cmd/compile: recognize OpVarDef and OpZero in cse isMemDef

recognize OpVarDef (width 0, always skippable) and OpZero (similar to
OpStore, it should be disjoint), so these ops do not prevent loads cse.

This change slightly improves code size:

Executable           Base .text linux_arm64     Change
----------------------------------------------------
asm                     2133284     2132900     -0.02%
cgo                     1742996     1742868     -0.01%
compile                10567620    10566852     -0.01%
cover                   1906740     1906100     -0.03%
fix                     3131284     3131012     -0.01%
link                    2667604     2667076     -0.02%
preprofile               877908      877876     -0.00%
vet                     3010372     3010084     -0.01%

Change-Id: I428f73008a817d0e302d438c020504c560ae1653
Reviewed-on: https://go-review.googlesource.com/c/go/+/769000
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>

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

@@ -472,10 +472,15 @@ func isMemUser(v *Value) (int, int, int64, bool) {
 
 // Query if the given "memory"-defining instruction's memory destination can be analyzed for aliasing with a memory "user" instructions.
 // Return index of pointer argument, index of "memory" argument, the access width and true on such instructions, otherwise return (-1, -1, 0, false).
+// If the access width is 0, the pointer index may be -1 (no pointer operand is needed).
 func isMemDef(v *Value) (int, int, int64, bool) {
 	switch v.Op {
 	case OpStore:
 		return 0, 2, auxToType(v.Aux).Size(), true
+	case OpVarDef:
+		return -1, 0, 0, true
+	case OpZero:
+		return 0, 1, v.AuxInt, true
 	default:
 		return -1, -1, 0, false
 	}
@@ -525,6 +530,10 @@ func skipDisjointMemDefs(user *Value, idxUserPtr, idxUserMem int, useWidth int64
 				// Skipping a memory def with a lot of uses may potentially increase register pressure.
 				break
 			}
+			if width == 0 {
+				mem = mem.Args[idxMem]
+				continue
+			}
 			defPtr := mem.Args[idxPtr]
 			if disjoint(defPtr, width, usePtr, useWidth) {
 				mem = mem.Args[idxMem]

test/codegen/memcse.go

@@ -15,3 +15,12 @@ func loadsAroundMemEqual(p *int, s1, s2 string) (int, bool) {
 	// arm64:"MOVD ZR, R0"
 	return x - y, eq
 }
+
+func loadsAroundVarDefAndZero(p *int) (int, *int) {
+	x := *p
+	var s [2]*int
+	s[0] = p
+	y := *p
+	// arm64:"MOVD ZR, R0"
+	return x - y, s[0]
+}