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

Roman Lebedev via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 18 13:26:07 PDT 2020

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/showbug.cgi?id=46271 / https://reviews.llvm.org/D81698 https://bugs.llvm.org/showbug.cgi?id=45539 / https://reviews.llvm.org/rG01bcc3e93714 https://bugs.llvm.org/showbug.cgi?id=44835 / https://reviews.llvm.org/D74278 https://reviews.llvm.org/D68408#1976760 https://reviews.llvm.org/D59378 https://bugs.llvm.org/showbug.cgi?id=38915 / https://reviews.llvm.org/D51964 https://bugs.llvm.org/showbug.cgi?id=37526 / https://reviews.llvm.org/rL332855 misc: https://bugs.llvm.org/showbug.cgi?id=44025 https://bugs.llvm.org/showbug.cgi?id=43310 / https://reviews.llvm.org/rL372510 https://bugs.llvm.org/showbug.cgi?id=35607 https://bugs.llvm.org/showbug.cgi?id=35642 / https://reviews.llvm.org/D41136 https://bugs.llvm.org/showbug.cgi?id=41083 / https://reviews.llvm.org/D74285 https://reviews.llvm.org/D70148 https://bugs.llvm.org/showbug.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

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