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e1ac460951
the algorithm easily degrades into quadratic memory and time complexity. The easiest example is a long chain of BBs that don't otherwise use a location. The caching will add an entry for every intermediate block and limiting the number of results doesn't help as no results are produced until a definition is found. Introduce a limit similar to the existing instructions-per-block limit. This limit counts the total number of blocks checked. If the limit is reached, entries are considered unknown. The initial value is 1000, which avoids regressions for normal sized functions while still limiting edge cases to reasnable memory consumption and execution time. Differential Revision: http://reviews.llvm.org/D16123 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261430 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))
//===---------------------------------------------------------------------===//