[PATCH] D88429: [InstCombine] visitTrunc - trunc (*shr (trunc A), C) --> trunc(*shr A, C)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:859
+    unsigned AWidth = AType->getScalarSizeInBits();
+    unsigned MaxShiftAmt = std::min<unsigned>(DestWidth, AWidth - DestWidth);
+
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RKSimon wrote:
> lebedev.ri wrote:
> > I do not understand why this isn't just `unsigned MaxShiftAmt = AWidth - DestWidth;`
> I tried that and hit issues with cases like this: https://alive2.llvm.org/ce/z/qV29xP truncating i64 -> i9 -> i8 - so I think there's a typo and it should be std::min<unsigned>(DestWidth, SrcWidth - DestWidth)
It is correct that  https://alive2.llvm.org/ce/z/qV29x9 is a micompile, it should not be getting folded:
https://rise4fun.com/Alive/RTe7 

However to me it still looks like that the precondition i proposed is correct:
https://rise4fun.com/Alive/saBR



Repository:
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  https://reviews.llvm.org/D88429/new/

https://reviews.llvm.org/D88429