cmd/compile: update default ABI choices for calls and bodyless fn stack maps

After recent discussion about bodyless functions, their wrappers,
their stack maps, nosplit, and callbacks, I was inspired to go and
be sure that more defaults were sensible.  This may not be all --
currently rtcall is "ABIDefault" which I think is correct, but I
am not 100% certain.

Updates #40724.

Change-Id: I95b14ee0e5952fa53e7fea9f6f5192358aa24f23
Reviewed-on: https://go-review.googlesource.com/c/go/+/304549
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/gc/compile.go

@@ -44,7 +44,7 @@ func enqueueFunc(fn *ir.Func) {
 		// Initialize ABI wrappers if necessary.
 		ssagen.InitLSym(fn, false)
 		types.CalcSize(fn.Type()) // TODO register args; remove this once all is done by abiutils
-		a := ssagen.AbiForFunc(fn)
+		a := ssagen.AbiForBodylessFuncStackMap(fn)
 		abiInfo := a.ABIAnalyze(fn.Type(), true) // will set parameter spill/home locations correctly
 		liveness.WriteFuncMap(fn, abiInfo)
 		return

src/cmd/compile/internal/ssagen/ssa.go

@@ -209,12 +209,15 @@ func InitConfig() {
 	ir.Syms.SigPanic = typecheck.LookupRuntimeFunc("sigpanic")
 }
 
-// AbiForFunc returns the ABI for a function, used to figure out arg/result mapping for rtcall and bodyless functions.
-// This follows policy for GOEXPERIMENT=regabi, //go:registerparams, and currently defined ABIInternal.
-// Policy is subject to change....
+// AbiForBodylessFuncStackMap returns the ABI for a bodyless function's stack map.
+// This is not necessarily the ABI used to call it.
+// Currently (1.17 dev) such a stack map is always ABI0;
+// any ABI wrapper that is present is nosplit, hence a precise
+// stack map is not needed there (the parameters survive only long
+// enough to call the wrapped assembly function).
 // This always returns a freshly copied ABI.
-func AbiForFunc(fn *ir.Func) *abi.ABIConfig {
-	return abiForFunc(fn, ssaConfig.ABI0, ssaConfig.ABI1).Copy() // No idea what races will result, be safe
+func AbiForBodylessFuncStackMap(fn *ir.Func) *abi.ABIConfig {
+	return ssaConfig.ABI0.Copy() // No idea what races will result, be safe
 }
 
 // TODO (NLT 2021-04-15) This must be changed to a name that cannot match; it may be helpful to other register ABI work to keep the trigger-logic
@@ -4842,21 +4845,21 @@ func (s *state) call(n *ir.CallExpr, k callKind, returnResultAddr bool) *ssa.Val
 	var ACArgs []*types.Type       // AuxCall args
 	var ACResults []*types.Type    // AuxCall results
 	var callArgs []*ssa.Value      // For late-expansion, the args themselves (not stored, args to the call instead).
-	inRegisters := false
+	inRegistersForTesting := false // If a call uses register ABI for one of the testing reasons, pragma, magic types, magic names
 
 	var magicFnNameSym *types.Sym
 	if fn.Name() != nil {
 		magicFnNameSym = fn.Name().Sym()
 		ss := magicFnNameSym.Name
 		if strings.HasSuffix(ss, magicNameDotSuffix) {
-			inRegisters = true
+			inRegistersForTesting = true
 		}
 	}
 	if magicFnNameSym == nil && n.Op() == ir.OCALLINTER {
 		magicFnNameSym = fn.(*ir.SelectorExpr).Sym()
 		ss := magicFnNameSym.Name
 		if strings.HasSuffix(ss, magicNameDotSuffix[1:]) {
-			inRegisters = true
+			inRegistersForTesting = true
 		}
 	}
 
@@ -4872,7 +4875,7 @@ func (s *state) call(n *ir.CallExpr, k callKind, returnResultAddr bool) *ssa.Val
 			// TODO(register args) remove after register abi is working
 			inRegistersImported := fn.Pragma()&ir.RegisterParams != 0
 			inRegistersSamePackage := fn.Func != nil && fn.Func.Pragma&ir.RegisterParams != 0
-			inRegisters = inRegisters || inRegistersImported || inRegistersSamePackage
+			inRegistersForTesting = inRegistersForTesting || inRegistersImported || inRegistersSamePackage
 			break
 		}
 		closure = s.expr(fn)
@@ -4898,13 +4901,13 @@ func (s *state) call(n *ir.CallExpr, k callKind, returnResultAddr bool) *ssa.Val
 	}
 
 	if regAbiForFuncType(n.X.Type().FuncType()) {
-		// fmt.Printf("Saw magic last type in call %v\n", n)
-		inRegisters = true
+		// Magic last type in input args to call
+		inRegistersForTesting = true
 	}
 
-	callABI := s.f.ABI1
-	if !inRegisters {
-		callABI = s.f.ABI0
+	callABI := s.f.ABIDefault
+	if inRegistersForTesting {
+		callABI = s.f.ABI1
 	}
 
 	params := callABI.ABIAnalyze(n.X.Type(), false /* Do not set (register) nNames from caller side -- can cause races. */)