[llvm-dev] EuroLLVM Numerics info

Fri Apr 5 08:20:08 PDT 2019

Hi Sanjay,

Thanks,  yes I was also thinking of looking at re association pass to see
if it can catch few more cases.

regards,
Venkat.

On Fri, 5 Apr 2019 at 19:48, Sanjay Patel <spatel at rotateright.com> wrote:

> [adding back the mailing list]
>
> Thanks for the example!
>
> define double @_Z12sqrt_squaredd(double) {
> %2 = tail call fast double @llvm.sqrt.f64(double %0)
> %3 = fdiv fast double 1.000000e+00, %2
> %4 = fmul fast double %3, %3
> ret double %4
> }
>
> So at first look, this has nothing to do with sqrt specifically. We are
> missing a basic factorization / re-association for fmul and fdiv:
>       // (L1 / L2) * (R1 / R2) --> (L1 * R1) / (L2 * R2)
>
> If we do that, then existing transforms should kick in to reduce the sqrt
> example. I'll try to fix this in instcombine soon. If you have more
> complicated examples where we miss this, that would suggest that we need to
> make an enhancement to the "reassociate" pass too.
>
> On Fri, Apr 5, 2019 at 12:26 AM Venkataramanan Kumar <
> venkataramanan.kumar.llvm at gmail.com> wrote:
>
>> Hi Sanjay,
>>
>> On Thu, 4 Apr 2019 at 21:21, Sanjay Patel <spatel at rotateright.com> wrote:
>>
>>> 1. Do you have a larger code example that shows the missed sqrt/div
>>> optimization?
>>>
>>> We optimize the example you provided already in IR:
>>> #include <math.h>
>>> float sqrt_squared(float x) {
>>>   return 1.0f/sqrtf(x) * 1.0f/sqrtf(x);
>>> }
>>>
>>> Something like this
>> #include <math.h>
>> double sqrt_squared(double x) {
>> double y = 1.0/sqrt(x) ;
>> double z = y * y ;
>> return z;
>> }
>> REF: https://godbolt.org/z/2lhnJq
>>
>>
>>> $ clang -O1 -ffast-math -emit-llvm  sqrt.c -S -o -
>>> define float @sqrt_squared(float) local_unnamed_addr #0 {
>>>   %2 = fdiv fast float 1.000000e+00, %0
>>>   ret float %2
>>> }
>>>
>>> 2. Yes, transforms like the above should work with vector types. If not,
>>> please file a bug.
>>>
>> Oh! thanks let me check.
>>
>> regards,
>> Venkat.
>>
>>>
>>> On Wed, Apr 3, 2019 at 11:47 PM Venkataramanan Kumar via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi Micheal,
>>>>
>>>> Thanks for the blog post.  Just like to point out few things that I
>>>> thought is related to FP Numerics.
>>>> LLVM could do some additional transformation with "sqrt" and "division"
>>>> under fast math on X86 like 1/sqrt(x)* 1/sqrt(x) to 1/x.   These are long
>>>> latency instructions and could get benefit if enabled under unsafe math.
>>>>
>>>> Also are we considering doing such FP transforms on vector floating
>>>> point types?
>>>>
>>>> regards,
>>>> Venkat.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Mon, 1 Apr 2019 at 19:56, Cameron McInally via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hey Michael,
>>>>>
>>>>> Thank you for working on this!
>>>>>
>>>>> I'd like to touch on a topic mentioned in the blog post. The
>>>>> constrained intrinsics work is at a road block on how to proceed with the
>>>>> constrained  implementation in the backends, i.e. D55506. Reviews/ideas in
>>>>> this area would be greatly appreciated (attn: target code owners).
>>>>>
>>>>> Thanks,
>>>>> Cameron
>>>>>
>>>>>
>>>>> On Thu, Mar 28, 2019 at 5:54 PM Michael Berg via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> All:  There will be a BoF talk at the EuroLLVM conference regarding
>>>>>> Numerics (FMF and module flags which control fp behavior and optimization).
>>>>>>
>>>>>> Even if you are not going to be in attendance, please reply to this
>>>>>> thread as we are collecting open issues and ideas for future direction in
>>>>>> all layers of LLVM for which optimizations are controlled by numerics flags.
>>>>>> Please read over the numerics blog if you like for reference material:
>>>>>>
>>>>>>
>>>>>> https://urldefense.proofpoint.com/v2/url?u=http-3A__blog.llvm.org_2019_03_llvm-2Dnumerics-2Dblog.html&d=DwIGaQ&c=slrrB7dE8n7gBJbeO0g-IQ&r=O_4M49EtSpZ_-BQYeigzGv0P4__noMcSu2RYEjS1vKs&m=CHHCGUAnxC91rtvrnhhoN0jXGXgXv8EQb8C4XiUEoxY&s=xzUyP83nqSY1h9GrDdHch4INrIFIU-Ycij4S4Tmxdko&e=
>>>>>>
>>>>>> Regards,
>>>>>> Michael
>>>>>> _______________________________________________
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>>>>>>
>>>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.llvm.org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=DwIGaQ&c=slrrB7dE8n7gBJbeO0g-IQ&r=O_4M49EtSpZ_-BQYeigzGv0P4__noMcSu2RYEjS1vKs&m=CHHCGUAnxC91rtvrnhhoN0jXGXgXv8EQb8C4XiUEoxY&s=5CVfTGSHNKAmAzx4dc0HQzDTXMucnDgx_NPimota5m8&e=
>>>>>>
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