cmd/compile: allow load-op merging in additional situations

x += *p

We want to do this with a single load+add operation on amd64.
The tricky part is that we don't want to combine if there are
other uses of x after this instruction.

Implement a simple detector that seems to capture a common situation -
x += *p is in a loop, and the other use of x is after loop exit.
In that case, it does not hurt to do the load+add combo.

Change-Id: I466174cce212e78bde83f908cc1f2752b560c49c
Reviewed-on: https://go-review.googlesource.com/c/go/+/672957
Reviewed-by: Keith Randall <khr@google.com>
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/rewrite.go

@@ -286,7 +286,18 @@ func canMergeLoadClobber(target, load, x *Value) bool {
 	// approximate x dying with:
 	//  1) target is x's only use.
 	//  2) target is not in a deeper loop than x.
-	if x.Uses != 1 {
+	switch {
+	case x.Uses == 2 && x.Op == OpPhi && len(x.Args) == 2 && (x.Args[0] == target || x.Args[1] == target) && target.Uses == 1:
+		// This is a simple detector to determine that x is probably
+		// not live after target. (It does not need to be perfect,
+		// regalloc will issue a reg-reg move to save it if we are wrong.)
+		// We have:
+		//   x = Phi(?, target)
+		//   target = Op(load, x)
+		// Because target has only one use as a Phi argument, we can schedule it
+		// very late. Hopefully, later than the other use of x. (The other use died
+		// between x and target, or exists on another branch entirely).
+	case x.Uses > 1:
 		return false
 	}
 	loopnest := x.Block.Func.loopnest()

test/codegen/memcombine.go

@@ -396,6 +396,15 @@ func load_op_no_merge(p, q *int) {
 	}
 }
 
+func load_op_in_loop(a []int) int {
+	r := 0
+	for _, x := range a {
+		// amd64:`ADDQ\t\([A-Z]+\)\([A-Z]+\*8\), [A-Z]+`
+		r += x
+	}
+	return r
+}
+
 // Make sure offsets are folded into loads and stores.
 func offsets_fold(_, a [20]byte) (b [20]byte) {
 	// arm64:`MOVD\tcommand-line-arguments\.a\+[0-9]+\(FP\), R[0-9]+`,`MOVD\tR[0-9]+, command-line-arguments\.b\+[0-9]+\(FP\)`