(original) (raw)

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

Hi Sanjoy,

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

On 8/14/2017 1:36 AM, Sanjoy Das wrote:

Hi Geoff,
On Wed, Aug 9, 2017 at 8:58 AM, Geoff Berry <gberry@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@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.