https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79389
Bug ID: 79389
Summary: 30% performance regression in SciMark2 MonteCarlo
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 40676
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40676&action=edit
The relevant source code and generated assembler before/after this change
The MonteCarlo test from the old SciMark2 (http://math.nist.gov/scimark) used
in the Phoronix compiler test suite has regressed 30% compared to GCC 6.3 when
run on Intel i7 6800K Broadwell (compiled with "-O3 -march=native").
The regression was introduced in r238005:
------------------------------------------------------------------------
r238005 | rguenth | 2016-07-05 15:25:47 +0200 (tis, 05 jul 2016) | 7 lines
2016-07-05 Richard Biener <[email protected]>
* gimple-ssa-split-paths.c (find_block_to_duplicate_for_splitting_pa):
Handle empty else block.
(is_feasible_trace): Likewise.
(split_paths): Likewise.
------------------------------------------------------------------------
and has the effect that the if-statement in the loop
for (count=0; count<Num_samples; count++)
{
double x= Random_nextDouble(R);
double y= Random_nextDouble(R);
if ( x*x + y*y <= 1.0)
under_curve ++;
}
is changed from a cmov to a branch, which mispredicts.
