[llvm-dev] [RFC] Integer Intrinsics for abs, in unsigned/signed min/max

[Roman Lebedev via llvm-dev]

As per popular demand i've dropped misleading "reduction"
wording/naming from them, updated https://reviews.llvm.org/D81829

So far all the responses are favorable to this proposal.

Roman.

On Mon, Jun 15, 2020 at 11:58 PM Nikita Popov <nikita.ppv at gmail.com> wrote:
>
> On Mon, Jun 15, 2020 at 10:44 AM Roman Lebedev via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Hello all.
>>
>> This is a proposal to introduce 5 new integer intrinsics:
>> * absolute value
>> * signed min
>> * signed max
>> * unsigned min
>> * unsigned max
>>
>> This is motivated by the fact that we keep working around
>> not having these intrinsics, and that constantly leads us into
>> having more workarounds, and causes infinite combine loops.
>>
>> Here's a (likely incomplete!) list of motivational bugs:
>>
>> infinite loops:
>> https://bugs.llvm.org/show_bug.cgi?id=46271 / https://reviews.llvm.org/D81698
>> https://bugs.llvm.org/show_bug.cgi?id=45539 /
>> https://reviews.llvm.org/rG01bcc3e93714
>> https://bugs.llvm.org/show_bug.cgi?id=44835 / https://reviews.llvm.org/D74278
>> https://reviews.llvm.org/D68408#1976760
>> https://reviews.llvm.org/D59378
>> https://bugs.llvm.org/show_bug.cgi?id=38915 / https://reviews.llvm.org/D51964
>> https://bugs.llvm.org/show_bug.cgi?id=37526 / https://reviews.llvm.org/rL332855
>>
>> misc:
>> https://bugs.llvm.org/show_bug.cgi?id=44025
>> https://bugs.llvm.org/show_bug.cgi?id=43310 / https://reviews.llvm.org/rL372510
>> https://bugs.llvm.org/show_bug.cgi?id=35607
>> https://bugs.llvm.org/show_bug.cgi?id=35642 / https://reviews.llvm.org/D41136
>> https://bugs.llvm.org/show_bug.cgi?id=41083 / https://reviews.llvm.org/D74285
>> https://reviews.llvm.org/D70148
>> https://bugs.llvm.org/show_bug.cgi?id=31751 /
>> https://reviews.llvm.org/D26096 / https://reviews.llvm.org/rL293345
>>
>> I believe we can do better than that if we stop just treating some IR patterns
>> as being canonical and desperately trying not to break/loose track of them,
>> but instead do a sensible thing and actually make them first class citizens,
>> by introducing intrinsics and use then throughout.
>>
>> This has been previously discussed in:
>> https://lists.llvm.org/pipermail/llvm-dev/2016-November/106868.html
>>
>> Proposed LangRef semantics: https://reviews.llvm.org/D81829
>> Proposed alive2 implementation: https://github.com/AliveToolkit/alive2/pull/353
>
>
> I'm also strongly in favor of this proposal.
>
> Next to the issues already mentioned, this also fixes issues related to undef handling. For example, umax(%x, C) is not actually guaranteed to be >= C. That's because the current umax representation has two uses of %x, which may take on independent values if %x is undef. This makes a number of "common sense" folds invalid. Having dedicated min/max intrinsics avoids that problem.
>
> Regards,
> Nikita