[all-commits] [llvm/llvm-project] 8db514: [InstCombine] Optimize overflow check base on uadd...

[Nikita Popov via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 8db5143b1a1521915c842ebef23cb9fe8fe607ce
      https://github.com/llvm/llvm-project/commit/8db5143b1a1521915c842ebef23cb9fe8fe607ce
  Author: Nikita Popov <nikita.ppv at gmail.com>
  Date:   2019-12-11 (Wed, 11 Dec 2019)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
    M llvm/test/Transforms/InstCombine/with_overflow.ll

  Log Message:
  -----------
  [InstCombine] Optimize overflow check base on uadd.with.overflow result

Fix for https://bugs.llvm.org/show_bug.cgi?id=40846.

This adds a combine for cases where a (a + b) < a style overflow
check is performed, but with a + b being the result of
uadd.with.overflow, so the overflow result is also already available
and we can just use it. Subsequently GVN/CSE will deduplicate the extracts.

We can run into this situation if you have both a uadd.with.overflow
and a manual add + overflow check in the same function (on the same
operands), in which case GVN will rewrite the add to the with.overflow
result and leave you with this pattern.

The implementation is a bit ugly because I'm handling the various
canonicalization edge cases.

This does not yet handle the negated version of this pattern.

Differential Revision: https://reviews.llvm.org/D58644