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Sanjoy Das
b500839596
Max backedge taken count is always expected to be a constant; and this is usually true by construction -- it is a SCEV expression with constant inputs. However, if the max backedge expression ends up being computed to be a udiv with a constant zero denominator[0], SCEV does not fold the result to a constant since there is no constant it can fold it to (SCEV has no representation for "infinity" or "undef"). However, in computeMaxBECountForLT we already know the denominator is positive, and thus at least 1; and we can use this fact to avoid dividing by zero. [0]: We can end up with a constant zero denominator if the signed range of the stride is more precise than the unsigned range. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316615 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))
//===---------------------------------------------------------------------===//