[llvm-dev] [ScalarEvolution][SCEV] no-wrap flags dependent on order of getSCEV() calls (original) (raw)

Adam Nemet via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 14 15:52:35 PDT 2017

On Aug 14, 2017, at 7:35 AM, Geoff Berry <gberry at codeaurora.org> wrote:

Hi Sanjoy, [adding Adam since I believe he added the original FIXME to preserve SCEV in LoopDataPrefetch]

For record, that wasn’t me. It was there from the beginning when Hal added the PPC-specific pass.

Adam

On 8/14/2017 1:36 AM, Sanjoy Das wrote: Hi Geoff, On Wed, Aug 9, 2017 at 8:58 AM, Geoff Berry <gberry at codeaurora.org> wrote:

On 8/8/2017 8:38 PM, Sanjoy Das wrote:

Hi, On Tue, Aug 8, 2017 at 12:58 PM, Friedman, Eli <efriedma at codeaurora.org> wrote:

Oh, I see... yes, we do stupid things involving mutating NoWrap flags after a SCEV is created. (grep for setNoWrapFlags in ScalarEvolution.cpp.)

That's really a compile time hack -- we defer some expensive tricks to prove nsw/nuw on an add recurrences to when we've been asked to sign/zero extend said add recurrence. If that is the case, preserving SCEV analysis where we didn't before should only result in later passes seeing more nsw/nuw flags, which should theoretically be beneficial? There are downsides to preserving SCEV for "too long" though -- if you simplified a loop in a way that SCEV can compute its trip count when it couldn't before (or it could compute a trip count before, but now it can compute a more precise trip count) then invalidating the cache can be an improvement. However, in this case judicious use of forgetLoop() (as opposed to re-constructing the whole of SCEV) will also do the job. In this particular case, neither of the passes that I'm marking as newly preserving SCEV modify any loop structure or any instructions that should impact any of the existing SCEV values. All they do is add prefetch instructions (and some new instructions to compute the addresses to prefetch) and annotate some load instructions with metadata. That's why I was surprised to see changes in generated code from marking them as preserving SCEV. I've done some performance testing and didn't see any significant changes in performance due to these minor SCEV changes, so I think I'm going to post a patch to enable SCEV preservation in these passes and move on. -- Sanjoy I would be okay if you wanted to expose those tricks via a helper in SCEV that could be called independently of creating zero or sign extensions. That way SCEV clients could trade-off compile time for> more precise information in a granular way. I'm not concerned with getting better SCEV results at the moment, I just want to make sure that changing these passes to preserve SCEV analysis is behaving as expected (which it sounds like it is). -- Sanjoy

-Eli -- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -- Geoff Berry Employee of Qualcomm Datacenter Technologies, Inc. Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project. -- Geoff Berry Employee of Qualcomm Datacenter Technologies, Inc. Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

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