In 35676a4f9a I've added handling for
non-trivial dominating conditions that imply non-zero on the true
branch. This adds the same support for the false branch.
The changes in pr45360.ll change block ordering and naming, but
don't change the control flow. The urem is still guaraded by a
non-zero check correctly.
Add tests for non-trivial conditions that imply non-zero on the
false branch rather than the true branch.
The last case already folds due to canonicalization.
The dominating condition handling in isKnownNonZero() currently
only takes into account conditions of the form "x != 0" or "x == 0".
However, there are plenty of other conditions that imply non-zero,
a common one being "x s> 0".
Peculiarly, the handling for assumes was already dealing with more
general non-zero-ness conditions, so this just reuses the same
logic for the dominating condition case.
Check conditions that imply non-zero, even if they are not literally
"x != 0".
Using ctlz for testing, as explicit comparison might get folded by
other reasoning.
Summary:
Same as D60846 and D69571 but with a fix for the problem encountered
after them. Both times it was a missing context adjustment in the
handling of PHI nodes.
The reproducers created from the bugs that caused the old commits to be
reverted are included.
Reviewers: nikic, nlopes, mkazantsev, spatel, dlrobertson, uabelho, hakzsam, hans
Subscribers: hiraditya, bollu, asbirlea, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71181
This caused miscompiles of Chromium (https://crbug.com/1023818). The reduced
repro is small enough to fit here:
$ cat /tmp/a.c
unsigned char f(unsigned char *p) {
unsigned char result = 0;
for (int shift = 0; shift < 1; ++shift)
result |= p[0] << (shift * 8);
return result;
}
$ bin/clang -O2 -S -o - /tmp/a.c | grep -A4 f:
f: # @f
.cfi_startproc
# %bb.0: # %entry
xorl %eax, %eax
retq
That's nicely optimized, but I don't think it's the right result :-)
> Same as D60846 but with a fix for the problem encountered there which
> was a missing context adjustment in the handling of PHI nodes.
>
> The test that caused D60846 to be reverted was added in e15ab8f277.
>
> Reviewers: nikic, nlopes, mkazantsev,spatel, dlrobertson, uabelho, hakzsam
>
> Subscribers: hiraditya, bollu, llvm-commits
>
> Tags: #llvm
>
> Differential Revision: https://reviews.llvm.org/D69571
This reverts commit 57dd4b03e4.
Same as D60846 but with a fix for the problem encountered there which
was a missing context adjustment in the handling of PHI nodes.
The test that caused D60846 to be reverted was added in e15ab8f277.
Reviewers: nikic, nlopes, mkazantsev,spatel, dlrobertson, uabelho, hakzsam
Subscribers: hiraditya, bollu, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69571
Nikita Popov