| Issue |
71429
|
| Summary |
need a way to report branch removed due to poison values
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
yxsamliu
|
https://godbolt.org/z/bb3We4ozP
the following code results in an empty function:
`int foo(int *x) {
int sum = 0;
for (int i = 0; i < 19; i++) {
unsigned mask = 1U << (i - 1);
sum += x[i & mask];
}
return sum;
}`
This is because when i is 0, `1U << (i - 1)` is poison. BDCEPass will remove the branches with poison conditions.
However, clang is unable to diagnose this. Since only after loop unrolling the compiler knows `mask` is poison. We cannot simply diagnose any `unsigned << signed` by default since such uses are too common.
This kind of error is pretty common and is difficult for users to debug. Therefore better have a way to let LLVM mid-end to report it. Maybe by some -R options.
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