Sanjay Patel fc3b34c508 [InstSimplify] remove shift that is redundant with part of funnel shift ...

In D111530, I suggested that we add some relatively basic pattern-matching
folds for shifts and funnel shifts and avoid a more specialized solution
if possible.

We can start by implementing at least one of these in IR because it's
easier to write the code and verify with Alive2:
https://alive2.llvm.org/ce/z/qHpmNn

This will need to be adapted/extended for SDAG to handle the motivating
bug ( #49541 ) because the patterns only appear later with that example
(added some tests: bb850d422b)

This can be extended within InstSimplify to handle cases where we 'and'
with a shift too (in that case, kill the funnel shift).
We could also handle patterns where the shift and funnel shift directions
are inverted, but I think it's better to canonicalize that instead to
avoid pattern-match case explosion.

Differential Revision: https://reviews.llvm.org/D120253

2022-02-23 09:10:01 -05:00

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