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f864d89ef7
For memmove/memclr using jump tables only reduces overall
function performance for both amd64 and 386.
Benchmarks for 32-bit memclr:
name old time/op new time/op delta
Memclr/5-8 8.01ns ± 0% 8.94ns ± 2% +11.59% (p=0.000 n=9+9)
Memclr/16-8 9.05ns ± 0% 9.49ns ± 0% +4.81% (p=0.000 n=8+8)
Memclr/64-8 9.15ns ± 0% 9.49ns ± 0% +3.76% (p=0.000 n=9+10)
Memclr/256-8 16.6ns ± 0% 16.6ns ± 0% ~ (p=1.140 n=10+9)
Memclr/4096-8 179ns ± 0% 166ns ± 0% -7.26% (p=0.000 n=9+8)
Memclr/65536-8 3.36µs ± 1% 3.31µs ± 1% -1.48% (p=0.000 n=10+9)
Memclr/1M-8 59.5µs ± 3% 60.5µs ± 2% +1.67% (p=0.009 n=10+10)
Memclr/4M-8 239µs ± 3% 245µs ± 0% +2.49% (p=0.004 n=10+8)
Memclr/8M-8 618µs ± 2% 614µs ± 1% ~ (p=0.315 n=10+8)
Memclr/16M-8 1.49ms ± 2% 1.47ms ± 1% -1.11% (p=0.029 n=10+10)
Memclr/64M-8 7.06ms ± 1% 7.05ms ± 0% ~ (p=0.573 n=10+8)
[Geo mean] 3.36µs 3.39µs +1.14%
For less predictable data, like loop iteration dependant sizes,
branch table still shows 2-5% worse results.
It also makes code slightly more complicated.
This CL removes TODO note that directly suggest trying this
optimization out. That encourages people to spend their time
in a quite hopeless endeavour.
The code used to implement branch table used a 32/64-entry table
with pointers to TEXT blocks that implemented every associated
label work. Most last entries point to "loop" code that is
a fallthrough for all other sizes that do not map into specialized
routines. The only inefficiency is extra MOVL/MOVQ required
to fetch table pointer itself as MOVL $sym<>(SB)(AX*4) is not valid
in Go asm (it works in other assemblers):
TEXT ·memclrNew(SB), NOSPLIT, $0-8
MOVL ptr+0(FP), DI
MOVL n+4(FP), BX
// Handle 0 separately.
TESTL BX, BX
JEQ _0
LEAL -1(BX), CX // n-1
BSRL CX, CX
// AX or X0 zeroed inside every text block.
MOVL $memclrTable<>(SB), AX
JMP (AX)(CX*4)
_0:
RET
Change-Id: I4f706931b8127f85a8439b95834d5c2485a5d1bf
Reviewed-on: https://go-review.googlesource.com/115678
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
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