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From: "michelemartone at users dot sourceforge.net" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/102461] New: overflow in omp parallel for schedule (static,chunk_size) Date: Wed, 22 Sep 2021 22:56:05 +0000 [thread overview] Message-ID: <bug-102461-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102461 Bug ID: 102461 Summary: overflow in omp parallel for schedule (static,chunk_size) Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: michelemartone at users dot sourceforge.net Target Milestone: --- Created attachment 51499 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51499&action=edit `git diff` output of gcc/omp-expand.c against revision 7ca388565af aka tag: releases/gcc-11.2.0 Hi, I am the author of the LIBRSB library [http://librsb.sourceforge.net/] for Sparse BLAS (sparse linear algebra_ computations. I've received a bug report [https://savannah.gnu.org/bugs/?60042#comment4] where an OpenMP-parallelized loop between i=0..<near to INT_MAX> ended up executing the loop body with i=-1. The original loop in question had static scheduling and chunk_size=10000, and even one thread is sufficient to cause the problem. I reduced the problem and can reproduce it with gcc-11.2.0 (built from sources, GIT tag releases/gcc-11.2.0) and older ones; arch x86_64. I could not reproduce the problem with clang-11.0.1 or icc-19.0.5.281. // # gcc -Wall -Wextra -pedantic -fopenmp -std=c11 -O0 overflow.c #include <stdlib.h> // abort #include <limits.h> // INT_MAX #include <omp.h> // compile with -fopenmp int main () { const int chunk_size = 1000; const int n = INT_MAX - 100; // 2147483547 int l = 0; #pragma omp parallel for schedule (static,chunk_size) num_threads (1) for (int i = 0; i < n; ++i) { l = i; if(i < 0) abort (); } if ( l != n-1 ) abort (); return 0; } I made some experiments in gcc/omp-expand.c and came up with a fix to insert overflow detection code when computing the lower and upper boundaries of the current chunk, thus avoiding the loop body from being executed with i=-1. Invoking patched gcc like: FIX1=1 FIX2=1 FIX3=1 gcc -fopenmp -Wall -pedantic -O0 overflow_mini.c -o overflow -fdump-tree-all and looking at the *.ompexp file it dumps, one can get an idea of what was going wrong with the original flow. Unfortunately I was not able to make my patch generate correct code for -O1 or more. I am attaching `git diff` output of gcc/omp-expand.c against revision 7ca388565af aka tag: releases/gcc-11.2.0 -- so one may use this as a base to fix the bug fully. I hope this information is enough for you GCC/OpenMP folks to fix this problem! Michele
next reply other threads:[~2021-09-22 22:56 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-22 22:56 michelemartone at users dot sourceforge.net [this message] 2021-09-22 22:59 ` [Bug c/102461] " michelemartone at users dot sourceforge.net 2021-09-22 23:00 ` michelemartone at users dot sourceforge.net 2021-09-22 23:02 ` michelemartone at users dot sourceforge.net 2021-09-23 7:54 ` [Bug middle-end/102461] " jakub at gcc dot gnu.org 2022-01-24 10:28 ` michelemartone at users dot sourceforge.net
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