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 <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: Jorropo

src/cmd/compile/internal/ssa/_gen/generic.rules

@@ -2855,3 +2855,10 @@
 // if b { x-- } => x -= b
 (CondSelect (Add8 <t> x (Const8 [-1])) x bool) => (Sub8 x (CvtBoolToUint8 <t> bool))
 (CondSelect (Add(64|32|16) <t> x (Const(64|32|16) [-1])) x bool) => (Sub(64|32|16) x (ZeroExt8to(64|32|16) <t> (CvtBoolToUint8 <types.Types[types.TUINT8]> bool)))
+
+// if b { x <<= 1 } => x <<= b
+(CondSelect (Lsh(64|32|16|8)x64 x (Const64 [1])) x bool) => (Lsh(64|32|16|8)x8 [true] x (CvtBoolToUint8 <types.Types[types.TUINT8]> bool))
+
+// if b { x >>= 1 } => x >>= b
+(CondSelect (Rsh(64|32|16|8)x64 x (Const64 [1])) x bool) => (Rsh(64|32|16|8)x8 [true] x (CvtBoolToUint8 <types.Types[types.TUINT8]> bool))
+(CondSelect (Rsh(64|32|16|8)Ux64 x (Const64 [1])) x bool) => (Rsh(64|32|16|8)Ux8 [true] x (CvtBoolToUint8 <types.Types[types.TUINT8]> bool))

src/cmd/compile/internal/ssa/rewritegeneric.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
+}