[PATCH] D142234: [ConstantRange] Handle Intrinsic::ctlz
Antonio Frighetto via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 7 01:12:21 PST 2023
antoniofrighetto added inline comments.
================
Comment at: llvm/test/Analysis/LazyValueAnalysis/lvi-for-ctlz.ll:1
+; RUN: opt < %s -passes=jump-threading -print-lvi-after-jump-threading -disable-output 2>&1 | FileCheck %s
+
----------------
antoniofrighetto wrote:
> StephenFan wrote:
> > antoniofrighetto wrote:
> > > nikic wrote:
> > > > nikic wrote:
> > > > > Having an IR test is fine, but please do not test LVI debug output. Just check the resulting IR change using update_test_checks.py.
> > > > I don't think that as written, these tests really test anything. There needs to be a comparison involving the ctlz that can be folded away, or similar.
> > > I feel like a suitable test could be the following one:
> > > ```
> > > int lol(int b) {
> > > if (b < 65536) {
> > > int n = __builtin_clz(b);
> > > if (n < 8)
> > > return 0;
> > > else
> > > return 1;
> > > }
> > > return 2;
> > > }
> > > ```
> > > which now could be simplified in a `return (b < 65536) ? 1 : 2;`. However, as far as I understand this, CVP needs to be extended as well in order to be able to obtain the above constant folding. Is that correct?
> > Is the simplification to `return (b < 65536) ? 1:2;` correct? Since if `b` is negative, `clz` returns 0.
> True, sorry, I originally intended the first argument to be `unsigned`, whose optimization seems to occur successfully in this case.
@nikic, perhaps a better example could be the following one? That would still require a change in CVP if I'm not mistaken though, correct?
```
int test_ctlz(int b) {
if (b < 65536) {
int n = __builtin_clz(b);
if (n < 8 && n > 2)
return 0;
else
return 1;
}
return 2;
}
```
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https://reviews.llvm.org/D142234/new/
https://reviews.llvm.org/D142234
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