cmd/compile: optimize abi.Type.GCData loads

This fires in just one place; stkframe.go's stkobjinit. But, it does
make the struct literal entirely constants.

Change-Id: Ice76cb3cddd97adee011fdaab40597839da2e89f
Reviewed-on: https://go-review.googlesource.com/c/go/+/701300
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: jakebailey

src/cmd/compile/internal/liveness/plive.go

@@ -1493,7 +1493,7 @@ func (lv *Liveness) emitStackObjects() *obj.LSym {
 		if sz != int64(int32(sz)) {
 			base.Fatalf("stack object too big: %v of type %v, size %d", v, t, sz)
 		}
-		lsym, ptrBytes := reflectdata.GCSym(t)
+		lsym, ptrBytes := reflectdata.GCSym(t, false)
 		off = objw.Uint32(x, off, uint32(sz))
 		off = objw.Uint32(x, off, uint32(ptrBytes))
 		off = objw.SymPtrOff(x, off, lsym)

src/cmd/compile/internal/reflectdata/reflect.go

@@ -1229,15 +1229,15 @@ func typesStrCmp(a, b typeAndStr) int {
 // GC information is always a bitmask, never a gc program.
 // GCSym may be called in concurrent backend, so it does not emit the symbol
 // content.
-func GCSym(t *types.Type) (lsym *obj.LSym, ptrdata int64) {
+func GCSym(t *types.Type, onDemandAllowed bool) (lsym *obj.LSym, ptrdata int64) {
 	// Record that we need to emit the GC symbol.
 	gcsymmu.Lock()
 	if _, ok := gcsymset[t]; !ok {
 		gcsymset[t] = struct{}{}
 	}
 	gcsymmu.Unlock()
 
-	lsym, _, ptrdata = dgcsym(t, false, false)
+	lsym, _, ptrdata = dgcsym(t, false, onDemandAllowed)
 	return
 }
 

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

@@ -2067,7 +2067,7 @@ func isFixedLoad(v *Value, sym Sym, off int64) bool {
 		for _, f := range rttype.Type.Fields() {
 			if f.Offset == off && copyCompatibleType(v.Type, f.Type) {
 				switch f.Sym.Name {
-				case "Size_", "PtrBytes", "Hash", "Kind_":
+				case "Size_", "PtrBytes", "Hash", "Kind_", "GCData":
 					return true
 				default:
 					// fmt.Println("unknown field", f.Sym.Name)
@@ -2147,6 +2147,12 @@ func rewriteFixedLoad(v *Value, sym Sym, sb *Value, off int64) *Value {
 					v.reset(OpConst8)
 					v.AuxInt = int64(reflectdata.ABIKindOfType(t))
 					return v
+				case "GCData":
+					gcdata, _ := reflectdata.GCSym(t, true)
+					v.reset(OpAddr)
+					v.Aux = symToAux(gcdata)
+					v.AddArg(sb)
+					return v
 				default:
 					base.Fatalf("unknown field %s for fixedLoad of %s at offset %d", f.Sym.Name, lsym.Name, off)
 				}