33146857e9
This adjusts mayHaveSideEffect() to return true for !willReturn() instructions. Just like other side-effects, non-willreturn calls (aka "divergence") cannot be removed and cannot be reordered relative to other side effects. This fixes a number of bugs where non-willreturn calls are either incorrectly dropped or moved. In particular, it also fixes the last open problem in https://bugs.llvm.org/show_bug.cgi?id=50511. I performed a cursory review of all current mayHaveSideEffect() uses, which convinced me that these are indeed the desired default semantics. Places that do not want to consider non-willreturn as a sideeffect generally do not want mayHaveSideEffect() semantics at all. I identified two such cases, which are addressed by D106591 and D106742. Finally, there is a use in SCEV for which we don't really have an appropriate API right now -- what it wants is basically "would this be considered forward progress". I've just spelled out the previous semantics there. Differential Revision: https://reviews.llvm.org/D106749
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))
//===---------------------------------------------------------------------===//