src/cmd/compile/internal/ssa/_gen/S390XOps.go
index c002d5bcc38..9e67a06ce8c 100644
--- a/src/cmd/compile/internal/ssa/_gen/S390XOps.go
+++ b/src/cmd/compile/internal/ssa/_gen/S390XOps.go
@@ -484,10 +484,10 @@ func init() {
 
 		{name: "CLEAR", argLength: 2, reg: regInfo{inputs: []regMask{ptr, 0}}, asm: "CLEAR", aux: "SymValAndOff", typ: "Mem", clobberFlags: true, faultOnNilArg0: true, symEffect: "Write"},
 
-		{name: "CALLstatic", argLength: 1, reg: regInfo{clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true},                                                // call static function aux.(*obj.LSym).  arg0=mem, auxint=argsize, returns mem
-		{name: "CALLtail", argLength: 1, reg: regInfo{clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true, tailCall: true},                                  // tail call static function aux.(*obj.LSym).  arg0=mem, auxint=argsize, returns mem
-		{name: "CALLclosure", argLength: 3, reg: regInfo{inputs: []regMask{ptrsp, buildReg("R12"), 0}, clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true}, // call function via closure.  arg0=codeptr, arg1=closure, arg2=mem, auxint=argsize, returns mem
-		{name: "CALLinter", argLength: 2, reg: regInfo{inputs: []regMask{ptr}, clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true},                         // call fn by pointer.  arg0=codeptr, arg1=mem, auxint=argsize, returns mem
+		{name: "CALLstatic", argLength: -1, reg: regInfo{clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true},                                                // call static function aux.(*obj.LSym).  last arg=mem, auxint=argsize, returns mem
+		{name: "CALLtail", argLength: -1, reg: regInfo{clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true, tailCall: true},                                  // tail call static function aux.(*obj.LSym).  last arg=mem, auxint=argsize, returns mem
+		{name: "CALLclosure", argLength: -1, reg: regInfo{inputs: []regMask{ptrsp, buildReg("R12"), 0}, clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true}, // call function via closure.  arg0=codeptr, arg1=closure, last arg=mem, auxint=argsize, returns mem
+		{name: "CALLinter", argLength: -1, reg: regInfo{inputs: []regMask{ptr}, clobbers: callerSave}, aux: "CallOff", clobberFlags: true, call: true},                         // call fn by pointer.  arg0=codeptr, last arg=mem, auxint=argsize, returns mem
 
 		// (InvertFlags (CMP a b)) == (CMP b a)
 		// InvertFlags is a pseudo-op which can't appear in assembly output.
@@ -812,16 +812,18 @@ func init() {
 	}
 
 	archs = append(archs, arch{
-		name:            "S390X",
-		pkg:             "cmd/internal/obj/s390x",
-		genfile:         "../../s390x/ssa.go",
-		ops:             S390Xops,
-		blocks:          S390Xblocks,
-		regnames:        regNamesS390X,
-		gpregmask:       gp,
-		fpregmask:       fp,
-		framepointerreg: -1, // not used
-		linkreg:         int8(num["R14"]),
+		name:               "S390X",
+		pkg:                "cmd/internal/obj/s390x",
+		genfile:            "../../s390x/ssa.go",
+		ops:                S390Xops,
+		blocks:             S390Xblocks,
+		regnames:           regNamesS390X,
+		ParamIntRegNames:   "R2 R3 R4 R5 R6 R7 R8 R9",
+		ParamFloatRegNames: "F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15",
+		gpregmask:          gp,
+		fpregmask:          fp,
+		framepointerreg:    -1, // not used
+		linkreg:            int8(num["R14"]),
 		imports: []string{
 			"cmd/internal/obj/s390x",
 		},
diff --git a/src/cmd/compile/internal/ssa/config.go b/src/cmd/compile/internal/ssa/config.go
index 3850fbf9616..9cfaa58839f 100644
--- a/src/cmd/compile/internal/ssa/config.go
+++ b/src/cmd/compile/internal/ssa/config.go
@@ -313,6 +313,8 @@ func NewConfig(arch string, types Types, ctxt *obj.Link, optimize, softfloat boo
 		c.registers = registersS390X[:]
 		c.gpRegMask = gpRegMaskS390X
 		c.fpRegMask = fpRegMaskS390X
+		c.intParamRegs = paramIntRegS390X
+		c.floatParamRegs = paramFloatRegS390X
 		c.FPReg = framepointerRegS390X
 		c.LinkReg = linkRegS390X
 		c.hasGReg = true
diff --git a/src/cmd/compile/internal/ssa/debug_lines_test.go b/src/cmd/compile/internal/ssa/debug_lines_test.go
index 79dbd91c2fc..5dbfdeb7f6e 100644
--- a/src/cmd/compile/internal/ssa/debug_lines_test.go
+++ b/src/cmd/compile/internal/ssa/debug_lines_test.go
@@ -45,7 +45,7 @@ func testGoArch() string {
 
 func hasRegisterABI() bool {
 	switch testGoArch() {
-	case "amd64", "arm64", "loong64", "ppc64", "ppc64le", "riscv":
+	case "amd64", "arm64", "loong64", "ppc64", "ppc64le", "riscv", "s390x":
 		return true
 	}
 	return false
diff --git a/src/cmd/compile/internal/ssa/opGen.go b/src/cmd/compile/internal/ssa/opGen.go
index 1ad737263a2..966d15b83ca 100644
--- a/src/cmd/compile/internal/ssa/opGen.go
+++ b/src/cmd/compile/internal/ssa/opGen.go
@@ -81087,7 +81087,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "CALLstatic",
 		auxType:      auxCallOff,
-		argLen:       1,
+		argLen:       -1,
 		clobberFlags: true,
 		call:         true,
 		reg: regInfo{
@@ -81097,7 +81097,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "CALLtail",
 		auxType:      auxCallOff,
-		argLen:       1,
+		argLen:       -1,
 		clobberFlags: true,
 		call:         true,
 		tailCall:     true,
@@ -81108,7 +81108,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "CALLclosure",
 		auxType:      auxCallOff,
-		argLen:       3,
+		argLen:       -1,
 		clobberFlags: true,
 		call:         true,
 		reg: regInfo{
@@ -81122,7 +81122,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "CALLinter",
 		auxType:      auxCallOff,
-		argLen:       2,
+		argLen:       -1,
 		clobberFlags: true,
 		call:         true,
 		reg: regInfo{
@@ -93219,8 +93219,8 @@ var registersS390X = [...]Register{
 	{31, s390x.REG_F15, "F15"},
 	{32, 0, "SB"},
 }
-var paramIntRegS390X = []int8(nil)
-var paramFloatRegS390X = []int8(nil)
+var paramIntRegS390X = []int8{2, 3, 4, 5, 6, 7, 8, 9}
+var paramFloatRegS390X = []int8{16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31}
 var gpRegMaskS390X = regMask(23551)
 var fpRegMaskS390X = regMask(4294901760)
 var specialRegMaskS390X = regMask(0)
diff --git a/src/cmd/internal/obj/s390x/a.out.go b/src/cmd/internal/obj/s390x/a.out.go
index caf5ec09358..6b16d7a9bd5 100644
--- a/src/cmd/internal/obj/s390x/a.out.go
+++ b/src/cmd/internal/obj/s390x/a.out.go
@@ -139,8 +139,8 @@ const (
 	REG_RESERVED // end of allocated registers
 
