[PATCH] D143368: [InstCombine] Look through truncate to fold icmp with intrinsics

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

General proofs with Alive2 are probably not possible for a bitwidth constrained transform like this, but you can include proofs for the specific changes here. 
How can we test `ctpop`?
What happens if the trunc has an extra use?



================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:4920-4921
+    if (II->getIntrinsicID() == Intrinsic::ctpop ||
+        II->getIntrinsicID() == Intrinsic::cttz ||
+        II->getIntrinsicID() == Intrinsic::ctlz) {
+      // Make sure the dest bits is enough to save the intrinsic output's range
----------------
Require one less bit for cttz/ctlz when the "is_zero_poison" argument is true?


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Comment at: llvm/test/Transforms/InstCombine/cmp-intrinsic.ll:548
   %tz = tail call i33 @llvm.cttz.i33(i33 %x, i1 false)
   %trunc = trunc i33 %tz to i15
   %cmp = icmp ult i15 %trunc, 7
----------------
Why truncate to i15? 

IIUC, the transform is valid on this example for truncate down to 6 bits, but not 5 bits. So we should have both of those variations. 

The source type width should be tested similarly, so if we start with i32, it is ok to truncate to i5, but not i4?


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  https://reviews.llvm.org/D143368/new/

https://reviews.llvm.org/D143368