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

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev marked an inline comment as done.
anton-afanasyev added inline comments.


================
Comment at: llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp:282
+    Instruction *I = Itr.first;
+    if (I->getOpcode() == Instruction::Shl) {
+      KnownBits KnownRHS = computeKnownBits(I->getOperand(1), DL);
----------------
spatel wrote:
> This logic is getting more complicated in D108201, so I want to step back to this patch to check my understanding:
> Can SrcBitWidth be different than OrigBitWidth?
> If so, can we write a test for that?
> If not, then assert that I->getOperand(1)->getType()->getScalarSizeInBits == OrigBitWidth. Then simplify this code to something like:
>       KnownBits KnownRHS = computeKnownBits(I->getOperand(1), DL);
>       unsigned MinBitWidth = KnownRHS.getMaxValue()
>                                  .uadd_sat(APInt(OrigBitWidth, 1))
>                                  .getZExtValue();
>       if (MinBitWidth >= OrigBitWidth)
>         return nullptr;
> 
Yes, `SrcBitWidth` can be different than `OrigBitWidth` (smaller or larger). `OrigBitWidth` is bitwidth of original (dominating) `trunc` operand (before truncation) whereas `SrcBitWidth` is bitwidth of shift instruction. Added tests for such cases: https://reviews.llvm.org/rG0988488ed461


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

https://reviews.llvm.org/D108091