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tune for better branch prediction by avoiding more than 3 in a 16-byte block

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amd_d_nrv2b.S amd_d_nrv2e.S l_lx_elf64amd.S

committer: jreiser <jreiser> 1131994471 +0000
This commit is contained in:
John Reiser
2005-11-14 18:54:31 +00:00
parent 2558243c83
commit beb4319d1e
3 changed files with 26 additions and 13 deletions
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src/stub/l_lx_elf64amd.S
+22 -9
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@@ -91,17 +91,29 @@ decompress: # (uchar const *src, size_t lsrc, uchar *dst, u32 &ldst, uint metho
xorl bits,bits # empty; force refill
xorl len,len # create loop invariant
orq $~0,disp # -1: initial displacement
jmp setup
call setup # push &getbit [TUNED]
ra_setup:
/* AMD64 branch prediction is much worse if there are more than 3 branches
per 16-byte block. The jnextb would suffer unless inlined. getnextb is OK
using closed subroutine to save space, and should be OK on cycles because
CALL+RET should be predicted. getnextb could partially expand, using closed
subroutine only for refill.
*/
/* jump on next bit {0,1} with prediction {y==>likely, n==>unlikely} */
/* Prediction omitted for now. */
#define jnextb0n jnextb0y
#define jnextb0y addl bits,bits; jnz 0f; call refill; 0: jnc
#define jnextb0y GETBIT; jnc
#define jnextb1n jnextb1y
#define jnextb1y addl bits,bits; jnz 0f; call refill; 0: jc
#define jnextb1y GETBIT; jc
#define GETBIT \
addl bits,bits; jnz 0f; \
movl (%rsi),bits; subq $-4,%rsi; \
adcl bits,bits; movb (%rsi),%dl; \
0:
/* rotate next bit into bottom bit of reg */
#define getnextb(reg) addl bits,bits; jnz 0f; call refill; 0: adcl reg,reg
#define getnextb(reg) call *%r11; adcl reg,reg
ALIGN(1<<3)
getbit:
@@ -110,13 +122,13 @@ getbit:
refill:
movl (%rsi),bits; subq $-4,%rsi # next 32 bits; set Carry
adcl bits,bits # LSB= 1 (CarryIn); CarryOut= next bit
movzbl (%rsi),%edx # speculate: literal, or bottom 8 bits of offset
movb (%rsi),%dl # speculate: literal, or bottom 8 bits of offset
rep; ret
copy: # In: len, %rdi, disp; Out: 0==len, %rdi, disp; trashes %rax, %rdx
leaq (%rdi,disp),%rax; movb (%rax),%dl
cmpl $ 3,len; jbe copy1 # perhaps extend this to length 5 or less?
cmpq $-4,disp; ja copy1 # 4-byte chunks would overlap
leaq (%rdi,disp),%rax; cmpl $5,len # <=3 is forced
movb (%rax),%dl; jbe copy1 # <=5 for better branch predict
cmpq $-4,disp; ja copy1 # 4-byte chunks would overlap
subl $4,len # adjust for termination cases
copy4:
movl (%rax),%edx; addq $4, %rax; subl $4,len
@@ -134,7 +146,8 @@ copy0:
setup:
cld
cmpl $ M_NRV2E_LE32,meth; je bot_n2e
pop %r11 # addq $ getbit - ra_setup,%r11 # &getbit
cmpl $ M_NRV2E_LE32,meth; je top_n2e
cmpl $ M_NRV2B_LE32,meth; je top_n2b
eof:
pop %rcx # &input_eof