cmd/compile: optimize LeadingZeros(16|32) on amd64
Introduce Len8 and Len16 ops and provide optimized lowerings for them.
amd64 only for this CL, although it wouldn't surprise me
if other architectures also admit of optimized lowerings.
Also use and optimize the Len32 lowering, along the same lines.
Leave Len8 unused for the moment; a subsequent CL will enable it.
For 16 and 32 bits, this leads to a speed-up.
name old time/op new time/op delta
LeadingZeros16-8 1.42ns ± 5% 1.23ns ± 5% -13.42% (p=0.000 n=20+20)
LeadingZeros32-8 1.25ns ± 5% 1.03ns ± 5% -17.63% (p=0.000 n=20+16)
Code:
func f16(x uint16) { z = bits.LeadingZeros16(x) }
func f32(x uint32) { z = bits.LeadingZeros32(x) }
Before:
"".f16 STEXT nosplit size=38 args=0x8 locals=0x0
0x0000 00000 (x.go:8) TEXT "".f16(SB), NOSPLIT, $0-8
0x0000 00000 (x.go:8) FUNCDATA $0, gclocals·
2a5305abe05176240e61b8620e19a815(SB)
0x0000 00000 (x.go:8) FUNCDATA $1, gclocals·
33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (x.go:8) MOVWLZX "".x+8(SP), AX
0x0005 00005 (x.go:8) MOVWLZX AX, AX
0x0008 00008 (x.go:8) BSRQ AX, AX
0x000c 00012 (x.go:8) MOVQ $-1, CX
0x0013 00019 (x.go:8) CMOVQEQ CX, AX
0x0017 00023 (x.go:8) ADDQ $-15, AX
0x001b 00027 (x.go:8) NEGQ AX
0x001e 00030 (x.go:8) MOVQ AX, "".z(SB)
0x0025 00037 (x.go:8) RET
"".f32 STEXT nosplit size=34 args=0x8 locals=0x0
0x0000 00000 (x.go:9) TEXT "".f32(SB), NOSPLIT, $0-8
0x0000 00000 (x.go:9) FUNCDATA $0, gclocals·
2a5305abe05176240e61b8620e19a815(SB)
0x0000 00000 (x.go:9) FUNCDATA $1, gclocals·
33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (x.go:9) MOVL "".x+8(SP), AX
0x0004 00004 (x.go:9) BSRQ AX, AX
0x0008 00008 (x.go:9) MOVQ $-1, CX
0x000f 00015 (x.go:9) CMOVQEQ CX, AX
0x0013 00019 (x.go:9) ADDQ $-31, AX
0x0017 00023 (x.go:9) NEGQ AX
0x001a 00026 (x.go:9) MOVQ AX, "".z(SB)
0x0021 00033 (x.go:9) RET
After:
"".f16 STEXT nosplit size=30 args=0x8 locals=0x0
0x0000 00000 (x.go:8) TEXT "".f16(SB), NOSPLIT, $0-8
0x0000 00000 (x.go:8) FUNCDATA $0, gclocals·
2a5305abe05176240e61b8620e19a815(SB)
0x0000 00000 (x.go:8) FUNCDATA $1, gclocals·
33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (x.go:8) MOVWLZX "".x+8(SP), AX
0x0005 00005 (x.go:8) MOVWLZX AX, AX
0x0008 00008 (x.go:8) LEAL 1(AX)(AX*1), AX
0x000c 00012 (x.go:8) BSRL AX, AX
0x000f 00015 (x.go:8) ADDQ $-16, AX
0x0013 00019 (x.go:8) NEGQ AX
0x0016 00022 (x.go:8) MOVQ AX, "".z(SB)
0x001d 00029 (x.go:8) RET
"".f32 STEXT nosplit size=28 args=0x8 locals=0x0
0x0000 00000 (x.go:9) TEXT "".f32(SB), NOSPLIT, $0-8
0x0000 00000 (x.go:9) FUNCDATA $0, gclocals·
2a5305abe05176240e61b8620e19a815(SB)
0x0000 00000 (x.go:9) FUNCDATA $1, gclocals·
33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (x.go:9) MOVL "".x+8(SP), AX
0x0004 00004 (x.go:9) LEAQ 1(AX)(AX*1), AX
0x0009 00009 (x.go:9) BSRQ AX, AX
0x000d 00013 (x.go:9) ADDQ $-32, AX
0x0011 00017 (x.go:9) NEGQ AX
0x0014 00020 (x.go:9) MOVQ AX, "".z(SB)
0x001b 00027 (x.go:9) RET
Change-Id: I6c93c173752a7bfdeab8be30777ae05a736e1f4b
Reviewed-on: https://go-review.googlesource.com/108941
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Keith Randall <khr@golang.org>