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

[Sanjay Patel via llvm-dev]

Thanks for putting this together! I am strongly in favor of this proposal.

My informal proposal in the llvm-dev link from 2016 was at least the 2nd
time this has come up. On each of the previous attempts, we decided that
the cost of analysis for what is usually a 2-instruction icmp+select
sequence was low enough that min/max intrinsics were not worth their
weight.

But that has proven wrong over time - the corner-cases change with each
fix, so we hit a new min/max infinite loop or missed optimization seemingly
every month or so. As noted, the list of problems shown here is only a
small sampling of the total.

We've shown that we can adapt IR analysis to use intrinsics for things like
overflowing/saturating math, and min/max/abs should be about the same level
of work. SelectionDAG already has equivalent nodes for these ops, so
connecting those with IR intrinsics is trivial.

On Mon, Jun 15, 2020 at 4:44 AM Roman Lebedev <lebedev.ri at gmail.com> 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
>
> Roman.
>
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