[PATCH] D51985: [InstCombine] Inefficient pattern for high-bits checking (PR38708)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, craig.topper.

It is sometimes important to check that some newly-computed value
is non-negative and only `n` bits wide (where `n` is a variable.)
There are **many** ways to check that:
https://godbolt.org/z/o4RB8D
The last variant seems best?
(I'm sure there are some other variations i haven't thought of..)

Let's handle the second variant first, since it is much simpler.
https://rise4fun.com/Alive/LYjY

https://bugs.llvm.org/show_bug.cgi?id=38708


Repository:
  rL LLVM

https://reviews.llvm.org/D51985

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp-ugt-of-shl-1-by-bits-and-val-to-icmp-eq-of-lshr-val-by-bits-and-0.ll
  test/Transforms/InstCombine/icmp-ule-of-shl-1-by-bits-and-val-to-icmp-ne-of-lshr-val-by-bits-and-0.ll

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