 	REGARG  = -1      // -1 disables passing the first argument in register
-	REGRT1  = REG_R3  // used during zeroing of the stack - not reserved
-	REGRT2  = REG_R4  // used during zeroing of the stack - not reserved
+	REGRT1  = REG_R1  // used during zeroing of the stack - not reserved
+	REGRT2  = REG_R10 // used during zeroing of the stack - not reserved
 	REGTMP  = REG_R10 // scratch register used in the assembler and linker
 	REGTMP2 = REG_R11 // scratch register used in the assembler and linker
 	REGCTXT = REG_R12 // context for closures
diff --git a/src/cmd/internal/obj/s390x/objz.go b/src/cmd/internal/obj/s390x/objz.go
index 44c1a7d586d..4bfc1f7b2c0 100644
--- a/src/cmd/internal/obj/s390x/objz.go
+++ b/src/cmd/internal/obj/s390x/objz.go
@@ -506,7 +506,13 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (pPre, pPreempt, pCh
 		// Save LR and REGCTXT
 		const frameSize = 16
 		p = c.ctxt.StartUnsafePoint(p, c.newprog)
+
+		// Spill arguments. This has to happen before we open
+		// any more frame space.
+		p = c.cursym.Func().SpillRegisterArgs(p, c.newprog)
+
 		// MOVD LR, -16(SP)
+
 		p = obj.Appendp(p, c.newprog)
 		p.As = AMOVD
 		p.From = obj.Addr{Type: obj.TYPE_REG, Reg: REG_LR}
@@ -549,10 +555,12 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (pPre, pPreempt, pCh
 		p.To = obj.Addr{Type: obj.TYPE_REG, Reg: REGSP}
 		p.Spadj = -frameSize
 
+		// Unspill arguments
+		p = c.cursym.Func().UnspillRegisterArgs(p, c.newprog)
 		p = c.ctxt.EndUnsafePoint(p, c.newprog, -1)
 	}
 
-	// MOVD	g_stackguard(g), R3
+	// MOVD	g_stackguard(g), R10
 	p = obj.Appendp(p, c.newprog)
 	// Jump back to here after morestack returns.
 	pCheck = p
@@ -565,7 +573,7 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (pPre, pPreempt, pCh
 		p.From.Offset = 3 * int64(c.ctxt.Arch.PtrSize) // G.stackguard1
 	}
 	p.To.Type = obj.TYPE_REG
-	p.To.Reg = REG_R3
+	p.To.Reg = REG_R10
 
 	// Mark the stack bound check and morestack call async nonpreemptible.
 	// If we get preempted here, when resumed the preemption request is
@@ -579,7 +587,7 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (pPre, pPreempt, pCh
 
 		p = obj.Appendp(p, c.newprog)
 		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R3
+		p.From.Reg = REG_R10
 		p.Reg = REGSP
 		p.As = ACMPUBGE
 		p.To.Type = obj.TYPE_BRANCH
@@ -598,40 +606,40 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (pPre, pPreempt, pCh
 		// stack guard to incorrectly succeed. We explicitly
 		// guard against underflow.
 		//
-		//	MOVD	$(framesize-StackSmall), R4
-		//	CMPUBLT	SP, R4, label-of-call-to-morestack
+		//	MOVD	$(framesize-StackSmall), R11
+		//	CMPUBLT	SP, R11, label-of-call-to-morestack
 
 		p = obj.Appendp(p, c.newprog)
 		p.As = AMOVD
 		p.From.Type = obj.TYPE_CONST
 		p.From.Offset = offset
 		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R4
+		p.To.Reg = REG_R11
 
 		p = obj.Appendp(p, c.newprog)
 		pPreempt = p
 		p.As = ACMPUBLT
 		p.From.Type = obj.TYPE_REG
 		p.From.Reg = REGSP
-		p.Reg = REG_R4
+		p.Reg = REG_R11
 		p.To.Type = obj.TYPE_BRANCH
 	}
 
 	// Check against the stack guard. We've ensured this won't underflow.
-	//	ADD $-(framesize-StackSmall), SP, R4
-	//	CMPUBGE stackguard, R4, label-of-call-to-morestack
+	//	ADD $-(framesize-StackSmall), SP, R11
+	//	CMPUBGE stackguard, R11, label-of-call-to-morestack
 	p = obj.Appendp(p, c.newprog)
 	p.As = AADD
 	p.From.Type = obj.TYPE_CONST
 	p.From.Offset = -offset
 	p.Reg = REGSP
 	p.To.Type = obj.TYPE_REG
-	p.To.Reg = REG_R4
+	p.To.Reg = REG_R11
 
 	p = obj.Appendp(p, c.newprog)
 	p.From.Type = obj.TYPE_REG
-	p.From.Reg = REG_R3
-	p.Reg = REG_R4
+	p.From.Reg = REG_R10
+	p.Reg = REG_R11
 	p.As = ACMPUBGE
 	p.To.Type = obj.TYPE_BRANCH
 
@@ -654,18 +662,22 @@ func (c *ctxtz) stacksplitPost(p *obj.Prog, pPre, pPreempt, pCheck *obj.Prog, fr
 
 	pcdata := c.ctxt.EmitEntryStackMap(c.cursym, spfix, c.newprog)
 	pcdata = c.ctxt.StartUnsafePoint(pcdata, c.newprog)
+	if pPreempt != nil {
+		pPreempt.To.SetTarget(pcdata)
+	}
+	pPre.To.SetTarget(pcdata)
+
+	// Spill the register args that could be clobbered by the
+	// morestack code.
+	spill := c.cursym.Func().SpillRegisterArgs(pcdata, c.newprog)
 
 	// MOVD	LR, R5
-	p = obj.Appendp(pcdata, c.newprog)
-	pPre.To.SetTarget(p)
+	p = obj.Appendp(spill, c.newprog)
 	p.As = AMOVD
 	p.From.Type = obj.TYPE_REG
 	p.From.Reg = REG_LR
 	p.To.Type = obj.TYPE_REG
 	p.To.Reg = REG_R5
-	if pPreempt != nil {
-		pPreempt.To.SetTarget(p)
-	}
 
 	// BL	runtime.morestack(SB)
 	p = obj.Appendp(p, c.newprog)
@@ -680,10 +692,12 @@ func (c *ctxtz) stacksplitPost(p *obj.Prog, pPre, pPreempt, pCheck *obj.Prog, fr
 		p.To.Sym = c.ctxt.Lookup("runtime.morestack")
 	}
 
+	// The instructions which unspill regs should be preemptible.
 	p = c.ctxt.EndUnsafePoint(p, c.newprog, -1)
+	unspill := c.cursym.Func().UnspillRegisterArgs(p, c.newprog)
 
 	// BR	pCheck
-	p = obj.Appendp(p, c.newprog)
+	p = obj.Appendp(unspill, c.newprog)
 
 	p.As = ABR
 	p.To.Type = obj.TYPE_BRANCH
diff --git a/src/internal/abi/abi_s390x.go b/src/internal/abi/abi_s390x.go
new file mode 100644
index 00000000000..9b07f27382d
--- /dev/null
+++ b/src/internal/abi/abi_s390x.go
@@ -0,0 +1,19 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.regabiargs
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// R2 - R9.
+	IntArgRegs = 8
+
+	// F0 - F15
+	FloatArgRegs = 16
+
+	EffectiveFloatRegSize = 8
+)
diff --git a/src/internal/buildcfg/exp.go b/src/internal/buildcfg/exp.go
index ddd05c6f287..df41f793388 100644
--- a/src/internal/buildcfg/exp.go
+++ b/src/internal/buildcfg/exp.go
@@ -65,6 +65,8 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 	case "amd64", "arm64", "loong64", "ppc64le", "ppc64", "riscv64":
 		regabiAlwaysOn = true
 		regabiSupported = true
+	case "s390x":
+		regabiSupported = true
 	}
 
