https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109062
Bug ID: 109062
Summary: [13 regression] Default value of GOMP_SPINCOUNT
changes since r13-2545
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgomp
Assignee: unassigned at gcc dot gnu.org
Reporter: wwwhhhyyy333 at gmail dot com
CC: jakub at gcc dot gnu.org
Target Milestone: ---
Recently we found several big regressions on Phoronix OpenMP benchmark on
GCC13. The regressions is caused by r13-2545-g9f2fca56593a2b
The issue is, the default value of GOMP_SPINCOUNT is now 0, instead of 300000
before this patch, which caused all Openmp program behaves like
OMP_WAIT_POLICY=passive.
As the comments in libgomp/env.c says:
/* Using a rough estimation of 100000 spins per msec,
use 5 min blocking for OMP_WAIT_POLICY=active,
3 msec blocking when OMP_WAIT_POLICY is not specificed
and 0 when OMP_WAIT_POLICY=passive.
Depending on the CPU speed, this can be e.g. 5 times longer
or 5 times shorter. */
The current code for wait_policy is
if (none != NULL && gomp_get_icv_flag (none->flags, GOMP_ICV_WAIT_POLICY))
wait_policy = none->icvs.wait_policy;
else if (all != NULL && gomp_get_icv_flag (all->flags, GOMP_ICV_WAIT_POLICY))
wait_policy = all->icvs.wait_policy;
If OMP_WAIT_POLICY not specified, non of the branch will be entered since
gomp_get_icv_flag will return 0 by default, then wait_policy remains its value
as uninitialized. While prior to this patch wait_policy will be set to -1 (not
specified) by parse_wait_policy ().
