[PATCH] D142234: [ConstantRange] Handle Intrinsic::ctlz

[Antonio Frighetto via Phabricator via llvm-commits]

antoniofrighetto added inline comments.


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Comment at: llvm/lib/IR/ConstantRange.cpp:1686
+    // 1) Lower is zero, handling cases of kind [0, 1), [0, 2), etc.
+    // 2) Upper is zero, wrapped set, handling cases of kind [3, 0], etc.
+    // 3) Zero contained in a wrapped set, e.g., [3, 2), [3, 1), etc.
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StephenFan wrote:
> IIUC, is `[3, 0]` the same as the `[3, 1)` in case 3? 
Correct.


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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
+
----------------
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.


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  https://reviews.llvm.org/D142234/new/

https://reviews.llvm.org/D142234