cmd/compile: optimize bool to int conversion

This CL teaches SSA to recognize code of the form

// b is a boolean value, i is an int of some flavor
if b {
	i = 1
} else {
	i = 0
}

and use b's underlying 0/1 representation for i
instead of generating jumps.

Unfortunately, it does not work on the obvious code:

func bool2int(b bool) int {
	if b {
		return 1
	}
	return 0
}

This is left for future work.
Note that the existing phiopt optimizations also don't work for:

func neg(b bool) bool {
	if b {
		return false
	}
	return true
}

In the meantime, runtime authors and the like can use:

func bool2int(b bool) int {
	var i int
	if b {
		i = 1
	} else {
		i = 0
	}
	return i
}

This compiles to:

"".bool2int t=1 size=16 args=0x10 locals=0x0
	0x0000 00000 (x.go:25)	TEXT	"".bool2int(SB), $0-16
	0x0000 00000 (x.go:25)	FUNCDATA	$0, gclocals·23e8278e2b69a3a75fa59b23c49ed6ad(SB)
	0x0000 00000 (x.go:25)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (x.go:32)	MOVBLZX	"".b+8(FP), AX
	0x0005 00005 (x.go:32)	MOVBQZX	AL, AX
	0x0008 00008 (x.go:32)	MOVQ	AX, "".~r1+16(FP)
	0x000d 00013 (x.go:32)	RET

The extraneous MOVBQZX is #15300.

This optimization also helps range and slice.
The compiler must protect against pointers pointing
to the end of a slice/string. It does this by increasing
a pointer by either 0 or 1 * elemsize, based on a condition.
This CL optimizes away a jump in that code.

This CL triggers 382 times while compiling the standard library.

Updating code to utilize this optimization is left for future CLs.

Updates #6011

Change-Id: Ia7c1185f8aa223c543f91a3cd6d4a2a09c691c70
Reviewed-on: https://go-review.googlesource.com/22711
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/phiopt.go

@@ -57,7 +57,16 @@ func phiopt(f *Func) {
 		}
 
 		for _, v := range b.Values {
-			if v.Op != OpPhi || !v.Type.IsBoolean() {
+			if v.Op != OpPhi {
+				continue
+			}
+
+			// Look for conversions from bool to 0/1.
+			if v.Type.IsInteger() {
+				phioptint(v, b0, reverse)
+			}
+
+			if !v.Type.IsBoolean() {
 				continue
 			}
 
@@ -110,5 +119,55 @@ func phiopt(f *Func) {
 			}
 		}
 	}
+}
+
+func phioptint(v *Value, b0 *Block, reverse int) {
+	a0 := v.Args[0]
+	a1 := v.Args[1]
+	if a0.Op != a1.Op {
+		return
+	}
+
+	switch a0.Op {
+	case OpConst8, OpConst16, OpConst32, OpConst64:
+	default:
+		return
+	}
 
+	negate := false
+	switch {
+	case a0.AuxInt == 0 && a1.AuxInt == 1:
+		negate = true
+	case a0.AuxInt == 1 && a1.AuxInt == 0:
+	default:
+		return
+	}
+
+	if reverse == 1 {
+		negate = !negate
+	}
+
+	switch v.Type.Size() {
+	case 1:
+		v.reset(OpCopy)
+	case 2:
+		v.reset(OpZeroExt8to16)
+	case 4:
+		v.reset(OpZeroExt8to32)
+	case 8:
+		v.reset(OpZeroExt8to64)
+	default:
+		v.Fatalf("bad int size %d", v.Type.Size())
+	}
+
+	a := b0.Control
+	if negate {
+		a = v.Block.NewValue1(v.Line, OpNot, a.Type, a)
+	}
+	v.AddArg(a)
+
+	f := b0.Func
+	if f.pass.debug > 0 {
+		f.Config.Warnl(v.Block.Line, "converted OpPhi bool -> int%d", v.Type.Size()*8)
+	}
 }