slp-vectorizer miscompile

[mikaelholmen]

llvm commit: 9801fdf
Reproduce with: opt -passes="slp-vectorizer" bbi-107513.ll -S -o -
Result:

define i16 @foo() {
entry:
  %cond3 = select i1 false, i16 1, i16 0
  ret i16 %cond3
}

The function should return the value 1, but with slp-vectorizer we get 0 instead.

This starts happening with d41e517

[SLP] Make getSameOpcode support interchangeable instructions. (#135797)

We use the term "interchangeable instructions" to refer to different
operators that have the same meaning (e.g., `add x, 0` is equivalent to
`mul x, 1`).
Non-constant values are not supported, as they may incur high costs with
little benefit.

---------

Co-authored-by: Alexey Bataev <a.bataev@gmx.com>

bbi-107513.ll.gz

[HanKuanChen]

I try 9d5e144 but it works.

[mikaelholmen]

I try 9d5e144 but it works.

It "works" in what way?

If I run the reproducer at 9d5e144 I get

define i16 @foo() {
entry:
  %cond3 = select i1 false, i16 1, i16 0
  ret i16 %cond3
}

which means it will return 0, which is wrong.

Compare
opt -passes="instcombine" bbi-107513.ll -S -o -
which correctly gives

entry:
  ret i16 1
}

and
opt -passes="slp-vectorizer,instcombine" bbi-107513.ll -S -o -
which gives

define i16 @foo() {
entry:
  ret i16 0
}

which is wrong.

[mikaelholmen]

Also see https://llvm.godbolt.org/z/8Yrx4ejGa which exposes the miscompile.

[bjope]

Here is another godbolt example showing that we get different result if reordering some instructions in the input IR: https://godbolt.org/z/z3aPhPnod

[HanKuanChen]

#143094