[LLVMdev] fmac generation for cortex-a9

[Renato Golin]

Hi Sebastien,

ARMv7-M has VFMA and LLVM's "triple" is far from perfect.

Wikipedia tells me NovaThor can also be A15, or STE could have cramped
a VFPv4 in it? ;) Or possibly, your code never branches into the VFMA.
Many things could be happening, but usually, VFMA shouldn't be
generated for A9.

A GCC bug, maybe?


On 9 November 2012 16:51, Sebastien DELDON-GNB <sebastien.deldon at st.com> wrote:
> Hi Bastien,
>
>
>
> Weird gcc is generating fma for my platform STEricsson Novathor with Linaro,
> code works. It also works when I use LLVM to generate fma (using llc
> –mtriple=armv7-eabi). Maybe someone from ARM can answer the question ?
>
>
>
> Seb
>
>
>
> From: JF Bastien [mailto:jfb at google.com]
> Sent: Friday, November 09, 2012 5:36 PM
> To: Sebastien DELDON-GNB
> Cc: Anitha Boyapati; llvmdev at cs.uiuc.edu
>
>
> Subject: Re: [LLVMdev] fmac generation for cortex-a9
>
>
>
> AFAIK A9 doesn't have VFPv4 or AdvSIMDv2, so it doesn't have VFMA. I don't
> know what LLVM does, but it shouldn't emit VFMA when you target A9. VMLA
> isn't a fused multiply-add, it's a multiply followed by an add and has
> different latency as well as precision.
>
>
>
> On Thu, Nov 8, 2012 at 4:57 AM, Sebastien DELDON-GNB
> <sebastien.deldon at st.com> wrote:
>
> Hi Anitha,
>
> Thanks for your answer but -mcpu=cortex-a9 -mattr=+vfp4 doesn' t enable
> fused mac generation for me.
> I would like just to understand why -mtriple=armv7-eabi enables it while
> -mcpu=cortex-a9 seems to disable it ?
>
> Seb
>
>
>> -----Original Message-----
>> From: Anitha Boyapati [mailto:anitha.boyapati at gmail.com]
>> Sent: Thursday, November 08, 2012 10:22 AM
>> To: Sebastien DELDON-GNB
>> Cc: llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] fmac generation for cortex-a9
>>
>> On 8 November 2012 13:56, Sebastien DELDON-GNB
>> <sebastien.deldon at st.com> wrote:
>> > Hi all,
>> >
>> >
>> >
>> >
>> >
>> > I've a .ll code that use double precision fmul/fadd or fmul/fsub. When
>
>> > I compile it using llc -mcpu=cortex-a9 I couldn't get vmla/vmls
>> > generated even using -fp-contract=fast, but when I use option
>> > -mtriple=armv7-eabi instead of -mcpu=cortex-a9 fused mac are
>
>> generated. Can someone explain me why  ?
>> >
>>
>> Perhaps you need to use some attributes. -mattr=+vfp4 Check fusedMAC.ll
>> from ARM codegen tests.
>>
>>
>> --
>>  Anitha
>
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-- 
cheers,
--renato

http://systemcall.org/