[PATCH] D41939: [LV] Fix incorrect detection of type-promoted Phis

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: hfinkel, dorit, anemet, anna, reames.

Loop vectorizer may recognize the following pattern

  %iv = phi i32 ...
  %x = add i32 %iv, %mask

where `%mask` is `2^N - 1` as Phi node of type `iN` which was type-promoted
during canonicalizing transforms. In this case we can vectorize the loop using
its original type and fit more elements into one vector.

The logic that is responsible for this is broken. It only demands that a Phi is
masked, but does not check that after all calculations the result is actually
truncated to type `iN`. As result, it may falsely detect the following code as
being somehow connected to type-promoted Phis:

  char foo(char c) {
      for (int i = 1; i < 193; i++) {
            c &= (char) 1;
            c += (char) 3;
      }
      return c;
  }

Vectorizer tries to calculate the result of this loop in vector of `i1`, what lead
to incorrect calculations (answer is always `0` or `1` instead of `3` or `4).

This patch strengthens the detecor of this situation. If we have shrinked the
type due to masking, we should later check that the result of all calculations
is either dead or is properly truncated to this type.


https://reviews.llvm.org/D41939

Files:
  lib/Transforms/Utils/LoopUtils.cpp
  test/Transforms/LoopVectorize/and-plus.ll

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