cmd/compile: use prove pass to detect Ctz of non-zero values

On amd64, Ctz must include special handling of zeros.
But the prove pass has enough information to detect whether the input
is non-zero, allowing a more efficient lowering.

Introduce new CtzNonZero ops to capture and use this information.

Benchmark code:

func BenchmarkVisitBits(b *testing.B) {
	b.Run("8", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			x := uint8(0xff)
			for x != 0 {
				sink = bits.TrailingZeros8(x)
				x &= x - 1
			}
		}
	})

    // and similarly so for 16, 32, 64
}

name            old time/op  new time/op  delta
VisitBits/8-8   7.27ns ± 4%  5.58ns ± 4%  -23.35%  (p=0.000 n=28+26)
VisitBits/16-8  14.7ns ± 7%  10.5ns ± 4%  -28.43%  (p=0.000 n=30+28)
VisitBits/32-8  27.6ns ± 8%  19.3ns ± 3%  -30.14%  (p=0.000 n=30+26)
VisitBits/64-8  44.0ns ±11%  38.0ns ± 5%  -13.48%  (p=0.000 n=30+30)

Fixes #25077

Change-Id: Ie6e5bd86baf39ee8a4ca7cadcf56d934e047f957
Reviewed-on: https://go-review.googlesource.com/109358
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/rewriteMIPS.go b/src/cmd/compile/internal/ssa/rewriteMIPS.go
index ad5033176e..b33afcc73d 100644
--- a/src/cmd/compile/internal/ssa/rewriteMIPS.go
+++ b/src/cmd/compile/internal/ssa/rewriteMIPS.go
@@ -85,6 +85,8 @@ func rewriteValueMIPS(v *Value) bool {
 		return rewriteValueMIPS_OpConstNil_0(v)
 	case OpCtz32:
 		return rewriteValueMIPS_OpCtz32_0(v)
+	case OpCtz32NonZero:
+		return rewriteValueMIPS_OpCtz32NonZero_0(v)
 	case OpCvt32Fto32:
 		return rewriteValueMIPS_OpCvt32Fto32_0(v)
 	case OpCvt32Fto64F:
@@ -1190,6 +1192,17 @@ func rewriteValueMIPS_OpCtz32_0(v *Value) bool {
 		return true
 	}
 }
+func rewriteValueMIPS_OpCtz32NonZero_0(v *Value) bool {
+	// match: (Ctz32NonZero x)
+	// cond:
+	// result: (Ctz32 x)
+	for {
+		x := v.Args[0]
+		v.reset(OpCtz32)
+		v.AddArg(x)
+		return true
+	}
+}
 func rewriteValueMIPS_OpCvt32Fto32_0(v *Value) bool {
 	// match: (Cvt32Fto32 x)
 	// cond: