cmd/compile: using a single Store op for non-pointer non-skip store

This makes fewer Values around until decomposing, reducing
allocation in compiler.

name       old alloc/op    new alloc/op    delta
Template      41.4MB ± 0%     40.8MB ± 0%  -1.29%  (p=0.000 n=10+10)
Unicode       30.3MB ± 0%     30.2MB ± 0%  -0.24%  (p=0.000 n=10+10)
GoTypes        118MB ± 0%      115MB ± 0%  -2.23%  (p=0.000 n=10+10)
Compiler       505MB ± 0%      493MB ± 0%  -2.47%  (p=0.000 n=10+10)
SSA            881MB ± 0%      872MB ± 0%  -1.03%  (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        401k ± 1%       400k ± 1%    ~     (p=0.631 n=10+10)
Unicode         321k ± 0%       321k ± 1%    ~     (p=0.684 n=10+10)
GoTypes        1.18M ± 0%      1.17M ± 0%  -0.34%  (p=0.000 n=10+10)
Compiler       4.63M ± 0%      4.61M ± 0%  -0.43%  (p=0.000 n=10+10)
SSA            7.83M ± 0%      7.82M ± 0%  -0.13%  (p=0.000 n=10+10)

Change-Id: I8f736396294444248a439bd4c90be1357024ce88
Reviewed-on: https://go-review.googlesource.com/38294
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -3417,9 +3417,17 @@ func (s *state) rtcall(fn *obj.LSym, returns bool, results []*Type, args ...*ssa
 
 // do *left = right for type t.
 func (s *state) storeType(t *Type, left, right *ssa.Value, skip skipMask) {
+	if skip == 0 && (!haspointers(t) || ssa.IsStackAddr(left)) {
+		// Known to not have write barrier. Store the whole type.
+		s.vars[&memVar] = s.newValue3A(ssa.OpStore, ssa.TypeMem, t, left, right, s.mem())
+		return
+	}
+
 	// store scalar fields first, so write barrier stores for
 	// pointer fields can be grouped together, and scalar values
 	// don't need to be live across the write barrier call.
+	// TODO: if the writebarrier pass knows how to reorder stores,
+	// we can do a single store here as long as skip==0.
 	s.storeTypeScalars(t, left, right, skip)
 	if skip&skipPtr == 0 && haspointers(t) {
 		s.storeTypePtrs(t, left, right)

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

@@ -19,7 +19,7 @@ func needwb(v *Value) bool {
 	if !t.HasPointer() {
 		return false
 	}
-	if isStackAddr(v.Args[0]) {
+	if IsStackAddr(v.Args[0]) {
 		return false // write on stack doesn't need write barrier
 	}
 	return true
@@ -316,8 +316,8 @@ func round(o int64, r int64) int64 {
 	return (o + r - 1) &^ (r - 1)
 }
 
-// isStackAddr returns whether v is known to be an address of a stack slot
-func isStackAddr(v *Value) bool {
+// IsStackAddr returns whether v is known to be an address of a stack slot
+func IsStackAddr(v *Value) bool {
 	for v.Op == OpOffPtr || v.Op == OpAddPtr || v.Op == OpPtrIndex || v.Op == OpCopy {
 		v = v.Args[0]
 	}