https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94366
Bug ID: 94366
Summary: OpenMP Parallel Reduction with "&&" operator does not
compute correct result
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: satzeni at nvidia dot com
Target Milestone: ---
Created attachment 48137
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48137&action=edit
reproducer
Consider this program:
#include <assert.h>
#include <omp.h>
#include <stdio.h>
static int expect = 1;
static int result;
int main() {
int a = 2; /* or any another even number */
#pragma omp parallel reduction(&& : a)
{
a = a && 1;
}
result = a ? 1 : 0;
assert(result == 1);
}
Compiler and run with:
$ gcc -fopenmp test.c
$ OMP_NUM_THREADS=2 ./a.out
GCC is not correctly computing the logical-and reduction on variable "a".
Inside the parallel region "a" value is 1, but right after the parallel region
is 0 failing the assertion. The correct result should be 1 as the initial value
is 2 and the logical-and between 2 && 1 should be non zero.
Looking at the assembly code, GCC is using a bitwise “and” instruction to
perform the “&&” operation:
<main._omp_fn.0+26> mov (%rbx),%edx
<main._omp_fn.0+28> jmp 0x400782 <main._omp_fn.0+34>
<main._omp_fn.0+30> xchg %ax,%ax
<main._omp_fn.0+32> mov %eax,%edx
<main._omp_fn.0+34> mov %edx,%ecx
<main._omp_fn.0+36> mov %edx,%eax
<main._omp_fn.0+38> and $0x1,%ecx
<main._omp_fn.0+41> lock cmpxchg %ecx,(%rbx)
However the the value of “a” is not reduced to “1” before it is passed to the
parallel region.
<main+15> movl $0x2,(%rsp)
<main+22> callq 0x4005c0 <GOMP_parallel_start@plt>
