[LoopInterchange] Loops should not interchanged due to dependencies

[llvmbot]

Bugzilla Link	47915
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DMG862

Extended Description

$ clang -mllvm -enable-loopinterchange -O2 reproducer.c -w && ./a.out
4096
0
4096
0

$ clang -O0 reproducer.c -w && ./a.out
4096
4096
4096
4096

reproducer.c:

#include <stdio.h>
int a;
int c[4][4];
*f = &a;

void test_deps() {
for (int i = 0; i <= 3; i++) {
for (int j = 0; j <= 3; j++) {
*f ^= 0x1000;
c[j][i] = *f;
}
}
}

main() {
test_deps();
for (int k = 0; k < 4; k++)
printf("%d\n", c[0][k]);
}

[llvmbot]

I looked into gcc11 with -fdump-tree-linterchange-missed, it gives me the following infomation:

missed: failed: evolution of base is not affine.

It seems to think the access of *f is not an affine access and stops doing interchange. Not even reach the dependency checking phase.

[sjoerdmeijer]

Should have been fixed by #119345