cmd/compile: optimize math/bits Len32 intrinsic on arm64

Arm64 has a 32-bit CLZ instruction CLZW, which can be used for intrinsic Len32.
Function LeadingZeros32 calls Len32, with this change, the assembly code of
LeadingZeros32 becomes more concise.

Go code:

func f32(x uint32) { z = bits.LeadingZeros32(x) }

Before:

"".f32 STEXT size=32 args=0x8 locals=0x0 leaf
        0x0000 00000 (test.go:7)        TEXT    "".f32(SB), LEAF|NOFRAME|ABIInternal, $0-8
        0x0004 00004 (test.go:7)        MOVWU   "".x(FP), R0
        0x0008 00008 ($GOROOT/src/math/bits/bits.go:30) CLZ     R0, R0
        0x000c 00012 ($GOROOT/src/math/bits/bits.go:30) SUB     $32, R0, R0
        0x0010 00016 (test.go:7)        MOVD    R0, "".z(SB)
        0x001c 00028 (test.go:7)        RET     (R30)

After:

"".f32 STEXT size=32 args=0x8 locals=0x0 leaf
        0x0000 00000 (test.go:7)        TEXT    "".f32(SB), LEAF|NOFRAME|ABIInternal, $0-8
        0x0004 00004 (test.go:7)        MOVWU   "".x(FP), R0
        0x0008 00008 ($GOROOT/src/math/bits/bits.go:30) CLZW    R0, R0
        0x000c 00012 (test.go:7)        MOVD    R0, "".z(SB)
        0x0018 00024 (test.go:7)        RET     (R30)

Benchmarks:
name              old time/op  new time/op  delta
LeadingZeros-8    2.53ns ± 0%  2.55ns ± 0%   +0.67%  (p=0.000 n=10+10)
LeadingZeros8-8   3.56ns ± 0%  3.56ns ± 0%     ~     (all equal)
LeadingZeros16-8  3.55ns ± 0%  3.56ns ± 0%     ~     (p=0.465 n=10+10)
LeadingZeros32-8  3.55ns ± 0%  2.96ns ± 0%  -16.71%  (p=0.000 n=10+7)
LeadingZeros64-8  2.53ns ± 0%  2.54ns ± 0%     ~     (p=0.059 n=8+10)

Change-Id: Ie5666bb82909e341060e02ffd4e86c0e5d67e90a
Reviewed-on: https://go-review.googlesource.com/c/157000
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: erifan01

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

@@ -3327,7 +3327,7 @@ func init() {
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			return s.newValue1(ssa.OpBitLen32, types.Types[TINT], args[0])
 		},
-		sys.AMD64)
+		sys.AMD64, sys.ARM64)
 	addF("math/bits", "Len32",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			if s.config.PtrSize == 4 {
@@ -3336,7 +3336,7 @@ func init() {
 			x := s.newValue1(ssa.OpZeroExt32to64, types.Types[TUINT64], args[0])
 			return s.newValue1(ssa.OpBitLen64, types.Types[TINT], x)
 		},
-		sys.ARM64, sys.ARM, sys.S390X, sys.MIPS, sys.PPC64)
+		sys.ARM, sys.S390X, sys.MIPS, sys.PPC64)
 	addF("math/bits", "Len16",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			if s.config.PtrSize == 4 {

src/cmd/compile/internal/ssa/gen/ARM64.rules

@@ -123,6 +123,7 @@
 (FMOVSload [off] {sym} ptr (MOVWstore [off] {sym} ptr val _)) -> (FMOVSgpfp val)
 
 (BitLen64 x) -> (SUB (MOVDconst [64]) (CLZ <typ.Int> x))
+(BitLen32 x) -> (SUB (MOVDconst [32]) (CLZW <typ.Int> x))
 
 (Bswap64 x) -> (REV x)
 (Bswap32 x) -> (REVW x)

src/cmd/compile/internal/ssa/rewriteARM64.go

@@ -31,7 +31,7 @@ func LeadingZeros64(n uint64) int {
 func LeadingZeros32(n uint32) int {
 	// amd64:"BSRQ","LEAQ",-"CMOVQEQ"
 	// s390x:"FLOGR"
-	// arm:"CLZ" arm64:"CLZ"
+	// arm:"CLZ" arm64:"CLZW"
 	// mips:"CLZ"
 	return bits.LeadingZeros32(n)
 }