https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79390
Bug ID: 79390
Summary: 10% performance drop in SciMark2 LU after r242550
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: krister.walfridsson at gmail dot com
Target Milestone: ---
Created attachment 40677
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40677&action=edit
The relevant source code and generated asm before/after this change
The dense LU matrix factorization test from the old SciMark2
(http://math.nist.gov/scimark) used in the Phoronix compiler test suite has
regressed 10% compared to the November trunk when run on Intel i7 6800K
Broadwell (compiled with "-O3 -march=native"). GCC 6 generated much slower
code, so this is not a regression compared to released versions of the
compiler.
The regression was introduced in r242550:
------------------------------------------------------------------------
r242550 | wschmidt | 2016-11-17 15:22:17 +0100 (tor, 17 nov 2016) | 18 lines
[gcc]
2016-11-17 Bill Schmidt <[email protected]>
Richard Biener <[email protected]>
PR tree-optimization/77848
* tree-if-conv.c (tree_if_conversion): Always version loops unless
the user specified -ftree-loop-if-convert.
[gcc/testsuite]
2016-11-17 Bill Schmidt <[email protected]>
Richard Biener <[email protected]>
PR tree-optimization/77848
* gfortran.dg/vect/pr77848.f: New test.
------------------------------------------------------------------------
and has the effect that the pivot-finding loop
int LU_factor(int M, int N, double **A, int *pivot)
{
int minMN = M < N ? M : N;
int j=0;
for (j=0; j<minMN; j++)
{
/* find pivot in column j and test for singularity. */
int jp=j;
int i;
double t = fabs(A[j][j]);
for (i=j+1; i<M; i++)
{
double ab = fabs(A[i][j]);
if ( ab > t)
{
jp = i;
t = ab;
}
}
pivot[j] = jp;
...
is transformed. The perf output seems to say that this is due to bad branch
prediction, but I do not understand x86 assembler enough to be able to
determine its cause (or to say if it really is a bug or just some random thing
the compiler cannot know about...)