 	// Older versions (anything before V16) of dsymutil don't handle
@@ -141,7 +143,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		flags.RegabiWrappers = true
 		flags.RegabiArgs = true
 	}
-	// regabi is only supported on amd64, arm64, loong64, riscv64, ppc64 and ppc64le.
+	// regabi is only supported on amd64, arm64, loong64, riscv64, s390x, ppc64 and ppc64le.
 	if !regabiSupported {
 		flags.RegabiWrappers = false
 		flags.RegabiArgs = false
diff --git a/src/internal/bytealg/compare_s390x.s b/src/internal/bytealg/compare_s390x.s
index 539454870d3..64f537be99e 100644
--- a/src/internal/bytealg/compare_s390x.s
+++ b/src/internal/bytealg/compare_s390x.s
@@ -5,65 +5,93 @@
 #include "go_asm.h"
 #include "textflag.h"
 
-TEXT ·Compare(SB),NOSPLIT|NOFRAME,$0-56
-	MOVD	a_base+0(FP), R3
-	MOVD	a_len+8(FP), R4
-	MOVD	b_base+24(FP), R5
-	MOVD	b_len+32(FP), R6
-	LA	ret+48(FP), R7
+TEXT ·Compare<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-56
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	a_base+0(FP), R2
+	MOVD	a_len+8(FP), R3
+	MOVD	b_base+24(FP), R4
+	MOVD	b_len+32(FP), R5
+	LA	ret+48(FP), R6
+#else
+	// R2 = a_base
+	// R3 = a_len
+	// R4 = a_cap (unused)
+	// R5 = b_base (want in R4)
+	// R6 = b_len (want in R5)
+	// R7 = b_cap (unused)
+	MOVD	R5, R4
+	MOVD	R6, R5
+#endif
 	BR	cmpbody<>(SB)
 
-TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
-	MOVD	a_base+0(FP), R3
-	MOVD	a_len+8(FP), R4
-	MOVD	b_base+16(FP), R5
-	MOVD	b_len+24(FP), R6
-	LA	ret+32(FP), R7
+TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	a_base+0(FP), R2
+	MOVD	a_len+8(FP), R3
+	MOVD	b_base+16(FP), R4
+	MOVD	b_len+24(FP), R5
+	LA	ret+32(FP), R6
+#endif
+	// R2 = a_base
+	// R3 = a_len
+	// R4 = b_base
+	// R5 = b_len
+
 	BR	cmpbody<>(SB)
 
 // input:
-//   R3 = a
-//   R4 = alen
-//   R5 = b
-//   R6 = blen
-//   R7 = address of output word (stores -1/0/1 here)
+//   R2 = a
+//   R3 = alen
+//   R4 = b
+//   R5 = blen
+//   For regabiargs output value( -1/0/1 ) stored in R2
+//   For !regabiargs address of output word( stores -1/0/1 ) stored in R6
 TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
-	CMPBEQ	R3, R5, cmplengths
-	MOVD	R4, R8
-	CMPBLE	R4, R6, amin
-	MOVD	R6, R8
+	CMPBEQ	R2, R4, cmplengths
+	MOVD	R3, R7
+	CMPBLE	R3, R5, amin
+	MOVD	R5, R7
 amin:
-	CMPBEQ	R8, $0, cmplengths
-	CMP	R8, $256
+	CMPBEQ	R7, $0, cmplengths
+	CMP	R7, $256
 	BLE	tail
 loop:
-	CLC	$256, 0(R3), 0(R5)
+	CLC	$256, 0(R2), 0(R4)
 	BGT	gt
 	BLT	lt
-	SUB	$256, R8
-	MOVD	$256(R3), R3
-	MOVD	$256(R5), R5
-	CMP	R8, $256
+	SUB	$256, R7
+	MOVD	$256(R2), R2
+	MOVD	$256(R4), R4
+	CMP	R7, $256
 	BGT	loop
 tail:
-	SUB	$1, R8
-	EXRL	$cmpbodyclc<>(SB), R8
+	SUB	$1, R7
+	EXRL	$cmpbodyclc<>(SB), R7
 	BGT	gt
 	BLT	lt
 cmplengths:
-	CMP	R4, R6
+	CMP	R3, R5
 	BEQ	eq
 	BLT	lt
 gt:
-	MOVD	$1, 0(R7)
+	MOVD	$1, R2
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	R2, 0(R6)
+#endif
 	RET
 lt:
-	MOVD	$-1, 0(R7)
+	MOVD	$-1, R2
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	R2, 0(R6)
+#endif
 	RET
 eq:
-	MOVD	$0, 0(R7)
+	MOVD	$0, R2
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	R2, 0(R6)
+#endif
 	RET
 
 TEXT cmpbodyclc<>(SB),NOSPLIT|NOFRAME,$0-0
-	CLC	$1, 0(R3), 0(R5)
+	CLC	$1, 0(R2), 0(R4)
 	RET
diff --git a/src/internal/bytealg/equal_s390x.s b/src/internal/bytealg/equal_s390x.s
index 67f814dfc1c..48e8d0f1547 100644
--- a/src/internal/bytealg/equal_s390x.s
+++ b/src/internal/bytealg/equal_s390x.s
@@ -6,80 +6,92 @@
 #include "textflag.h"
 
 // memequal(a, b unsafe.Pointer, size uintptr) bool
-TEXT runtime·memequal(SB),NOSPLIT|NOFRAME,$0-25
-	MOVD	a+0(FP), R3
-	MOVD	b+8(FP), R5
-	MOVD	size+16(FP), R6
-	LA	ret+24(FP), R7
+TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-25
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	a+0(FP), R2
+	MOVD	b+8(FP), R3
+	MOVD	size+16(FP), R4
+	LA	ret+24(FP), R5
+#endif
 	BR	memeqbody<>(SB)
 
 // memequal_varlen(a, b unsafe.Pointer) bool
-TEXT runtime·memequal_varlen(SB),NOSPLIT|NOFRAME,$0-17
-	MOVD	a+0(FP), R3
-	MOVD	b+8(FP), R5
-	MOVD	8(R12), R6    // compiler stores size at offset 8 in the closure
-	LA	ret+16(FP), R7
+TEXT runtime·memequal_varlen<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-17
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD	a+0(FP), R2
+	MOVD	b+8(FP), R3
+	LA	ret+16(FP), R5
+#endif
+
+	MOVD	8(R12), R4    // compiler stores size at offset 8 in the closure
 	BR	memeqbody<>(SB)
 
