cmd/compile: remove residual register GC map code

We used to generate register GC maps as an experimental approach
for asynchronous preemption, which later we chose not to take.
Most of the register GC map code are already removed. One
exception is that the ssa.Register type still contains a field
for the register map index. Remove it.

Change-Id: Ib177ebce9548aa5ffbcaedd4b507240ea7df8afe
Reviewed-on: https://go-review.googlesource.com/c/go/+/651076
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ssa/_gen/main.go

@@ -426,7 +426,6 @@ func genOp() {
 			continue
 		}
 		fmt.Fprintf(w, "var registers%s = [...]Register {\n", a.name)
-		var gcRegN int
 		num := map[string]int8{}
 		for i, r := range a.regnames {
 			num[r] = int8(i)
@@ -443,14 +442,7 @@ func genOp() {
 			default:
 				objname = pkg + ".REG_" + r
 			}
-			// Assign a GC register map index to registers
-			// that may contain pointers.
-			gcRegIdx := -1
-			if a.gpregmask&(1<<uint(i)) != 0 {
-				gcRegIdx = gcRegN
-				gcRegN++
-			}
-			fmt.Fprintf(w, "  {%d, %s, %d, \"%s\"},\n", i, objname, gcRegIdx, r)
+			fmt.Fprintf(w, "  {%d, %s, \"%s\"},\n", i, objname, r)
 		}
 		parameterRegisterList := func(paramNamesString string) []int8 {
 			paramNamesString = strings.TrimSpace(paramNamesString)
@@ -477,10 +469,6 @@ func genOp() {
 		paramIntRegs := parameterRegisterList(a.ParamIntRegNames)
 		paramFloatRegs := parameterRegisterList(a.ParamFloatRegNames)
 
-		if gcRegN > 32 {
-			// Won't fit in a uint32 mask.
-			log.Fatalf("too many GC registers (%d > 32) on %s", gcRegN, a.name)
-		}
 		fmt.Fprintln(w, "}")
 		fmt.Fprintf(w, "var paramIntReg%s = %#v\n", a.name, paramIntRegs)
 		fmt.Fprintf(w, "var paramFloatReg%s = %#v\n", a.name, paramFloatRegs)

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

@@ -37,24 +37,23 @@ type Config struct {
 	floatParamRegs []int8         // register numbers of floating param (in/out) registers
 	ABI1           *abi.ABIConfig // "ABIInternal" under development // TODO change comment when this becomes current
 	ABI0           *abi.ABIConfig
-	GCRegMap       []*Register // garbage collector register map, by GC register index
-	FPReg          int8        // register number of frame pointer, -1 if not used
-	LinkReg        int8        // register number of link register if it is a general purpose register, -1 if not used
-	hasGReg        bool        // has hardware g register
-	ctxt           *obj.Link   // Generic arch information
-	optimize       bool        // Do optimization
-	noDuffDevice   bool        // Don't use Duff's device
-	useSSE         bool        // Use SSE for non-float operations
-	useAvg         bool        // Use optimizations that need Avg* operations
-	useHmul        bool        // Use optimizations that need Hmul* operations
-	SoftFloat      bool        //
-	Race           bool        // race detector enabled
-	BigEndian      bool        //
-	UseFMA         bool        // Use hardware FMA operation
-	unalignedOK    bool        // Unaligned loads/stores are ok
-	haveBswap64    bool        // architecture implements Bswap64
-	haveBswap32    bool        // architecture implements Bswap32
-	haveBswap16    bool        // architecture implements Bswap16
+	FPReg          int8      // register number of frame pointer, -1 if not used
+	LinkReg        int8      // register number of link register if it is a general purpose register, -1 if not used
+	hasGReg        bool      // has hardware g register
+	ctxt           *obj.Link // Generic arch information
+	optimize       bool      // Do optimization
+	noDuffDevice   bool      // Don't use Duff's device
+	useSSE         bool      // Use SSE for non-float operations
+	useAvg         bool      // Use optimizations that need Avg* operations
+	useHmul        bool      // Use optimizations that need Hmul* operations
+	SoftFloat      bool      //
+	Race           bool      // race detector enabled
+	BigEndian      bool      //
+	UseFMA         bool      // Use hardware FMA operation
+	unalignedOK    bool      // Unaligned loads/stores are ok
+	haveBswap64    bool      // architecture implements Bswap64
+	haveBswap32    bool      // architecture implements Bswap32
+	haveBswap16    bool      // architecture implements Bswap16
 }
 
 type (
@@ -385,21 +384,6 @@ func NewConfig(arch string, types Types, ctxt *obj.Link, optimize, softfloat boo
 		opcodeTable[Op386LoweredWB].reg.clobbers |= 1 << 3 // BX
 	}
 
-	// Create the GC register map index.
-	// TODO: This is only used for debug printing. Maybe export config.registers?
-	gcRegMapSize := int16(0)
-	for _, r := range c.registers {
-		if r.gcNum+1 > gcRegMapSize {
-			gcRegMapSize = r.gcNum + 1
-		}
-	}
-	c.GCRegMap = make([]*Register, gcRegMapSize)
-	for i, r := range c.registers {
-		if r.gcNum != -1 {
-			c.GCRegMap[r.gcNum] = &c.registers[i]
-		}
-	}
-
 	return c
 }
 

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

@@ -20,7 +20,6 @@ type Location interface {
 type Register struct {
 	num    int32 // dense numbering
 	objNum int16 // register number from cmd/internal/obj/$ARCH
-	gcNum  int16 // GC register map number (dense numbering of registers that can contain pointers)
 	name   string
 }
 
@@ -34,12 +33,6 @@ func (r *Register) ObjNum() int16 {
 	return r.objNum
 }
 
-// GCNum returns the runtime GC register index of r, or -1 if this
-// register can't contain pointers.
-func (r *Register) GCNum() int16 {
-	return r.gcNum
-}
-
 // A LocalSlot is a location in the stack frame, which identifies and stores
 // part or all of a PPARAM, PPARAMOUT, or PAUTO ONAME node.
 // It can represent a whole variable, part of a larger stack slot, or part of a