https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86291
Bug ID: 86291
Summary: OpenMP incorrect for-loop collapsing with iterators
and at least 5 nested loops
Product: gcc
Version: 8.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgomp
Assignee: unassigned at gcc dot gnu.org
Reporter: bonjour at matteodelabre dot me
CC: jakub at gcc dot gnu.org
Target Milestone: ---
Created attachment 44315
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44315&action=edit
Preprocessor output
When collapsing at least 5 nested for loops with the collapse() clause in a
`for` pragma, the body of the loop never executes.
In the testcase, 5 for loops on trivial iterators are nested. These iterators
only yield one value, such that the innermost statement should execute exactly
once.
When compiled without the -fopenmp flag, the program exhibits correct behaviour
and we see one line printed on standard output. When compiled with the -fopenmp
flag, the body of the loop never executes and the output stays blank.
This only happens when all of the loop variables are iterators and there is at
least 5 levels of nesting. If any of the loops is changed to operate on
non-iterator values (such as plain integers) or if the nesting is reduced below
5 levels, the statement executes.
g++ -v info:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 8.1.1 20180531 (GCC)
