runtime: remove TODO notes suggesting jump tables
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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