[PATCH] D12703: [InstCombine] Optimize icmp slt signum(x), 1 --> icmp sle x, 0.

[Sanjoy Das via llvm-commits]

sanjoy created this revision.
sanjoy added a reviewer: majnemer.
sanjoy added a subscriber: llvm-commits.

`signum(x)` is sometimes implemented as `(x >> 63) | (-x >>> 63)` (for
an `i64` `x`).  This change adds a pattern matcher for that pattern, and
adds an instcombine rule to optimize `signum(x) s< 1`.

Later, we can also consider optimizing:

  icmp slt signum(x), 0 --> icmp slt x, 0
  icmp sle signum(x), 1 --> true

etc.

http://reviews.llvm.org/D12703

Files:
  include/llvm/IR/PatternMatch.h
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/compare-signs.ll

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