[llvm-dev] FMA canonicalization in IR

[Philip Reames via llvm-dev]

On 11/19/2016 11:38 PM, Hal Finkel via llvm-dev wrote:
> ----- Original Message -----
>> From: "Sanjay Patel" <spatel at rotateright.com>
>> To: "Hal Finkel" <hfinkel at anl.gov>
>> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
>> Sent: Saturday, November 19, 2016 10:40:27 PM
>> Subject: Re: [llvm-dev] FMA canonicalization in IR
>>
>>
>> The potential advantage I was considering would be more accurate cost
>> modeling in the vectorizer, inliner, etc. Like min/max, this is
>> another case where the sum of the IR parts is greater than the
>> actual cost.
> This is indeed a problem, but is a much larger problem than just FMAs (as you note). Our cost-modeling interfaces should be extended to handle instruction patterns -- I don't see any other way of solving this in general.
This proposal - cost model instruction patterns, not just instructions - 
keeps coming up in a number of contexts.  We've seen a number of 
proposals recently to add intrinsics at various places in the pipeline 
to get around this limitation.  Investing in infrastructure to solve 
this problem via the cost model seems like a generally useful path 
forward which would benefit far more than FMA.
>
>> Beyond that, it seems odd to me that we'd choose the longer IR
>> expression of something that could be represented in a minimal form.
> My fear is that, by forming the FMAs earlier than necessary, you'll just end up limiting opportunities for CSE, reassociation, etc. without any corresponding benefit.
>
>> I know we make practical concessions in IR based on backend
>> deficiencies, but in this case I think the fix would be easy - if
>> we're in contract=fast mode, just split all of these intrinsics at
>> DAG creation time and let the DAG or other passes behave exactly
>> like they do today to fuse them back together again?
> This is a good point; we could do this in fp-contract=fast mode.
>
>   -Hal
>
>> On Sat, Nov 19, 2016 at 8:29 PM Hal Finkel < hfinkel at anl.gov > wrote:
>>
>>
>> ----- Original Message -----
>>> From: "Hal J. via llvm-dev Finkel" < llvm-dev at lists.llvm.org >
>>> To: "Sanjay Patel" < spatel at rotateright.com >
>>> Cc: "llvm-dev" < llvm-dev at lists.llvm.org >
>>> Sent: Saturday, November 19, 2016 10:58:27 AM
>>> Subject: Re: [llvm-dev] FMA canonicalization in IR
>>>
>>>
>>> Sent from my Verizon Wireless 4G LTE DROID
>>> On Nov 19, 2016 10:26 AM, Sanjay Patel < spatel at rotateright.com >
>>> wrote:
>>>> If I have my FMA intrinsics story straight now (thanks for the
>>>> explanation, Hal!), I think it raises another question about IR
>>>> canonicalization (and may affect the proposed revision to IR
>>>> FMF):
>>>
>>> No, I think that we specifically don't want to canonicalize to
>>> fmuladd at the IR level at all. If the backend has the freedom to
>>> form FMAs as it sees fit, then we should delay the decision until
>>> whenever the backend finds most appropriate. Some backends, for
>>> example, form FMAs using the MachineCombiner pass which considers
>>> critical path, latency, throughputs, etc. in order to find the best
>>> fusion opportunities. We only use fmuladd when required to restrict
>>> the backend to certain choices due to source-language semantics.
>> I'll also add that, in general, we canonicalize in order to enable
>> other transformations (and reduce the number of input forms those
>> transformations need to match in order to be effective). Forming
>> @llvm.fmulall at the IR level does not seem to further this goal.
>> Did you have something in mind that this canonicalization would
>> help?
>>
>> Thanks again,
>> Hal
>>
>>>
>>> Thanks again,
>>> Hal
>>>
>>>
>>>> define float @foo(float %a, float %b, float %c) {
>>>> %mul = fmul fast float %a, %b ; using 'fast' because there is no
>>>> 'fma' flag
>>>> %add = fadd fast float %mul, %c
>>>> ret float %add
>>>> }
>>>>
>>>> Should this be:
>>>>
>>>> define float @goo(float %a, float %b, float %c) {
>>>> % maybe.fma = call fast float @llvm.fmuladd.f32(float %a, float
>>>> %b,
>>>> float %c)
>>>> ret float % maybe.fma
>>>> }
>>>> declare float @llvm.fmuladd.f32(float %a, float %b, float %c)
>>>>
>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>