runtime: disable prefetching on 386

It doesn't seem to help on modern processors and it makes Go impossible to run
on Pentium MMX (which is the documented minimum hardware requirement.)

Old is with prefetch, new is w/o. Both are compiled with GO386=sse2.
Benchmarking is done on Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz.

name                     old time/op    new time/op    delta
BinaryTree17-4              2.89s ± 2%     2.87s ± 0%    ~     (p=0.061 n=11+10)
Fannkuch11-4                3.65s ± 0%     3.65s ± 0%    ~     (p=0.365 n=11+11)
FmtFprintfEmpty-4          52.1ns ± 0%    52.1ns ± 0%    ~      (p=0.065 n=10+9)
FmtFprintfString-4          168ns ± 0%     167ns ± 0%  -0.48%   (p=0.000 n=8+10)
FmtFprintfInt-4             167ns ± 0%     167ns ± 1%    ~      (p=0.591 n=9+10)
FmtFprintfIntInt-4          295ns ± 0%     292ns ± 0%  -0.99%   (p=0.000 n=9+10)
FmtFprintfPrefixedInt-4     327ns ± 0%     326ns ± 0%  -0.24%  (p=0.007 n=10+10)
FmtFprintfFloat-4           431ns ± 0%     431ns ± 0%  -0.07%  (p=0.000 n=10+11)
FmtManyArgs-4              1.13µs ± 0%    1.13µs ± 0%  -0.37%  (p=0.009 n=11+11)
GobDecode-4                9.36ms ± 1%    9.33ms ± 0%  -0.31%  (p=0.006 n=11+10)
GobEncode-4                7.38ms ± 1%    7.38ms ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                      394ms ± 0%     395ms ± 1%    ~     (p=0.519 n=11+11)
Gunzip-4                   65.4ms ± 0%    65.4ms ± 0%    ~     (p=0.739 n=10+10)
HTTPClientServer-4         52.4µs ± 1%    52.5µs ± 1%    ~     (p=0.748 n=11+11)
JSONEncode-4               19.0ms ± 0%    19.0ms ± 0%    ~      (p=0.780 n=9+10)
JSONDecode-4               59.6ms ± 0%    59.6ms ± 0%    ~      (p=0.720 n=9+10)
Mandelbrot200-4            4.09ms ± 0%    4.09ms ± 0%    ~      (p=0.295 n=11+9)
GoParse-4                  3.45ms ± 1%    3.43ms ± 1%  -0.35%  (p=0.040 n=11+11)
RegexpMatchEasy0_32-4       101ns ± 1%     101ns ± 1%    ~     (p=1.000 n=11+11)
RegexpMatchEasy0_1K-4       796ns ± 0%     796ns ± 0%    ~      (p=0.954 n=10+8)
RegexpMatchEasy1_32-4       110ns ± 0%     110ns ± 1%    ~      (p=0.289 n=9+11)
RegexpMatchEasy1_1K-4       991ns ± 0%     991ns ± 0%    ~      (p=0.784 n=10+8)
RegexpMatchMedium_32-4      131ns ± 0%     130ns ± 0%  -0.42%   (p=0.004 n=11+9)
RegexpMatchMedium_1K-4     41.9µs ± 1%    41.6µs ± 0%    ~      (p=0.067 n=11+9)
RegexpMatchHard_32-4       2.34µs ± 0%    2.34µs ± 0%    ~     (p=0.208 n=11+11)
RegexpMatchHard_1K-4       70.9µs ± 0%    71.0µs ± 0%    ~      (p=0.968 n=9+10)
Revcomp-4                   819ms ± 0%     818ms ± 0%    ~     (p=0.251 n=10+11)
Template-4                 73.9ms ± 0%    73.8ms ± 0%  -0.25%  (p=0.013 n=10+11)
TimeParse-4                 414ns ± 0%     414ns ± 0%    ~     (p=0.809 n=11+10)
TimeFormat-4                485ns ± 0%     485ns ± 0%    ~      (p=0.404 n=11+7)

name                     old speed      new speed      delta
GobDecode-4              82.0MB/s ± 1%  82.3MB/s ± 0%  +0.31%  (p=0.007 n=11+10)
GobEncode-4               104MB/s ± 1%   104MB/s ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                   49.2MB/s ± 0%  49.1MB/s ± 1%    ~     (p=0.507 n=11+11)
Gunzip-4                  297MB/s ± 0%   297MB/s ± 0%    ~     (p=0.670 n=10+10)
JSONEncode-4              102MB/s ± 0%   102MB/s ± 0%    ~      (p=0.794 n=9+10)
JSONDecode-4             32.6MB/s ± 0%  32.6MB/s ± 0%    ~       (p=0.334 n=9+9)
GoParse-4                16.8MB/s ± 1%  16.9MB/s ± 1%    ~     (p=0.052 n=11+11)
RegexpMatchEasy0_32-4     314MB/s ± 0%   314MB/s ± 1%    ~     (p=0.618 n=11+11)
RegexpMatchEasy0_1K-4    1.29GB/s ± 0%  1.29GB/s ± 0%    ~     (p=0.315 n=10+10)
RegexpMatchEasy1_32-4     290MB/s ± 1%   290MB/s ± 1%    ~     (p=0.667 n=10+11)
RegexpMatchEasy1_1K-4    1.03GB/s ± 0%  1.03GB/s ± 0%    ~      (p=0.829 n=10+8)
RegexpMatchMedium_32-4   7.63MB/s ± 0%  7.65MB/s ± 0%    ~     (p=0.142 n=11+11)
RegexpMatchMedium_1K-4   24.4MB/s ± 1%  24.6MB/s ± 0%    ~      (p=0.063 n=11+9)
RegexpMatchHard_32-4     13.7MB/s ± 0%  13.7MB/s ± 0%    ~     (p=0.302 n=11+11)
RegexpMatchHard_1K-4     14.4MB/s ± 0%  14.4MB/s ± 0%    ~      (p=0.784 n=9+10)
Revcomp-4                 310MB/s ± 0%   311MB/s ± 0%    ~     (p=0.243 n=10+11)
Template-4               26.2MB/s ± 0%  26.3MB/s ± 0%  +0.24%  (p=0.009 n=10+11)

Update golang#12970.

Change-Id: Id185080687a60c229a5cb2e5220e7ca1b53910e2
Reviewed-on: https://go-review.googlesource.com/15999
Reviewed-by: Austin Clements <[email protected]>
Reviewed-by: Dmitry Vyukov <[email protected]>

src/runtime/asm_386.s

@@ -1530,23 +1530,15 @@ TEXT runtime·goexit(SB),NOSPLIT,$0-0
 	// traceback from goexit1 must hit code range of goexit
 	BYTE	$0x90	// NOP
 
+// Prefetching doesn't seem to help.
 TEXT runtime·prefetcht0(SB),NOSPLIT,$0-4
-	MOVL	addr+0(FP), AX
-	PREFETCHT0	(AX)
 	RET
 
 TEXT runtime·prefetcht1(SB),NOSPLIT,$0-4
-	MOVL	addr+0(FP), AX
-	PREFETCHT1	(AX)
 	RET
 
-
 TEXT runtime·prefetcht2(SB),NOSPLIT,$0-4
-	MOVL	addr+0(FP), AX
-	PREFETCHT2	(AX)
 	RET
 
 TEXT runtime·prefetchnta(SB),NOSPLIT,$0-4
-	MOVL	addr+0(FP), AX
-	PREFETCHNTA	(AX)
 	RET