 // input:
-//   R3 = a
-//   R5 = b
-//   R6 = len
-//   R7 = address of output byte (stores 0 or 1 here)
+//   R2 = a
+//   R3 = b
+//   R4 = len
+//   For regabiargs output value( 0/1 ) stored in R2
+//   For !regabiargs address of output byte( stores 0/1 ) stored in R5
 //   a and b have the same length
 TEXT memeqbody<>(SB),NOSPLIT|NOFRAME,$0-0
-	CMPBEQ	R3, R5, equal
+	CMPBEQ	R2, R3, equal
 loop:
-	CMPBEQ	R6, $0, equal
-	CMPBLT	R6, $32, tiny
-	CMP	R6, $256
+	CMPBEQ	R4, $0, equal
+	CMPBLT	R4, $32, tiny
+	CMP	R4, $256
 	BLT	tail
-	CLC	$256, 0(R3), 0(R5)
+	CLC	$256, 0(R2), 0(R3)
 	BNE	notequal
-	SUB	$256, R6
+	SUB	$256, R4
+	LA	256(R2), R2
 	LA	256(R3), R3
-	LA	256(R5), R5
 	BR	loop
 tail:
-	SUB	$1, R6, R8
+	SUB	$1, R4, R8
 	EXRL	$memeqbodyclc<>(SB), R8
 	BEQ	equal
 notequal:
-	MOVB	$0, 0(R7)
+	MOVD	$0, R2
+#ifndef GOEXPERIMENT_regabiargs
+	MOVB	R2, 0(R5)
+#endif
 	RET
 equal:
-	MOVB	$1, 0(R7)
+	MOVD	$1, R2
+#ifndef GOEXPERIMENT_regabiargs
+	MOVB	R2, 0(R5)
+#endif
 	RET
 tiny:
-	MOVD	$0, R2
-	CMPBLT	R6, $16, lt16
-	MOVD	0(R3), R8
-	MOVD	0(R5), R9
+	MOVD	$0, R1
+	CMPBLT	R4, $16, lt16
+	MOVD	0(R2), R8
+	MOVD	0(R3), R9
 	CMPBNE	R8, R9, notequal
-	MOVD	8(R3), R8
-	MOVD	8(R5), R9
+	MOVD	8(R2), R8
+	MOVD	8(R3), R9
 	CMPBNE	R8, R9, notequal
-	LA	16(R2), R2
-	SUB	$16, R6
+	LA	16(R1), R1
+	SUB	$16, R4
 lt16:
-	CMPBLT	R6, $8, lt8
-	MOVD	0(R3)(R2*1), R8
-	MOVD	0(R5)(R2*1), R9
+	CMPBLT	R4, $8, lt8
+	MOVD	0(R2)(R1*1), R8
+	MOVD	0(R3)(R1*1), R9
 	CMPBNE	R8, R9, notequal
-	LA	8(R2), R2
-	SUB	$8, R6
+	LA	8(R1), R1
+	SUB	$8, R4
 lt8:
-	CMPBLT	R6, $4, lt4
-	MOVWZ	0(R3)(R2*1), R8
-	MOVWZ	0(R5)(R2*1), R9
+	CMPBLT	R4, $4, lt4
+	MOVWZ	0(R2)(R1*1), R8
+	MOVWZ	0(R3)(R1*1), R9
 	CMPBNE	R8, R9, notequal
-	LA	4(R2), R2
-	SUB	$4, R6
+	LA	4(R1), R1
+	SUB	$4, R4
 lt4:
 #define CHECK(n) \
-	CMPBEQ	R6, $n, equal \
-	MOVB	n(R3)(R2*1), R8 \
-	MOVB	n(R5)(R2*1), R9 \
+	CMPBEQ	R4, $n, equal \
+	MOVB	n(R2)(R1*1), R8 \
+	MOVB	n(R3)(R1*1), R9 \
 	CMPBNE	R8, R9, notequal
 	CHECK(0)
 	CHECK(1)
@@ -88,5 +100,5 @@ lt4:
 	BR	equal
 
 TEXT memeqbodyclc<>(SB),NOSPLIT|NOFRAME,$0-0
-	CLC	$1, 0(R3), 0(R5)
+	CLC	$1, 0(R2), 0(R3)
 	RET
diff --git a/src/internal/bytealg/indexbyte_s390x.s b/src/internal/bytealg/indexbyte_s390x.s
index cf88d92a24b..343ed672f76 100644
--- a/src/internal/bytealg/indexbyte_s390x.s
+++ b/src/internal/bytealg/indexbyte_s390x.s
@@ -5,104 +5,134 @@
 #include "go_asm.h"
 #include "textflag.h"
 
-TEXT ·IndexByte(SB),NOSPLIT|NOFRAME,$0-40
-	MOVD	b_base+0(FP), R3// b_base => R3
-	MOVD	b_len+8(FP), R4 // b_len => R4
-	MOVBZ	c+24(FP), R5    // c => R5
-	MOVD	$ret+32(FP), R2 // &ret => R9
-	BR	indexbytebody<>(SB)
 
-TEXT ·IndexByteString(SB),NOSPLIT|NOFRAME,$0-32
-	MOVD	s_base+0(FP), R3// s_base => R3
-	MOVD	s_len+8(FP), R4 // s_len => R4
-	MOVBZ	c+16(FP), R5    // c => R5
-	MOVD	$ret+24(FP), R2 // &ret => R9
-	BR	indexbytebody<>(SB)
+TEXT ·IndexByte<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD    b_base+0(FP), R2// b_base => R2
+	MOVD    b_len+8(FP), R3 // b_len => R3
+	MOVBZ   c+24(FP), R4    // c => R4
+	MOVD    $ret+32(FP), R5 // &ret => R5
+#else
+	MOVD    R5, R4
+	AND	$0xff, R4
+#endif
+	BR      indexbytebody<>(SB)
+
+TEXT ·IndexByteString<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-32
+#ifndef GOEXPERIMENT_regabiargs
+	MOVD    s_base+0(FP), R2 // s_base => R2
+	MOVD    s_len+8(FP), R3 // s_len => R3
+	MOVBZ   c+16(FP), R4    // c => R4
+	MOVD    $ret+24(FP), R5 // &ret => R5
+#else
+	AND	$0xff, R4
+#endif
+	BR      indexbytebody<>(SB)
 
 // input:
-// R3: s
-// R4: s_len
-// R5: c -- byte sought
-// R2: &ret -- address to put index into
+// R2: s
+// R3: s_len
+// R4: c -- byte sought
+// For regabiargs output value(index) stored in R2
+// For !regabiargs address of output value(index) stored in R5
 TEXT indexbytebody<>(SB),NOSPLIT|NOFRAME,$0
-	CMPBEQ	R4, $0, notfound
-	MOVD	R3, R6          // store base for later
-	ADD	R3, R4, R8      // the address after the end of the string
-	//if the length is small, use loop; otherwise, use vector or srst search
-	CMPBGE	R4, $16, large
+        CMPBEQ  R3, $0, notfound
+        MOVD    R2, R6          // store base for later
+        ADD     R2, R3, R8      // the address after the end of the string
+        //if the length is small, use loop; otherwise, use vector or srst search
+        CMPBGE  R3, $16, large
 
 residual:
-	CMPBEQ	R3, R8, notfound
-	MOVBZ	0(R3), R7
-	LA	1(R3), R3
-	CMPBNE	R7, R5, residual
+        CMPBEQ  R2, R8, notfound
+        MOVBZ   0(R2), R7
+        LA      1(R2), R2
+        CMPBNE  R7, R4, residual
 
 found:
-	SUB	R6, R3
-	SUB	$1, R3
-	MOVD	R3, 0(R2)
-	RET
+        SUB     R6, R2
+        SUB     $1, R2
+#ifndef GOEXPERIMENT_regabiargs
+        MOVD    R2, 0(R5)
+#endif
+        RET
 
 notfound:
-	MOVD	$-1, 0(R2)
-	RET
+#ifndef GOEXPERIMENT_regabiargs
+        MOVD    $-1, 0(R5)
+#else
+        MOVD    $-1, R2
+#endif
+        RET
 
 large:
-	MOVBZ	internal∕cpu·S390X+const_offsetS390xHasVX(SB), R1
-	CMPBNE	R1, $0, vectorimpl
+        MOVBZ   internal∕cpu·S390X+const_offsetS390xHasVX(SB), R1
+        CMPBNE  R1, $0, vectorimpl
 
 srstimpl:                       // no vector facility
-	MOVBZ	R5, R0          // c needs to be in R0, leave until last minute as currently R0 is expected to be 0
+        MOVBZ   R4, R0          // c needs to be in R0, leave until last minute as currently R0 is expected to be 0
 srstloop:
-	WORD	$0xB25E0083     // srst %r8, %r3 (search the range [R3, R8))
-	BVS	srstloop        // interrupted - continue
-	BGT	notfoundr0
+        WORD    $0xB25E0082     // srst %r8, %r2 (search the range [R2, R8))
+        BVS     srstloop        // interrupted - continue
+        BGT     notfoundr0
 foundr0:
-	XOR	R0, R0          // reset R0
-	SUB	R6, R8          // remove base
-	MOVD	R8, 0(R2)
-	RET
+        XOR     R0, R0          // reset R0
+        SUB     R6, R8          // remove base
+#ifndef GOEXPERIMENT_regabiargs
+        MOVD    R8, 0(R5)
+#else
+        MOVD    R8, R2
+#endif
+        RET
 notfoundr0:
-	XOR	R0, R0          // reset R0
-	MOVD	$-1, 0(R2)
-	RET
+        XOR     R0, R0          // reset R0
+#ifndef GOEXPERIMENT_regabiargs
+        MOVD    $-1, 0(R5)
+#else
+        MOVD    $-1, R2
+#endif
+        RET
 
 vectorimpl:
-	//if the address is not 16byte aligned, use loop for the header
-	MOVD	R3, R8
-	AND	$15, R8
-	CMPBGT	R8, $0, notaligned
+        //if the address is not 16byte aligned, use loop for the header
+        MOVD    R2, R8
+        AND     $15, R8
+        CMPBGT  R8, $0, notaligned
 
 aligned:
-	ADD	R6, R4, R8
-	MOVD	R8, R7
-	AND	$-16, R7
-	// replicate c across V17
-	VLVGB	$0, R5, V19
-	VREPB	$0, V19, V17
+        ADD     R6, R3, R8
+        MOVD    R8, R7
+        AND     $-16, R7
+        // replicate c across V17
+        VLVGB   $0, R4, V19
+        VREPB   $0, V19, V17
 
 vectorloop:
-	CMPBGE	R3, R7, residual
-	VL	0(R3), V16    // load string to be searched into V16
-	ADD	$16, R3
-	VFEEBS	V16, V17, V18 // search V17 in V16 and set conditional code accordingly
-	BVS	vectorloop
+        CMPBGE  R2, R7, residual
+        VL      0(R2), V16    // load string to be searched into V16
+        ADD     $16, R2
+        VFEEBS  V16, V17, V18 // search V17 in V16 and set conditional code accordingly
+        BVS     vectorloop
 
-	// when vector search found c in the string
-	VLGVB	$7, V18, R7   // load 7th element of V18 containing index into R7
-	SUB	$16, R3
-	SUB	R6, R3
-	ADD	R3, R7
-	MOVD	R7, 0(R2)
-	RET
+        // when vector search found c in the string
+        VLGVB   $7, V18, R7   // load 7th element of V18 containing index into R7
+        SUB     $16, R2
+        SUB     R6, R2
+        ADD     R2, R7
+#ifndef GOEXPERIMENT_regabiargs
+        MOVD    R7, 0(R5)
+#else
+        MOVD    R7, R2
+#endif
+        RET
 
 notaligned:
-	MOVD	R3, R8
-	AND	$-16, R8
-	ADD     $16, R8
+        MOVD    R2, R8
+        AND     $-16, R8
+        ADD     $16, R8
 notalignedloop:
-	CMPBEQ	R3, R8, aligned
-	MOVBZ	0(R3), R7
-	LA	1(R3), R3
-	CMPBNE	R7, R5, notalignedloop
-	BR	found
+        CMPBEQ  R2, R8, aligned
+        MOVBZ   0(R2), R7
+        LA      1(R2), R2
+        CMPBNE  R7, R4, notalignedloop
+        BR      found
+
diff --git a/src/internal/runtime/syscall/linux/asm_linux_s390x.s b/src/internal/runtime/syscall/linux/asm_linux_s390x.s
index 1b27f293907..c912afab649 100644
--- a/src/internal/runtime/syscall/linux/asm_linux_s390x.s
+++ b/src/internal/runtime/syscall/linux/asm_linux_s390x.s
@@ -5,7 +5,16 @@
 #include "textflag.h"
 
 // func Syscall6(num, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, errno uintptr)
-TEXT ·Syscall6(SB),NOSPLIT,$0-80
+TEXT ·Syscall6<ABIInternal>(SB),NOSPLIT,$0-80
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	R2, R1
+	MOVD	R3, R2
+	MOVD	R4, R3
+	MOVD	R5, R4
+	MOVD	R6, R5
+	MOVD	R7, R6
+	MOVD	R8, R7
+#else
 	MOVD	num+0(FP), R1	// syscall entry
 	MOVD	a1+8(FP), R2
 	MOVD	a2+16(FP), R3
@@ -13,16 +22,27 @@ TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	MOVD	a4+32(FP), R5
 	MOVD	a5+40(FP), R6
 	MOVD	a6+48(FP), R7
+#endif
 	SYSCALL
 	MOVD	$0xfffffffffffff001, R8
 	CMPUBLT	R2, R8, ok
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	$0, R3
+	NEG	R2, R4
+	MOVD    $-1, R2
+#else
 	MOVD	$-1, r1+56(FP)
 	MOVD	$0, r2+64(FP)
 	NEG	R2, R2
 	MOVD	R2, errno+72(FP)
+#endif
 	RET
 ok:
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	$0, R4
+#else
 	MOVD	R2, r1+56(FP)
 	MOVD	R3, r2+64(FP)
 	MOVD	$0, errno+72(FP)
+#endif
 	RET
diff --git a/src/reflect/asm_s390x.s b/src/reflect/asm_s390x.s
index 4bd6613004f..22cbd38ee5c 100644
--- a/src/reflect/asm_s390x.s
+++ b/src/reflect/asm_s390x.s
@@ -5,34 +5,82 @@
 #include "textflag.h"
 #include "funcdata.h"
 
+// The frames of each of the two functions below contain two locals, at offsets
+// that are known to the runtime.
+//
+// The first local is a bool called retValid with a whole pointer-word reserved
+// for it on the stack. The purpose of this word is so that the runtime knows
+// whether the stack-allocated return space contains valid values for stack
+// scanning.
+//
+// The second local is an abi.RegArgs value whose offset is also known to the
+// runtime, so that a stack map for it can be constructed, since it contains
+// pointers visible to the GC.
+
+#define LOCAL_RETVALID 40
+#define LOCAL_REGARGS 48
+
+// The frame size of the functions below is
+// 32 (args of callReflect/callMethod) + 8 (bool + padding) + 264 (abi.RegArgs) = 304.
+
 // makeFuncStub is the code half of the function returned by MakeFunc.
 // See the comment on the declaration of makeFuncStub in makefunc.go
 // for more details.
 // No arg size here, runtime pulls arg map out of the func value.
-TEXT ·makeFuncStub(SB),(NOSPLIT|WRAPPER),$40
+TEXT ·makeFuncStub(SB),(NOSPLIT|WRAPPER),$304
 	NO_LOCAL_POINTERS
+	ADD	$LOCAL_REGARGS, R15, R10 // spillArgs using R10
+	BL	runtime·spillArgs(SB)
+	MOVD	R12, 32(R15) // save context reg R12 > args of moveMakeFuncArgPtrs < LOCAL_REGARGS
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	R12, R2
+	MOVD	R10, R3
+#else
 	MOVD	R12, 8(R15)
-	MOVD	$argframe+0(FP), R3
-	MOVD	R3, 16(R15)
-	MOVB	$0, 40(R15)
-	ADD	$40, R15, R3
-	MOVD	R3, 24(R15)
-	MOVD	$0, 32(R15)
+	MOVD	R10, 16(R15)
+#endif
+	BL	·moveMakeFuncArgPtrs<ABIInternal>(SB)
+	MOVD	32(R15), R12 // restore context reg R12
+	MOVD	R12, 8(R15)
+	MOVD	$argframe+0(FP), R1
+	MOVD	R1, 16(R15)
+	MOVB	$0, LOCAL_RETVALID(R15)
+	ADD	$LOCAL_RETVALID, R15, R1
+	MOVD	R1, 24(R15)
+	ADD	$LOCAL_REGARGS, R15, R1
+	MOVD	R1, 32(R15)
 	BL	·callReflect(SB)
+	ADD	$LOCAL_REGARGS, R15, R10 // unspillArgs using R10
+	BL	runtime·unspillArgs(SB)
 	RET
 
