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

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev added inline comments.


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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
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spatel wrote:
> 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?
Hmm, yes, you're right. But this could be adjusted only if shift amount is variable, this case is covered by `@shl_var_not_commute()` function below. I can only remove this test at all.


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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
----------------
spatel wrote:
> 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.
Ok, done


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D108091/new/

https://reviews.llvm.org/D108091