8045ba8948
We were not summarizing a function alias in the vtable, leading to incorrect WPD in some cases, and missing WPD in others. Specifically, we would end up ignoring function aliases as they aren't summarized, so we could incorrectly devirtualize if there was a single other non-alias function in a compatible vtable. And if there was only one implementation, but it was an alias, we would not be able to identify and perform the single implementation devirtualization. Handling the alias summary correctly also required fixing the handling in mustBeUnreachableFunction, so that it is not incorrectly ignored. Regular LTO is conservatively correct because it will skip devirtualizing when any pointer within a vtable is not a function. However, it needs additional work to be able to take advantage of function alias within the vtable that is in fact the only implementation. For that reason, the Regular LTO testing in the second test case is currently disabled, and will be enabled along with a follow on enhancement fix for Regular LTO WPD. Differential Revision: https://reviews.llvm.org/D144209
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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