cmd/compile: extend condselect into math code to handle other constants than 1

On amd64 along:
  if b { x += 1 } => x += b

We can also implement constants 2 4 and 8:
  if b { x += 2 } => x += b * 2

This compiles to a displacement LEA.

Change-Id: Ib00fcc5059acb0ebb346e056c4a656f164cc63df
Reviewed-on: https://go-review.googlesource.com/c/go/+/760841
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 45 insertions, 0 deletions

diff --git a/test/codegen/condmove.go b/test/codegen/condmove.go
index ec1b971ed7..2b28077758 100644
--- a/test/codegen/condmove.go
+++ b/test/codegen/condmove.go
@@ -475,6 +475,51 @@ func cmovmathaddelse(a uint, b bool) uint {
 	return a
 }
 
+func cmovmathadd2(a uint, b bool) uint {
+	if b {
+		a += 2
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+func cmovmathadd2else(a uint, b bool) uint {
+	if !b {
+		a += 2
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+
+func cmovmathadd4(a uint, b bool) uint {
+	if b {
+		a += 4
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+func cmovmathadd4else(a uint, b bool) uint {
+	if !b {
+		a += 4
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+
+func cmovmathadd8(a uint, b bool) uint {
+	if b {
+		a += 8
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+func cmovmathadd8else(a uint, b bool) uint {
+	if !b {
+		a += 8
+	}
+	// amd64:"LEAQ" -"CMOV" -"MUL"
+	return a
+}
+
 func cmovmathsub(a uint, b bool) uint {
 	if b {
 		a--