 // methodValueCall is the code half of the function returned by makeMethodValue.
 // See the comment on the declaration of methodValueCall in makefunc.go
 // for more details.
 // No arg size here; runtime pulls arg map out of the func value.
-TEXT ·methodValueCall(SB),(NOSPLIT|WRAPPER),$40
+TEXT ·methodValueCall(SB),(NOSPLIT|WRAPPER),$304
 	NO_LOCAL_POINTERS
+	ADD	$LOCAL_REGARGS, R15, R10 // spillArgs using R10
+	BL	runtime·spillArgs(SB)
+	MOVD	R12, 32(R15) // save context reg R12 > args of moveMakeFuncArgPtrs < LOCAL_REGARGS
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	R12, R2
+	MOVD	R10, R3
+#else
 	MOVD	R12, 8(R15)
-	MOVD	$argframe+0(FP), R3
-	MOVD	R3, 16(R15)
-	MOVB	$0, 40(R15)
-	ADD	$40, R15, R3
-	MOVD	R3, 24(R15)
-	MOVD	$0, 32(R15)
+	MOVD	R10, 16(R15)
+#endif
+	BL	·moveMakeFuncArgPtrs<ABIInternal>(SB)
+	MOVD	32(R15), R12 // restore context reg R12
+	MOVD	R12, 8(R15)
+	MOVD	$argframe+0(FP), R1
+	MOVD	R1, 16(R15)
+	MOVB	$0, LOCAL_RETVALID(R15)
+	ADD	$LOCAL_RETVALID, R15, R1
+	MOVD	R1, 24(R15)
+	ADD	$LOCAL_REGARGS, R15, R1
+	MOVD	R1, 32(R15)
 	BL	·callMethod(SB)
+	ADD	$LOCAL_REGARGS, R15, R10 // unspillArgs using R10
+	BL	runtime·unspillArgs(SB)
 	RET
diff --git a/src/reflect/float32reg_generic.go b/src/reflect/float32reg_generic.go
index 23ad4bf285b..efbc3edc3d5 100644
--- a/src/reflect/float32reg_generic.go
+++ b/src/reflect/float32reg_generic.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !ppc64 && !ppc64le && !riscv64
+//go:build !ppc64 && !ppc64le && !riscv64 && !s390x
 
 package reflect
 
diff --git a/src/reflect/float32reg_s390x.s b/src/reflect/float32reg_s390x.s
new file mode 100644
index 00000000000..bcf55823267
--- /dev/null
+++ b/src/reflect/float32reg_s390x.s
@@ -0,0 +1,30 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build s390x
+
+#include "textflag.h"
+
+// On s390x, the float32 becomes a float64
+// when loaded in a register, different from
+// other platforms. These functions are
+// needed to ensure correct conversions on s390x.
+
+// Convert float32->uint64
+TEXT ·archFloat32ToReg(SB),NOSPLIT,$0-16
+       FMOVS   val+0(FP), F1
+       FMOVD   F1, ret+8(FP)
+       RET
+
+// Convert uint64->float32
+TEXT ·archFloat32FromReg(SB),NOSPLIT,$0-12
+       FMOVD   reg+0(FP), F1
+       // Normally a float64->float32 conversion
+       // would need rounding, but that is not needed
+       // here since the uint64 was originally converted
+       // from float32, and should be avoided to
+       // preserve SNaN values.
+       FMOVS   F1, ret+8(FP)
+       RET
+
diff --git a/src/reflect/stubs_s390x.go b/src/reflect/stubs_s390x.go
new file mode 100644
index 00000000000..03504d7904b
--- /dev/null
+++ b/src/reflect/stubs_s390x.go
@@ -0,0 +1,10 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build s390x
+
+package reflect
+
+func archFloat32FromReg(reg uint64) float32
+func archFloat32ToReg(val float32) uint64
diff --git a/src/runtime/asm_s390x.s b/src/runtime/asm_s390x.s
index bb29845f583..791ea80bc29 100644
--- a/src/runtime/asm_s390x.s
+++ b/src/runtime/asm_s390x.s
@@ -160,7 +160,7 @@ nocgo:
 	MOVD	$0, 1(R0)
 	RET
 
-DATA	runtime·mainPC+0(SB)/8,$runtime·main(SB)
+DATA	runtime·mainPC+0(SB)/8,$runtime·main<ABIInternal>(SB)
 GLOBL	runtime·mainPC(SB),RODATA,$8
 
 TEXT runtime·breakpoint(SB),NOSPLIT|NOFRAME,$0-0
@@ -205,25 +205,29 @@ TEXT gogo<>(SB), NOSPLIT|NOFRAME, $0
 // Switch to m->g0's stack, call fn(g).
 // Fn must never return.  It should gogo(&g->sched)
 // to keep running g.
-TEXT runtime·mcall(SB), NOSPLIT, $-8-8
+TEXT runtime·mcall<ABIInternal>(SB), NOSPLIT, $-8-8
+#ifdef GOEXPERIMENT_regabiargs
+	MOVD	R2, R12				// context
+#else
+	MOVD	fn+0(FP), R12			// context
+#endif
 	// Save caller state in g->sched
 	MOVD	R15, (g_sched+gobuf_sp)(g)
 	MOVD	LR, (g_sched+gobuf_pc)(g)
 	MOVD	$0, (g_sched+gobuf_lr)(g)
 
 	// Switch to m->g0 & its stack, call fn.
-	MOVD	g, R3
-	MOVD	g_m(g), R8
-	MOVD	m_g0(R8), g
+	MOVD	g, R2
+	MOVD	g_m(g), R4
+	MOVD	m_g0(R4), g
 	BL	runtime·save_g(SB)
-	CMP	g, R3
+	CMP	g, R2
 	BNE	2(PC)
 	BR	runtime·badmcall(SB)
-	MOVD	fn+0(FP), R12			// context
 	MOVD	0(R12), R4			// code pointer
 	MOVD	(g_sched+gobuf_sp)(g), R15	// sp = m->g0->sched.sp
 	SUB	$16, R15
-	MOVD	R3, 8(R15)
+	MOVD	R2, 8(R15)
 	MOVD	$0, 0(R15)
 	BL	(R4)
 	BR	runtime·badmcall2(SB)
@@ -292,18 +296,18 @@ noswitch:
 
 // func switchToCrashStack0(fn func())
 TEXT runtime·switchToCrashStack0<ABIInternal>(SB), NOSPLIT, $0-8
-	MOVD	fn+0(FP), R12	// context
-	MOVD	g_m(g), R4	// curm
+	MOVD	R2, R12		// context
+	MOVD	g_m(g), R2	// curm
 
