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5733100450
Summary: Some optimizations such as jump threading and loop unswitching can negatively affect performance when applied to divergent branches. The divergence analysis added in this patch conservatively estimates which branches in a GPU program can diverge. This information can then help LLVM to run certain optimizations selectively. Test Plan: test/Analysis/DivergenceAnalysis/NVPTX/diverge.ll Reviewers: resistor, hfinkel, eliben, meheff, jholewinski Subscribers: broune, bjarke.roune, madhur13490, tstellarAMD, dberlin, echristo, jholewinski, llvm-commits Differential Revision: http://reviews.llvm.org/D8576 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234567 91177308-0d34-0410-b5e6-96231b3b80d8
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))
//===---------------------------------------------------------------------===//