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>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/runtime/memclr_amd64.s b/src/runtime/memclr_amd64.s
index 63730eebfb..d79078fd00 100644
--- a/src/runtime/memclr_amd64.s
+++ b/src/runtime/memclr_amd64.s
@@ -17,6 +17,7 @@ TEXT runtime·memclrNoHeapPointers(SB), NOSPLIT, $0-16
 
 	// MOVOU seems always faster than REP STOSQ.
 tail:
+	// BSR+branch table make almost all memmove/memclr benchmarks worse. Not worth doing.
 	TESTQ	BX, BX
 	JEQ	_0
 	CMPQ	BX, $2
@@ -39,7 +40,6 @@ tail:
 	JBE	_129through256
 	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
 	JE loop_preheader_avx2
-	// TODO: use branch table and BSR to make this just a single dispatch
 	// TODO: for really big clears, use MOVNTDQ, even without AVX2.
 
 loop: