cmd/compile: eliminate unnecessary type conversions in TrailingZeros(16|8) for arm64

This CL eliminates unnecessary type conversion operations: OpZeroExt16to64 and OpZeroExt8to64.
If the input argrument is a nonzero value, then ORconst operation can also be eliminated.

Benchmarks:

name               old time/op  new time/op  delta
TrailingZeros-8    2.75ns ± 0%  2.75ns ± 0%     ~     (all equal)
TrailingZeros8-8   3.49ns ± 1%  2.93ns ± 0%  -16.00%  (p=0.000 n=10+10)
TrailingZeros16-8  3.49ns ± 1%  2.93ns ± 0%  -16.05%  (p=0.000 n=9+10)
TrailingZeros32-8  2.67ns ± 1%  2.68ns ± 1%     ~     (p=0.468 n=10+10)
TrailingZeros64-8  2.67ns ± 1%  2.65ns ± 0%   -0.62%  (p=0.022 n=10+9)

code:

func f16(x uint) { z = bits.TrailingZeros16(uint16(x)) }

Before:

"".f16 STEXT size=48 args=0x8 locals=0x0 leaf
        0x0000 00000 (test.go:7)        TEXT    "".f16(SB), LEAF|NOFRAME|ABIInternal, $0-8
        0x0000 00000 (test.go:7)        FUNCDATA        ZR, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        FUNCDATA        $3, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        PCDATA  $2, ZR
        0x0000 00000 (test.go:7)        PCDATA  ZR, ZR
        0x0000 00000 (test.go:7)        MOVD    "".x(FP), R0
        0x0004 00004 (test.go:7)        MOVHU   R0, R0
        0x0008 00008 (test.go:7)        ORR     $65536, R0, R0
        0x000c 00012 (test.go:7)        RBIT    R0, R0
        0x0010 00016 (test.go:7)        CLZ     R0, R0
        0x0014 00020 (test.go:7)        MOVD    R0, "".z(SB)
        0x0020 00032 (test.go:7)        RET     (R30)

This line of code is unnecessary:
        0x0004 00004 (test.go:7)        MOVHU   R0, R0

After:

"".f16 STEXT size=32 args=0x8 locals=0x0 leaf
        0x0000 00000 (test.go:7)        TEXT    "".f16(SB), LEAF|NOFRAME|ABIInternal, $0-8
        0x0000 00000 (test.go:7)        FUNCDATA        ZR, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        FUNCDATA        $3, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (test.go:7)        PCDATA  $2, ZR
        0x0000 00000 (test.go:7)        PCDATA  ZR, ZR
        0x0000 00000 (test.go:7)        MOVD    "".x(FP), R0
        0x0004 00004 (test.go:7)        ORR     $65536, R0, R0
        0x0008 00008 (test.go:7)        RBITW   R0, R0
        0x000c 00012 (test.go:7)        CLZW    R0, R0
        0x0010 00016 (test.go:7)        MOVD    R0, "".z(SB)
        0x001c 00028 (test.go:7)        RET     (R30)

The situation of TrailingZeros8 is similar to TrailingZeros16.

Change-Id: I473bdca06be8460a0be87abbae6fe640017e4c9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/156999
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/test/codegen/mathbits.go b/test/codegen/mathbits.go
index 09939bb6be..c77b66c3f7 100644
--- a/test/codegen/mathbits.go
+++ b/test/codegen/mathbits.go
@@ -242,6 +242,7 @@ func RotateLeftVariable32(n uint32, m int) uint32 {
 
 func TrailingZeros(n uint) int {
 	// amd64:"BSFQ","MOVL\t\\$64","CMOVQEQ"
+	// arm64:"RBIT","CLZ"
 	// s390x:"FLOGR"
 	// ppc64:"ANDN","POPCNTD"
 	// ppc64le:"ANDN","POPCNTD"
@@ -250,6 +251,7 @@ func TrailingZeros(n uint) int {
 
 func TrailingZeros64(n uint64) int {
 	// amd64:"BSFQ","MOVL\t\\$64","CMOVQEQ"
+	// arm64:"RBIT","CLZ"
 	// s390x:"FLOGR"
 	// ppc64:"ANDN","POPCNTD"
 	// ppc64le:"ANDN","POPCNTD"
@@ -258,6 +260,7 @@ func TrailingZeros64(n uint64) int {
 
 func TrailingZeros32(n uint32) int {
 	// amd64:"BTSQ\\t\\$32","BSFQ"
+	// arm64:"RBITW","CLZW"
 	// s390x:"FLOGR","MOVWZ"
 	// ppc64:"ANDN","POPCNTW"
 	// ppc64le:"ANDN","POPCNTW"
@@ -266,6 +269,7 @@ func TrailingZeros32(n uint32) int {
 
 func TrailingZeros16(n uint16) int {
 	// amd64:"BSFL","BTSL\\t\\$16"
+	// arm64:"ORR\t\\$65536","RBITW","CLZW",-"MOVHU\tR",-"RBIT\t",-"CLZ\t"
 	// s390x:"FLOGR","OR\t\\$65536"
 	// ppc64:"POPCNTD","OR\\t\\$65536"
 	// ppc64le:"POPCNTD","OR\\t\\$65536"
@@ -274,6 +278,7 @@ func TrailingZeros16(n uint16) int {
 
 func TrailingZeros8(n uint8) int {
 	// amd64:"BSFL","BTSL\\t\\$8"
+	// arm64:"ORR\t\\$256","RBITW","CLZW",-"MOVBU\tR",-"RBIT\t",-"CLZ\t"
 	// s390x:"FLOGR","OR\t\\$256"
 	return bits.TrailingZeros8(n)
 }
@@ -314,6 +319,7 @@ func IterateBits16(n uint16) int {
 	i := 0
 	for n != 0 {
 		// amd64:"BSFL",-"BTSL"
+		// arm64:"RBITW","CLZW",-"ORR"
 		i += bits.TrailingZeros16(n)
 		n &= n - 1
 	}
@@ -324,6 +330,7 @@ func IterateBits8(n uint8) int {
 	i := 0
 	for n != 0 {
 		// amd64:"BSFL",-"BTSL"
+		// arm64:"RBITW","CLZW",-"ORR"
 		i += bits.TrailingZeros8(n)
 		n &= n - 1
 	}