f9f073a861
Summary: When inserting a new Def, MemorySSA may be have non-minimal number of Phis. While inserting, the walk to find the previous definition may cleanup minimal Phis. When the last definition is trivial to obtain, we do not cache it. It is possible while getting the previous definition for a Def to get two different answers: - one that was straight-forward to find when walking the first path (a trivial phi in this case), and - another that follows a cleanup of the trivial phi, it determines it may need additional Phi nodes, it inserts them and returns a new phi in the same position as the former trivial one. While the Phis added for the second path are all redundant, they are not complete (the walk is only done upwards), and they are not properly cleaned up afterwards. A way to fix this problem is to cache the straight-forward answer we got on the first walk. The caching is only kept for the duration of a getPreviousDef call, and for Phis we use TrackingVH, so removing the trivial phi will lead to replacing it with the next dominating phi in the cache. Resolves PR40749. Reviewers: george.burgess.iv Subscribers: jlebar, Prazek, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D60634 llvm-svn: 358313
Analysis Opportunities:
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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.
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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))
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