| Issue |
165878
|
| Summary |
[SLPVectorizer] Miscompile in slp vectorizer (shift produces poison)
|
| Labels |
miscompilation,
llvm:SLPVectorizer
|
| Assignees |
|
| Reporter |
danilaml
|
SLPVectorizer (`opt -passes=slp-vectorizer -slp-threshold=-99999`) turns the following IR:
```llvm
target triple = "x86_64-unknown-linux-gnu"
define float @foo(i8 %0) {
entry:
%1 = sext i8 0 to i32
%2 = lshr i32 %1, 27
%3 = sext i8 %0 to i32
%reass.add.epil = mul i32 %3, 2
%4 = or i32 %reass.add.epil, %2
switch i32 %4, label %exit [
i32 0, label %exit
i32 1, label %exit
]
exit: ; preds = %body, %body, %body
ret float 0.000000e+00
}
```
into
```llvm
define float @foo(i8 %0) {
entry:
%1 = insertelement <2 x i8> <i8 poison, i8 0>, i8 %0, i32 0
%2 = sext <2 x i8> %1 to <2 x i16>
%3 = mul <2 x i16> %2, <i16 2, i16 27>
%4 = lshr <2 x i16> %2, <i16 2, i16 27>
%5 = shufflevector <2 x i16> %3, <2 x i16> %4, <2 x i32> <i32 0, i32 3>
%6 = extractelement <2 x i16> %5, i32 0
%7 = sext i16 %6 to i32
%8 = extractelement <2 x i16> %5, i32 1
%9 = zext i16 %8 to i32
%10 = or i32 %7, %9
switch i32 %10, label %exit [
i32 0, label %exit
i32 1, label %exit
]
exit:
ret float 0.000000e+00
}
```
Note that it now has `i16` shifts by `27` which later get turned into poison.
Godbolt link: https://godbolt.org/z/shj5GK5nd
Alive2 link: https://alive2.llvm.org/ce/z/CM7Pmp
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