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SCEV expansion can invalidate previously expanded values. For example in SCEVExpander::ReuseOrCreateCast, if we already have the requested cast value but it's not at the desired location, a new cast is inserted and the old cast will be invalidated. Therefore, when expanding the bounds for the pointers, a later entry can invalidate the IR value for an earlier one. The fix is to store a value handle rather than the value itself. The newly added test has a more detailed description of how the bug triggers. This bug can have a negative but potentially highly variable performance impact in Loop Distribution. Because one of the bound values was invalidated and is an undef expression now, InstCombine is free to transform the array overlap check: Start0 <= End1 && Start1 <= End0 into: Start0 <= End1 So depending on the runtime location of the arrays, we would detect a conflict and fall back on the original loop of the versioned loop. Also tested compile time with SPEC2006 LTO bc files. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245760 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))
//===---------------------------------------------------------------------===//