https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82133
Bug ID: 82133
Summary: unroll-loops too aggressive
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: nickpapior at gmail dot com
Target Milestone: ---
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/generic/gcc/7.2.0/libexec/gcc/x86_64-pc-linux-gnu/7.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure --prefix /opt/generic/gcc/7.2.0
--with-gmp=/opt/generic/gcc/7.2.0 --with-mpfr=/opt/generic/gcc/7.2.0
--with-mpc=/opt/generic/gcc/7.2.0 --with-isl=/opt/generic/gcc/7.2.0
--enable-lto --enable-threads --with-quad --with-multilib-list=m64
--enable-languages=c,c++,fortran,objc,obj-c++,go
Thread model: posix
gcc version 7.2.0 (GCC)
The following bug has been confirmed by other parts and runned using these
variants:
- SuSE tumbleweed with system GCC 7.1.1, Atom core
- Debian stretch with custom 7.1.0 and 7.2.0 builds (according to flags
above), Haswell core
- Another distribution (see martin-frbg user in below link).
As several 7.X versions are found; assigned 7.0.
When using unroll-loops to compile OpenBLAS (0.2.20 and developer) it produces
seg-faults when running the tests.
It happens at these minimum compilation options:
-O1 -funroll-loops -fipa-ra
As long as -funroll-loops isn't in the compilation flags everything works as
expected (even for much higher optimization levels: -O3
-fexpensive-optimizations -ftree-vectorize -fprefetch-loop-arrays
-march=native)
It only happens with compilation with optimization, so a backtrace is unusable
(I am not experienced enough to run gdb efficiently).
An invalid memory reference is triggered:
Program received signal SIGSEGV: Segmentation fault - invalid memory reference.
I have filed a bug-report with OpenBLAS too and they indicate that it is
because GCC unrolls a loop one to many times.
For the discussion see here: https://github.com/xianyi/OpenBLAS/issues/1292
The bug can be reproduced by using this small script (in the OpenBLAS
directory):
###
#!/bin/bash
function run_all {
echo "CFLAGS = $CFLAGS"
echo "FCFLAGS = $FCFLAGS"
make clean > /dev/null
make COMMON_OPT="$CFLAGS" FCOMMON_OPT="$FCFLAGS" BINARY=64 SANITY_CHECK=1
MAX_STACK_ALLOC=2048 NUM_THREADS=48 USE_THREAD=0 libs netlib shared 2>
${1}_compilation.out > ${1}_compilation.out
make COMMON_OPT="$CFLAGS" FCOMMON_OPT="$FCFLAGS" BINARY=64 SANITY_CHECK=1
MAX_STACK_ALLOC=2048 NUM_THREADS=48 USE_THREAD=0 tests 2> $1.out > $1.out
}
which gcc
sleep 1
unset FCFLAGS
unset CFLAGS
export CFLAGS="-O1 -funroll-loops -fipa-ra"
export FCFLAGS="$CFLAGS"
run_all minimum
###
