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

[Ristow, Warren via llvm-dev]

I'm not aware of any additional bits needed.  But putting us right at the edge leaves me uncomfortable.  So an implementation that isn't limited by the 7 bits in SubclassOptionalData seems sensible.

Thanks,
-Warren

From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Monday, October 2, 2017 12:06 AM
To: Ristow, Warren
Cc: Hal Finkel; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Trouble when suppressing a portion of fast-math-transformations

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?


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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