 	// set g to gcrash
 	MOVD	$runtime·gcrash(SB), g	// g = &gcrash
 	BL	runtime·save_g(SB)
-	MOVD	R4, g_m(g)	// g.m = curm
-	MOVD	g, m_g0(R4)	// curm.g0 = g
+	MOVD	R2, g_m(g)	// g.m = curm
+	MOVD	g, m_g0(R2)	// curm.g0 = g
 
 	// switch to crashstack
-	MOVD	(g_stack+stack_hi)(g), R4
-	ADD	$(-4*8), R4, R15
+	MOVD	(g_stack+stack_hi)(g), R2
+	ADD	$(-4*8), R2, R15
 
 	// call target function
 	MOVD	0(R12), R3	// code pointer
@@ -446,10 +450,14 @@ tailArgs: /* copy remaining bytes */		\
 	EXRL	$callfnMVC<>(SB), R5;		\
 callFunction:					\
 	MOVD	f+8(FP), R12;			\
-	MOVD	(R12), R8;			\
+	MOVD    regArgs+40(FP), R10;		\
+	BL      ·unspillArgs(SB);		\
+	MOVD	(R12), R10;			\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
-	BL	(R8);				\
+	BL	(R10);				\
 	/* copy return values back */		\
+	MOVD    regArgs+40(FP), R10;		\
+	BL      ·spillArgs(SB);		\
 	MOVD	stackArgsType+0(FP), R7;		\
 	MOVD	stackArgs+16(FP), R6;			\
 	MOVWZ	stackArgsSize+24(FP), R5;			\
@@ -466,11 +474,12 @@ callFunction:					\
 // to reflectcallmove. It does not follow the Go ABI; it expects its
 // arguments in registers.
 TEXT callRet<>(SB), NOSPLIT, $40-0
+	NO_LOCAL_POINTERS;
 	MOVD	R7, 8(R15)
 	MOVD	R6, 16(R15)
 	MOVD	R4, 24(R15)
 	MOVD	R5, 32(R15)
-	MOVD	$0, 40(R15)
+	MOVD	R10, 40(R15)
 	BL	runtime·reflectcallmove(SB)
 	RET
 
@@ -754,15 +763,80 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-8
 	MOVD	R3, ret+0(FP)
 	RET
 
+#ifdef GOEXPERIMENT_regabiargs
+// spillArgs stores return values from registers to a *internal/abi.RegArgs in R10.
+TEXT runtime·spillArgs(SB),NOSPLIT,$0-0
+	MOVD	R2, 0(R10)
+	MOVD	R3, 8(R10)
+	MOVD	R4, 16(R10)
+	MOVD	R5, 24(R10)
+	MOVD	R6, 32(R10)
+	MOVD	R7, 40(R10)
+	MOVD	R8, 48(R10)
+	MOVD	R9, 56(R10)
+	FMOVD	F0, 64(R10)
+	FMOVD	F1, 72(R10)
+	FMOVD	F2, 80(R10)
+	FMOVD	F3, 88(R10)
+	FMOVD	F4, 96(R10)
+	FMOVD	F5, 104(R10)
+	FMOVD	F6, 112(R10)
+	FMOVD	F7, 120(R10)
+	FMOVD	F8, 128(R10)
+	FMOVD	F9, 136(R10)
+	FMOVD	F10, 144(R10)
+	FMOVD	F11, 152(R10)
+	FMOVD	F12, 160(R10)
+	FMOVD	F13, 168(R10)
+	FMOVD	F14, 176(R10)
+	FMOVD	F15, 184(R10)
+	RET
+
+// unspillArgs loads args into registers from a *internal/abi.RegArgs in R10.
+TEXT runtime·unspillArgs(SB),NOSPLIT,$0-0
+	MOVD	0(R10), R2
+	MOVD	8(R10), R3
+	MOVD	16(R10), R4
+	MOVD	24(R10), R5
+	MOVD	32(R10), R6
+	MOVD	40(R10), R7
+	MOVD	48(R10), R8
+	MOVD	56(R10), R9
+	FMOVD	64(R10), F0
+	FMOVD	72(R10), F1
+	FMOVD	80(R10), F2
+	FMOVD	88(R10), F3
+	FMOVD	96(R10), F4
+	FMOVD	104(R10), F5
+	FMOVD	112(R10), F6
+	FMOVD	120(R10), F7
+	FMOVD	128(R10), F8
+	FMOVD	136(R10), F9
+	FMOVD	144(R10), F10
+	FMOVD	152(R10), F11
+	FMOVD	160(R10), F12
+	FMOVD	168(R10), F13
+	FMOVD	176(R10), F14
+	FMOVD	184(R10), F15
+	RET
+#else
+
+TEXT runtime·spillArgs(SB),NOSPLIT,$0-0
+        RET
+
+TEXT runtime·unspillArgs(SB),NOSPLIT,$0-0
+        RET
+#endif
+
 // AES hashing not implemented for s390x
-TEXT runtime·memhash(SB),NOSPLIT|NOFRAME,$0-32
-	JMP	runtime·memhashFallback(SB)
-TEXT runtime·strhash(SB),NOSPLIT|NOFRAME,$0-24
-	JMP	runtime·strhashFallback(SB)
-TEXT runtime·memhash32(SB),NOSPLIT|NOFRAME,$0-24
-	JMP	runtime·memhash32Fallback(SB)
-TEXT runtime·memhash64(SB),NOSPLIT|NOFRAME,$0-24
-	JMP	runtime·memhash64Fallback(SB)
+TEXT runtime·memhash<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-32
+	JMP	runtime·memhashFallback<ABIInternal>(SB)
+TEXT runtime·strhash<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-24
+	JMP	runtime·strhashFallback<ABIInternal>(SB)
+TEXT runtime·memhash32<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-24
+	JMP	runtime·memhash32Fallback<ABIInternal>(SB)
+TEXT runtime·memhash64<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-24
+	JMP	runtime·memhash64Fallback<ABIInternal>(SB)
 
 // Called from cgo wrappers, this function returns g->m->curg.stack.hi.
 // Must obey the gcc calling convention.
@@ -902,8 +976,7 @@ TEXT runtime·panicBounds<ABIInternal>(SB),NOSPLIT,$144-0
 	// skip R14 aka LR @ 136
 	// skip R15 aka SP @ 144
 
-	MOVD	R14, 8(R15)	// PC immediately after call to panicBounds
-	ADD	$24, R15, R0	// pointer to save area
-	MOVD	R0, 16(R15)
+	MOVD	R14, R2		// PC immediately after call to panicBounds
+	ADD     $24, R15, R3	// pointer to save area
 	CALL	runtime·panicBounds64<ABIInternal>(SB)
 	RET
diff --git a/src/runtime/cgocall.go b/src/runtime/cgocall.go
index f01353ffa6c..55e7bdbdb55 100644
--- a/src/runtime/cgocall.go
+++ b/src/runtime/cgocall.go
@@ -796,6 +796,9 @@ func cgoCheckResult(val any) {
 
 	ep := efaceOf(&val)
 	t := ep._type
+	if t == nil {
+		return
+	}
 	cgoCheckArg(t, ep.data, !t.IsDirectIface(), false, cgoResultFail)
 }
 
diff --git a/src/runtime/memclr_s390x.s b/src/runtime/memclr_s390x.s
index 392057565e8..919423edf78 100644
--- a/src/runtime/memclr_s390x.s
+++ b/src/runtime/memclr_s390x.s
@@ -7,10 +7,14 @@
 // See memclrNoHeapPointers Go doc for important implementation constraints.
 
