[PATCH] D47928: [SimplifyIndVars] Eliminate redundant truncs

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: reames, etherzhhb, greened, sanjoy.

This patch adds logic to deal with the following constructions:

  %iv = phi i64 ...
  %trunc = trunc i64 %iv to i32
  %cmp = icmp <pred> i32 %trunc, %invariant

Replacing it with

  %iv = phi i64 ...
  %cmp = icmp <pred> i64 %iv, sext/zext(%invariant)

In case if it is legal. Specifically, if `%iv` has signed comparison users, it is
required that `sext(trunc(%iv)) == %iv`, and if it has unsigned comparison
uses then we require `zext(trunc(%iv)) == %iv`. The current implementation
bails if `%trunc` has other uses than `icmp`, but in theory we can handle more
cases here (e.g. if the user of trunc is bitcast).


https://reviews.llvm.org/D47928

Files:
  lib/Transforms/Utils/SimplifyIndVar.cpp
  test/Transforms/IndVarSimplify/eliminate-trunc.ll

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