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* [Bug target/50572] New: unstable performance on Atom due to loop alignment
@ 2011-09-30 9:03 sergos.gnu at gmail dot com
2011-10-20 19:30 ` [Bug target/50572] " hjl at gcc dot gnu.org
2012-01-07 22:28 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: sergos.gnu at gmail dot com @ 2011-09-30 9:03 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50572
Bug #: 50572
Summary: unstable performance on Atom due to loop alignment
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: target
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: sergos.gnu@gmail.com
After monitoring of Atom performance on trunk for some period of time I figured
out that we have a significant (up to 15%) instability because of loop
alignment. Currently for Atom we have the following alignments:
{&atom_cost, 16, 7, 16, 7, 16}
for
struct ptt
{
const struct processor_costs *cost; /* Processor costs */
const int align_loop; /* Default alignments. */
const int align_loop_max_skip;
const int align_jump;
const int align_jump_max_skip;
const int align_func;
};
Which means we try to align by 16, although if it takes no more than 7 bytes to
insert. This 'if' is the source of instability. For a reduction loop I observed
almost twice slowdown because it did not fit into 16bytes after being aligned
by 8.
I used the -falign-loops=16 option to measure code size impact using -m32-O2
-msse2 -mfpmath=sse -ffast-math -march=atom for SPEC2000:
SPEC2000
Test .text section size
-----------------------------------------
Aligned Current Increas %% increase
wupwise 630324 630084 240 0,04%
swim_ 602612 602548 64 0,01%
mgrid_ 608388 608212 176 0,03%
applu_ 641684 641412 272 0,04%
mesa_ 941444 938116 3328 0,35%
galgel_ 813508 811764 1744 0,21%
art_ 437572 437412 160 0,04%
equake_ 442228 442084 144 0,03%
facerec 694948 694596 352 0,05%
ammp_ 561428 560292 1136 0,20%
lucas_ 663236 662948 288 0,04%
fma3d_ 1565348 1560228 5120 0,33%
sixtrac 1537844 1534228 3616 0,24%
apsi_ 719172 718340 832 0,12%
gzip_ 480452 480020 432 0,09%
vpr_ 548164 547156 1008 0,18%
cc1_ 1554052 1546532 7520 0,49%
mcf_ 434036 433908 128 0,03%
crafty_ 592084 590836 1248 0,21%
parser_ 509476 508276 1200 0,24%
eon_ 1189348 1188852 496 0,04%
perlbmk 894292 891268 3024 0,34%
gap_ 845636 841124 4512 0,54%
vortex_ 969988 968788 1200 0,12%
bzip2_ 472596 472260 336 0,07%
twolf_ 607140 605044 2096 0,35%
Will it be acceptable to put -falign-loops=16 under -mtune=atom for O2?
^ permalink raw reply [flat|nested] 3+ messages in thread
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-- links below jump to the message on this page --
2011-09-30 9:03 [Bug target/50572] New: unstable performance on Atom due to loop alignment sergos.gnu at gmail dot com
2011-10-20 19:30 ` [Bug target/50572] " hjl at gcc dot gnu.org
2012-01-07 22:28 ` pinskia at gcc dot gnu.org
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