 // func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
-TEXT runtime·memclrNoHeapPointers(SB),NOSPLIT|NOFRAME,$0-16
+TEXT runtime·memclrNoHeapPointers<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-16
+#ifndef GOEXPERIMENT_regabiargs
 	MOVD	ptr+0(FP), R4
 	MOVD	n+8(FP), R5
-
+#else
+	MOVD	R2, R4
+	MOVD	R3, R5
+#endif
 	CMPBGE	R5, $32, clearge32
 
 start:
diff --git a/src/runtime/memmove_s390x.s b/src/runtime/memmove_s390x.s
index f4c2b87d929..28c6a5dab20 100644
--- a/src/runtime/memmove_s390x.s
+++ b/src/runtime/memmove_s390x.s
@@ -7,10 +7,16 @@
 // See memmove Go doc for important implementation constraints.
 
 // func memmove(to, from unsafe.Pointer, n uintptr)
-TEXT runtime·memmove(SB),NOSPLIT|NOFRAME,$0-24
+TEXT runtime·memmove<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-24
+#ifndef GOEXPERIMENT_regabiargs
 	MOVD	to+0(FP), R6
 	MOVD	from+8(FP), R4
 	MOVD	n+16(FP), R5
+#else
+	MOVD	R4, R5
+	MOVD	R3, R4
+	MOVD	R2, R6
+#endif
 
 	CMPBEQ	R6, R4, done
 
diff --git a/src/runtime/race_s390x.s b/src/runtime/race_s390x.s
index 3dfda9e733b..d5a0bbedace 100644
--- a/src/runtime/race_s390x.s
+++ b/src/runtime/race_s390x.s
@@ -25,10 +25,14 @@
 
 // func runtime·raceread(addr uintptr)
 // Called from instrumented code.
-TEXT	runtime·raceread(SB), NOSPLIT, $0-8
+TEXT	runtime·raceread<ABIInternal>(SB), NOSPLIT, $0-8
 	// void __tsan_read(ThreadState *thr, void *addr, void *pc);
 	MOVD	$__tsan_read(SB), R1
+#ifndef GOEXPERIMENT_regabiargs
 	MOVD	addr+0(FP), R3
+#else
+	MOVD	R2, R3
+#endif
 	MOVD	R14, R4
 	JMP	racecalladdr<>(SB)
 
@@ -46,10 +50,14 @@ TEXT	runtime·racereadpc(SB), NOSPLIT, $0-24
 
 // func runtime·racewrite(addr uintptr)
 // Called from instrumented code.
-TEXT	runtime·racewrite(SB), NOSPLIT, $0-8
+TEXT	runtime·racewrite<ABIInternal>(SB), NOSPLIT, $0-8
 	// void __tsan_write(ThreadState *thr, void *addr, void *pc);
 	MOVD	$__tsan_write(SB), R1
+#ifndef GOEXPERIMENT_regabiargs
 	MOVD	addr+0(FP), R3
+#else
+	MOVD	R2, R3
+#endif
 	MOVD	R14, R4
 	JMP	racecalladdr<>(SB)
 
@@ -67,10 +75,15 @@ TEXT	runtime·racewritepc(SB), NOSPLIT, $0-24
 
 // func runtime·racereadrange(addr, size uintptr)
 // Called from instrumented code.
-TEXT	runtime·racereadrange(SB), NOSPLIT, $0-16
+TEXT	runtime·racereadrange<ABIInternal>(SB), NOSPLIT, $0-16
 	// void __tsan_read_range(ThreadState *thr, void *addr, uintptr size, void *pc);
 	MOVD	$__tsan_read_range(SB), R1
+#ifndef GOEXPERIMENT_regabiargs
 	LMG	addr+0(FP), R3, R4
+#else
+	MOVD	R3, R4
+	MOVD	R2, R3
+#endif
 	MOVD	R14, R5
 	JMP	racecalladdr<>(SB)
 
@@ -91,10 +104,15 @@ TEXT	runtime·racereadrangepc1(SB), NOSPLIT, $0-24
 
 // func runtime·racewriterange(addr, size uintptr)
 // Called from instrumented code.
-TEXT	runtime·racewriterange(SB), NOSPLIT, $0-16
+TEXT	runtime·racewriterange<ABIInternal>(SB), NOSPLIT, $0-16
 	// void __tsan_write_range(ThreadState *thr, void *addr, uintptr size, void *pc);
 	MOVD	$__tsan_write_range(SB), R1
+#ifndef GOEXPERIMENT_regabiargs
 	LMG	addr+0(FP), R3, R4
+#else
+	MOVD	R3, R4
+	MOVD	R2, R3
+#endif
 	MOVD	R14, R5
 	JMP	racecalladdr<>(SB)
 
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 43e4c142362..64ee4c8d2e9 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -39,7 +39,7 @@ func gotraceback() (level int32, all, crash bool) {
 	gp := getg()
 	t := atomic.Load(&traceback_cache)
 	crash = t&tracebackCrash != 0
-	all = gp.m.throwing >= throwTypeUser || t&tracebackAll != 0
+	all = gp.m.throwing > throwTypeUser || t&tracebackAll != 0
 	if gp.m.traceback != 0 {
 		level = int32(gp.m.traceback)
 	} else if gp.m.throwing >= throwTypeRuntime {
diff --git a/src/runtime/stkframe.go b/src/runtime/stkframe.go
index 819b7f6c7d9..d6e7e0371c0 100644
--- a/src/runtime/stkframe.go
+++ b/src/runtime/stkframe.go
@@ -234,7 +234,7 @@ func (frame *stkframe) getStackMap(debug bool) (locals, args bitvector, objs []s
 	}
 
 	// stack objects.
-	if (GOARCH == "amd64" || GOARCH == "arm64" || GOARCH == "loong64" || GOARCH == "ppc64" || GOARCH == "ppc64le" || GOARCH == "riscv64") &&
+	if (GOARCH == "amd64" || GOARCH == "arm64" || GOARCH == "loong64" || GOARCH == "ppc64" || GOARCH == "ppc64le" || GOARCH == "riscv64" || GOARCH == "s390x") &&
 		unsafe.Sizeof(abi.RegArgs{}) > 0 && isReflect {
 		// For reflect.makeFuncStub and reflect.methodValueCall,
 		// we need to fake the stack object record.
diff --git a/src/runtime/stubs_s390x.go b/src/runtime/stubs_s390x.go
index a2b07ff8aa8..6d704e82003 100644
--- a/src/runtime/stubs_s390x.go
+++ b/src/runtime/stubs_s390x.go
@@ -8,6 +8,13 @@ package runtime
 func load_g()
 func save_g()
 
+// Used by reflectcall and the reflect package.
+//
+// Spills/loads arguments in registers to/from an internal/abi.RegArgs
+// respectively. Does not follow the Go ABI.
+func spillArgs()
+func unspillArgs()
+
 // getfp returns the frame pointer register of its caller or 0 if not implemented.
 // TODO: Make this a compiler intrinsic
 func getfp() uintptr { return 0 }
diff --git a/src/runtime/tls_s390x.s b/src/runtime/tls_s390x.s
index cb6a21c1143..388e7b88bd9 100644
--- a/src/runtime/tls_s390x.s
+++ b/src/runtime/tls_s390x.s
@@ -19,7 +19,7 @@
 //
 // If !iscgo, this is a no-op.
 //
-// NOTE: setg_gcc<> assume this clobbers only R10 and R11.
+// NOTE: setg_gcc<> and mcall assume this clobbers only R10 and R11.
 TEXT runtime·save_g(SB),NOSPLIT|NOFRAME,$0-0
 	MOVB	runtime·iscgo(SB),  R10
 	CMPBEQ	R10, $0, nocgo