[PATCH] D140851: [Patch 3/4]: Add cases for assume (X & Y != {0|Y})

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:939
+        // 1. is_pow2(b) && b == a
+        //      v.zeros[log2(b)] = 1
+        // 2. is_pow2(b) && 0 == a
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nikic wrote:
> goldstein.w.n wrote:
> > nikic wrote:
> > > It looks like we are missing this canonicalization: https://alive2.llvm.org/ce/z/MtveLU With that done, this becomes `v & b == 0` and is covered by existing handling.
> > > It looks like we are missing this canonicalization: https://alive2.llvm.org/ce/z/MtveLU With that done, this becomes `v & b == 0` and is covered by existing handling.
> > 
> > So would a better approach be to handle the `v & b != a` canonicalization in `InstCombine` and drop this case or leave this as is?
> Handling this in InstCombine would be preferred, it reduces the number of patterns other passes see.
> Handling this in InstCombine would be preferred, it reduces the number of patterns other passes see.

Do you know where I should look in `InstCombineCompares` for where to put this?


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Comment at: llvm/lib/Analysis/ValueTracking.cpp:950
+            KnownBits AKnown = computeKnownBits(A, Depth + 1, QueryNoAC)
+                                   .anyextOrTrunc(BitWidth);
+            if (AKnown.isZero()) {
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nikic wrote:
> This should just check whether A is zero, no need to compute known bits.
> This should just check whether A is zero, no need to compute known bits.

How can I check if a value is known zero w.o `computeKnownBits`? I see `isKnownNonZero` but not the inverse.


Repository:
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  https://reviews.llvm.org/D140851/new/

https://reviews.llvm.org/D140851