Files
llvm/lib/Analysis
Roman Lebedev 2f4e7f099b [IR] Value: add replaceUsesWithIf() utility
Summary:
While there is always a `Value::replaceAllUsesWith()`,
sometimes the replacement needs to be conditional.

I have only cleaned a few cases where `replaceUsesWithIf()`
could be used, to both add test coverage,
and show that it is actually useful.

Reviewers: jdoerfert, spatel, RKSimon, craig.topper

Reviewed By: jdoerfert

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, aheejin, george.burgess.iv, asbirlea, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65528

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367548 91177308-0d34-0410-b5e6-96231b3b80d8
2019-08-01 12:32:08 +00:00
..
2019-05-31 10:14:04 +00:00

Analysis Opportunities:

//===---------------------------------------------------------------------===//

In test/Transforms/LoopStrengthReduce/quadradic-exit-value.ll, the
ScalarEvolution expression for %r is this:

  {1,+,3,+,2}<loop>

Outside the loop, this could be evaluated simply as (%n * %n), however
ScalarEvolution currently evaluates it as

  (-2 + (2 * (trunc i65 (((zext i64 (-2 + %n) to i65) * (zext i64 (-1 + %n) to i65)) /u 2) to i64)) + (3 * %n))

In addition to being much more complicated, it involves i65 arithmetic,
which is very inefficient when expanded into code.

//===---------------------------------------------------------------------===//

In formatValue in test/CodeGen/X86/lsr-delayed-fold.ll,

ScalarEvolution is forming this expression:

((trunc i64 (-1 * %arg5) to i32) + (trunc i64 %arg5 to i32) + (-1 * (trunc i64 undef to i32)))

This could be folded to

(-1 * (trunc i64 undef to i32))

//===---------------------------------------------------------------------===//