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| author | Roman Lebedev <lebedev.ri@gmail.com> | 2020-02-25 16:48:36 +0300 |
|---|---|---|
| committer | Roman Lebedev <lebedev.ri@gmail.com> | 2020-02-25 18:23:51 +0300 |
| commit | 781d077afb0ed9771c513d064c40170c1ccd21c9 (patch) | |
| tree | 642ff6e62798d53108a801ec5e55f693eb741aeb /llvm/lib/Transforms/Utils/LoopUtils.cpp | |
| parent | 425ef999385058143bb927aefe81daddcd43f623 (diff) | |
| download | llvm-781d077afb0ed9771c513d064c40170c1ccd21c9.zip llvm-781d077afb0ed9771c513d064c40170c1ccd21c9.tar.gz llvm-781d077afb0ed9771c513d064c40170c1ccd21c9.tar.bz2 | |
[InstCombine] reassociateShiftAmtsOfTwoSameDirectionShifts(): fix miscompile (PR44802)
As input, we have the following pattern:
Sh0 (Sh1 X, Q), K
We want to rewrite that as:
Sh x, (Q+K) iff (Q+K) u< bitwidth(x)
While we know that originally (Q+K) would not overflow
(because 2 * (N-1) u<= iN -1), we may have looked past extensions of
shift amounts. so it may now overflow in smaller bitwidth.
To ensure that does not happen, we need to ensure that the total maximal
shift amount is still representable in that smaller bitwidth.
If the overflow would happen, (Q+K) u< bitwidth(x) check would be bogus.
https://bugs.llvm.org/show_bug.cgi?id=44802
Diffstat (limited to 'llvm/lib/Transforms/Utils/LoopUtils.cpp')
0 files changed, 0 insertions, 0 deletions