wrong code at -O3 on x86_64-linux-gnu #50873
Comments
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My bisection points to commit 9934a5b llvm/include/llvm/Transforms/Utils/Local.h | 5 +++ |
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https://godbolt.org/z/qn4Enffoc, I can not reproduce. Also this patch only handle switchinst, this testcase has no swtichinst. |
The C source is irrelevant; the IR has a switch, and https://reviews.llvm.org/D106056 appears to be at fault or exposing a bug somewhere else. Here's a reduced test for "opt -correlated-propagation" (it can probably be reduced further): define i8 @src(i8 %t0) { g.exit.1:; preds = %g.exit, %g.exit, %g.exit g.exit.2:; preds = %g.exit.1, %g.exit.1, %g.exit.1 .thread.2:; preds = %g.exit.2, %g.exit.2, %g.exit.2 .thread.3:; preds = %.thread.2, %.thread.2 |
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Alive2 appears to confirm that the switch transform is at fault: |
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I reverted the original patch, so we are not blocking anyone while investigating. Here's a further reduction that seems to make the problem obvious - if the default block is the same as any of the case blocks, we can't make it unreachable? |
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I think it's safe to resolve this report. I reverted the offending patch with: And added a minimal test based on this example: ...so we should not fail in the same way. There's still an open question about another potential bug discussed in: |
Extended Description
It appears to be a recent regression.
$ clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project.git 4f5ba46)
$ clang-trunk abc.c ; ./a.out
0
$ clang-trunk -O2 abc.c ; ./a.out
$ cat abc.c
int a, c, d;
char e, f;
char(g)(char h, char i) { return i == 0 || h == -128 && i == 1 ? h : h % i; }
char k() {
int j;
d = 3;
for (;; d--)
for (;;) {
short b = g(d, e | d);
j = !b + 1;
f = (1 == b) * j;
c = b;
if (d)
break;
return 1;
}
}
int main() {
k();
printf("%X\n", a);
}
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