[llvm-dev] LangRef semantics for shufflevector with undef mask is incorrect

Nuno Lopes via llvm-dev
2019-11-26T23:19:44Z

Hi,

This is a follow up on a discussion around shufflevector with undef mask in
https://reviews.llvm.org/D70641 and
https://bugs.llvm.org/show_bug.cgi?id=43958.

The current semantics of shufflevector in
http://llvm.org/docs/LangRef.html#shufflevector-instruction states:
"If the shuffle mask is undef, the result vector is undef. If any element
of
the mask operand is undef, that element of the result is undef."

We found this semantics to be problematic. TL;DR: instructions can't detect
if an operand is undef.
Long story:
Undef can be replaced with any value at any time. It's valid to replace
undef with 0 or 1 or anything else.

A possible sequence of optimizations with sufflevector could be as follows:
%v = shufflevector <2 x float> %x, <2 x float> undef, <2 x
i32> <i32 undef,
i32 0>
->
%v = shufflevector <2 x float> %x, <2 x float> undef, <2 x
i32> <i32 2, i32
0>
->
%v = <undef, %x[0]>

So this respects the semantics in LangRef: the mask is undef, so the
resulting element is undef.

However, there's an alternative sequence of optimizations:
%v = shufflevector <2 x float> %x, <2 x float> undef, <2 x
i32> <i32 undef,
i32 0>
->
%v = shufflevector <2 x float> %x, <2 x float> undef, <2 x
i32> <i32 1, i32
0>
->
%v = <%x[1], %x[0]>

So now it depends on what the value of %x[1] is. If it's poison, we obtain:
%v = <poison, %x[0]>

This result contradicts the semantics in LangRef, even though no individual
transformation we did above is wrong.
In summary, an instruction cannot detect undef and have special semantics
for it.

AFAICT, the only way to fix the semantics of shufflevector is to say that if
the mask is out-of-bounds, we yield poison. That's correct because
there's
nothing worse than poison.
Since we can replace undef with an OOB index, an undef mask can safely yield
poison. Or it would yield one of the input elements, which is poison in the
worst case. So we get poison in both cases.

I guess the issue to make this semantics a reality is that we would need to
introduce a poison value (which is a good thing IMHO). Otherwise we can't
continue doing some of the folds we have today since we don't have a poison
constant to replace undef when folding.

Any comments/suggestions appreciated!

Thanks,
Nuno