cmd/compile: invalidate zero-use values during rewrite

This helps remove uses that aren't needed any more.
That in turn helps other rules with Uses==1 conditions fire.

Update #39918

Change-Id: I68635b675472f1d59e59604e4d34b949a0016533
Reviewed-on: https://go-review.googlesource.com/c/go/+/249463
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index 7ead0ff300..7fc33772d3 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -460,3 +460,23 @@ func (v *Value) LackingPos() bool {
 	return v.Op == OpVarDef || v.Op == OpVarKill || v.Op == OpVarLive || v.Op == OpPhi ||
 		(v.Op == OpFwdRef || v.Op == OpCopy) && v.Type == types.TypeMem
 }
+
+// removeable reports whether the value v can be removed from the SSA graph entirely
+// if its use count drops to 0.
+func (v *Value) removeable() bool {
+	if v.Type.IsVoid() {
+		// Void ops, like nil pointer checks, must stay.
+		return false
+	}
+	if v.Type.IsMemory() {
+		// All memory ops aren't needed here, but we do need
+		// to keep calls at least (because they might have
+		// syncronization operations we can't see).
+		return false
+	}
+	if v.Op.HasSideEffects() {
+		// These are mostly synchronization operations.
+		return false
+	}
+	return true
+}