[PATCH] D108091: [AggressiveInstCombine] Add shift left instruction to `TruncInstCombine` DAG

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D108091#2945739 <https://reviews.llvm.org/D108091#2945739>, @lebedev.ri wrote:

> @spatel this appears correct, but is this the right place for this logic, or should it be in `getMinBitWidth()`?

Hmm...I haven't looked at this code much before, so I'm not sure yet.



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Comment at: llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll:36-37
   %zext = zext i8 %x to i32
   %shl = shl i32 %zext, 16
   %trunc = trunc i32 %shl to i16
   ret i16 %trunc
----------------
If the left shift amount >= the truncated bit-width, the result must be 0.
Looks like -instcombine gets this, but not -instsimplify. 
Can we adjust this test to something that does not completely fold away?


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Comment at: llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll:49
   %zext = zext i8 %x to i32
   %shl = shl i32 %zext, %zext
   %trunc = trunc i32 %shl to i16
----------------
If the intent is to check for a shift amount with no known bits, it would be better to just add a function argument `%y`, so we're not mixing up that constraint with number of uses or some other factor.


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  https://reviews.llvm.org/D108091/new/

https://reviews.llvm.org/D108091