[llvm-dev] Change undef to poison in a few operations

[Nemanja Ivanovic via llvm-dev]

I don't understand the long-term practical implications of this. One of
these operations is kind of important to me so I'll ask about it
specifically.
The shufflevector with undef elements in the shuffle mask seems to me like
quite a useful thing. We certainly make use of it in the PPC back end. It
seems reasonable to me that if I want the result to have a few elements of
the input vectors in specific element indices, that those should have well
defined values. For those that I don't care about, the back end should be
allowed to put any element there (and decision for which should be up to
the back end - i.e. which shuffle can be handled efficiently). On PPC, we
have a number of instructions that can perform a shuffle with a single
instruction - others are handled with a constant-pool load and a generic
shuffle instruction. We detect masks we can handle and utilize undef as a
"this certainly matches" type of value for any index.

So my question is whether any of this would change and if so, how it would
change.

N

On Tue, Jun 6, 2017 at 11:11 AM, Nuno Lopes via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Lately we have come to realize how undef makes our life complicated..
> Therefore in this email we propose to change the behavior of a few
> instruction to yield poison instead of undef in error cases.  This follows
> the suggestion of Eli in https://reviews.llvm.org/D33654.
>
> Why is undef so bad?
> - I believe it's not possible to make newgvn correct with undef. See for
> example the discussion here: https://bugs.llvm.org/show_bug.cgi?id=33165
> - A bunch of optimizations are correct with poison but not with undef (see
> John's blog posts, my talk at LLVM conf last year, our recent paper on
> PLDI,
> etc)
>
> This proposal is not very radical; it's just to change a few error
> behaviors
> to poison and shouldn't have any practical effect for the time being. (so
> the change is documentation only)
> Later we can continue the work until we have completely removed undef.
> Even
> if we don't go there, it's generally a good idea to reduce the usage of
> undef and replace it with posion whenever possible.
>
> So we propose to change the following from undef to poison:
> - insertelement/extractelement (element index out of range)
> - shufflevector (undef elements in shuffle mask)
> - alloca (allocating zero bytes) ?
> - load (load value with different type from written value)
> - GEP inrange
> - fptrunc/fptoui/fptosi/uitofp/sitofp (overflow) ?
> - llvm.ctlz.* and friends (is_zero_undef) ?
>
> Some of these are no brainers, others may need a bit of discussion.
>
> Please let us know if you have thoughts and/or concerns about this.
>
> Thanks,
> Nuno
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