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bzip2: work around bad compiler optimization

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gc-6.1.1 x86_64:
function                                             old     new   delta
generateMTFValues                                    380     367     -13

gcc-4.3.1 386:
function                                             old     new   delta
inner_loop                                             -      41     +41
generateMTFValues                                    357     294     -63
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 41/-63)            Total: -22 bytes

gcc-6.3.0 386:
function                                             old     new   delta
inner_loop                                             -      36     +36
generateMTFValues                                    363     250    -113
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 36/-113)           Total: -77 bytes

The last case, gcc-6.3.0, runs almost 3 times faster after this change.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko
2018-02-05 00:34:08 +01:00
parent f75a7c0439
commit c2a51b0cf1
1 changed files with 43 additions and 32 deletions
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75
archival/libarchive/bz/compress.c
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@@ -158,6 +158,38 @@ void makeMaps_e(EState* s)
/*---------------------------------------------------*/
/*
* This bit of code is performance-critical.
* On 32bit x86, gcc-6.3.0 was observed to spill ryy_j to stack,
* resulting in abysmal performance (x3 slowdown).
* Forcing it into a separate function alleviates register pressure,
* and spillage no longer happens.
* Other versions of gcc do not exhibit this problem, but out-of-line code
* seems to be helping them too (code is both smaller and faster).
* Therefore NOINLINE is enabled for the entire 32bit x86 arch for now,
* without a check for gcc version.
*/
static
#if defined __i386__
NOINLINE
#endif
int inner_loop(uint8_t *yy, uint8_t ll_i)
{
register uint8_t rtmp;
register uint8_t* ryy_j;
rtmp = yy[1];
yy[1] = yy[0];
ryy_j = &(yy[1]);
while (ll_i != rtmp) {
register uint8_t rtmp2;
ryy_j++;
rtmp2 = rtmp;
rtmp = *ryy_j;
*ryy_j = rtmp2;
}
yy[0] = rtmp;
return ryy_j - &(yy[0]);
}
static NOINLINE
void generateMTFValues(EState* s)
{
@@ -165,7 +197,6 @@ void generateMTFValues(EState* s)
int i;
int zPend;
int32_t wr;
int32_t EOB;
/*
* After sorting (eg, here),
@@ -189,15 +220,12 @@ void generateMTFValues(EState* s)
* compressBlock().
*/
uint32_t* ptr = s->ptr;
uint8_t* block = s->block;
uint16_t* mtfv = s->mtfv;
makeMaps_e(s);
EOB = s->nInUse+1;
wr = 0;
zPend = 0;
for (i = 0; i <= EOB; i++)
for (i = 0; i <= s->nInUse+1; i++)
s->mtfFreq[i] = 0;
for (i = 0; i < s->nInUse; i++)
@@ -211,7 +239,7 @@ void generateMTFValues(EState* s)
j = ptr[i] - 1;
if (j < 0)
j += s->nblock;
ll_i = s->unseqToSeq[block[j]];
ll_i = s->unseqToSeq[s->block[j]];
AssertD(ll_i < s->nInUse, "generateMTFValues(2a)");
if (yy[0] == ll_i) {
@@ -225,15 +253,15 @@ void generateMTFValues(EState* s)
while (1) {
#if 0
if (zPend & 1) {
mtfv[wr] = BZ_RUNB; wr++;
s->mtfv[wr] = BZ_RUNB; wr++;
s->mtfFreq[BZ_RUNB]++;
} else {
mtfv[wr] = BZ_RUNA; wr++;
s->mtfv[wr] = BZ_RUNA; wr++;
s->mtfFreq[BZ_RUNA]++;
}
#else /* same as above, since BZ_RUNA is 0 and BZ_RUNB is 1 */
unsigned run = zPend & 1;
mtfv[wr] = run;
s->mtfv[wr] = run;
wr++;
s->mtfFreq[run]++;
#endif
@@ -247,36 +275,19 @@ void generateMTFValues(EState* s)
goto end;
zPend = 0;
}
{
register uint8_t rtmp;
register uint8_t* ryy_j;
register uint8_t rll_i;
rtmp = yy[1];
yy[1] = yy[0];
ryy_j = &(yy[1]);
rll_i = ll_i;
while (rll_i != rtmp) {
register uint8_t rtmp2;
ryy_j++;
rtmp2 = rtmp;
rtmp = *ryy_j;
*ryy_j = rtmp2;
}
yy[0] = rtmp;
j = ryy_j - &(yy[0]);
mtfv[wr] = j+1;
wr++;
s->mtfFreq[j+1]++;
}
j = inner_loop(yy, ll_i);
s->mtfv[wr] = j+1;
wr++;
s->mtfFreq[j+1]++;
}
i = -1;
if (zPend > 0)
goto process_zPend; /* "process it and come back here" */
end:
mtfv[wr] = EOB;
s->mtfv[wr] = s->nInUse+1;
wr++;
s->mtfFreq[EOB]++;
s->mtfFreq[s->nInUse+1]++;
s->nMTF = wr;
}