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[InstSimplify] rotating all-ones is still all-ones #50917

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rotateright opened this issue Aug 22, 2021 · 4 comments
Closed

[InstSimplify] rotating all-ones is still all-ones #50917

rotateright opened this issue Aug 22, 2021 · 4 comments
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@rotateright
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Bugzilla Link 51575
Resolution FIXED
Resolved on Aug 24, 2021 12:54
Version trunk
OS All
CC @RKSimon
Fixed by commit(s) d41e308 dcf659e 204038d

Extended Description

I'm not sure if there's overlap between this and related rotate bugs, but filing it so we have a reference:

define i32 @​src(i32 %shamt) {
%r = lshr i32 -1, %shamt
%s = sub i32 32, %shamt
%l = shl i32 -1, %s
%o = or i32 %r, %l
ret i32 %o
}

define i32 @​tgt(i32 %shamt) {
ret i32 -1
}

https://alive2.llvm.org/ce/z/gDke_c

We do handle the sibling rotate-of-zero (probably via known bits or simpler folds since all bits are the same in that case).
@rotateright
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Also missing the easier pattern with a funnel shift intrinsic:

define i64 @​src(i64 %x) {
%r = tail call i64 @​llvm.fshl.i64(i64 -1, i64 -1, i64 %x)
ret i64 %r
}

define i64 @​tgt(i64 %x) {
ret i64 -1
}

https://alive2.llvm.org/ce/z/GpkFCt

@rotateright
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And we miss the intrinsic case even with zero:
https://alive2.llvm.org/ce/z/fjKwqv

@rotateright
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The intrinsic patterns should be fixed with:
https://reviews.llvm.org/rGd41e308f109e
https://reviews.llvm.org/rGdcf659e8219b

@rotateright
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Handle the original, non-intrinsic rotate pattern:
https://reviews.llvm.org/rG204038d52e03

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 11, 2021
This issue was closed.
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@rotateright