[llvm-dev] Trouble when suppressing a portion of fast-math-transformations

Tue Oct 3 13:08:03 PDT 2017

On Tue, Oct 3, 2017 at 10:08 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 10/03/2017 05:26 AM, Ristow, Warren wrote:
>
> > I agree that using SubclassOptionalData is going to be problematic when
> we run out of bits. ...
>
>
>
> I don't have a good view of the big-picture here (in terms of the cost of
> size and speed of the metadata approach, vs a member of Instruction, vs
> something else).
>
>
>
> We could tackle this problem now, or defer it hoping that we're not going
> to want to add more flags for finer granularity control of fast-math
> transformations in the future.  It seems the general sense is that we
> should tackle it now.
>
> Sanjay and Hal, is that what you're view is?
>
>
> I think that it might be a good idea, but I don't have an opinion on
> ordering. They should be separate patches if possible (and, if it is not
> possible because we've run out of bits, then obviously the decision has
> been made for us).
>
>
Yes, it seems like we're going to hit that wall sooner or later, so I'd try
to get that part done first since that should have no visible effect on the
final codegen.

I have no sense of what the fastest implementation would be or how anyone
would know without just trying something and timing it.

>  -Hal
>
>
>
>
> -Warren
>
>
>
> *From:* Hal Finkel [mailto:hfinkel at anl.gov <hfinkel at anl.gov>]
> *Sent:* Tuesday, October 3, 2017 1:49 AM
> *To:* Sanjay Patel; Ristow, Warren
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Trouble when suppressing a portion of
> fast-math-transformations
>
>
>
>
>
> On 10/01/2017 06:05 PM, Sanjay Patel wrote:
>
> Are we confident that we just need those 7 bits to represent all of the
> relaxed FP states that we need/want to support?
>
>
>
> I'm asking because FMF in IR is currently mapped onto the
> SubclassOptionalData of Value...and we have exactly 7 bits there. :)
>
> If we're redoing the definitions, I'm wondering if we can share the struct
> with the backend's SDNodeFlags, but that already has one extra bit for
> vector reduction. Should we give up on SubclassOptionalData for FMF? We
> have a MD_fpmath enum value for metadata, so we could move things over
> there?
>
>
> I agree that using SubclassOptionalData is going to be problematic when we
> run out of bits. As I recall, the reason that we didn't use metadata in the
> first place was because metadata is (generically) expensive. This case is
> very much like the case of debug info: in some modes, we add the debugging
> metadata to nearly every instruction. We use metadata for debug locations,
> syntactically, but we actually have a DebugLoc in each instruction that's
> used for the underlying representation. Here we'd have a similar problem:
> in some modes, we'd add this metadata to a large subset of all
> instructions. That could measurably slow down the optimizer. We may need to
> find some other place to put the data (e.g., an actual member variable of
> Instruction or more tail-allocated data in places)
>
>  -Hal
>
>
>
>
>
>
> On Fri, Sep 29, 2017 at 8:16 PM, Ristow, Warren via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi Hal,
>
> >> 4. To fix this, I think that additional fast-math-flags are likely
> >> needed in the IR.  Instead of the following set:
> >>
> >> 'nnan' + 'ninf' + 'nsz' + 'arcp' + 'contract'
> >>
> >> something like this:
> >>
> >> 'reassoc' + 'libm' + 'nnan' + 'ninf' + 'nsz' + 'arcp' + 'contract'
> >>
> >> would be more useful.  Related to this, the current 'fast' flag which
> acts
> >> as an umbrella (enabling 'nnan' + 'ninf' + 'nsz' + 'arcp' + 'contract')
> may
> >> not be needed.  A discussion on this point was raised last November on
> the
> >> mailing list:
> >>
> >> http://lists.llvm.org/pipermail/llvm-dev/2016-November/107104.html
> >
> > I agree. I'm happy to help review the patches. It will be best to have
> > only the finer-grained flags where there's no "fast" flag that implies
> > all of the others.
>
> Thanks for the quick response, and for the willingness to review.  I won't
> let
> this languish so long, like the post from last November.
>
> Happy to hear that you feel it's best not to have the umbrella "fast" flag.
>
> Thanks again,
>
> -Warren
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>
>
>
>
> --
>
> Hal Finkel
>
> Lead, Compiler Technology and Programming Languages
>
> Leadership Computing Facility
>
> Argonne National Laboratory
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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