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

[Hal Finkel via llvm-dev]

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).

  -Hal

> -Warren
>
> *From:*Hal Finkel [mailto: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 <mailto: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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