cmd/compile: rewrite condselects into doublings and halvings

For performance see CL 685676.

This allows something like:
  if y { x *= 2 }

To be compiled to:
  SHLXQ BX, AX, AX

Instead of:
  MOVQ    AX, CX
  SHLQ    $1, CX
  MOVBLZX BL, DX
  TESTQ   DX, DX
  CMOVQNE CX, AX

While ./make.bash uniqued per LOC, there is 2 doublings and 4 halvings.

Change-Id: Ic0727cbf429528a2dbf17cbfc3b0121db8387444
Reviewed-on: https://go-review.googlesource.com/c/go/+/685695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/test/codegen/condmove.go b/test/codegen/condmove.go
index 95a9d2cd23..5659972eed 100644
--- a/test/codegen/condmove.go
+++ b/test/codegen/condmove.go
@@ -473,3 +473,36 @@ func cmovmathsub(a uint, b bool) uint {
 	// wasm:"Sub", "-Select"
 	return a
 }
+
+func cmovmathdouble(a uint, b bool) uint {
+	if b {
+		a *= 2
+	}
+	// amd64:"SHL", -"CMOV"
+	// amd64/v3:"SHL", -"CMOV", -"MOV"
+	// arm64:"LSL", -"CSEL"
+	// wasm:"Shl", "-Select"
+	return a
+}
+
+func cmovmathhalvei(a int, b bool) int {
+	if b {
+		// For some reason on arm64 it attributes the ASR to inside this block rather than where the Phi node is.
+		// arm64:"ASR", -"CSEL"
+		a /= 2
+	}
+	// arm64:-"CSEL"
+	// wasm:"Shr", "-Select"
+	return a
+}
+
+func cmovmathhalveu(a uint, b bool) uint {
+	if b {
+		a /= 2
+	}
+	// amd64:"SHR", -"CMOV"
+	// amd64/v3:"SHR", -"CMOV", -"MOV"
+	// arm64:"LSR", -"CSEL"
+	// wasm:"Shr", "-Select"
+	return a
+}