[LLVMdev] RE : fmac generation for cortex-a9

[Sebastien DELDON-GNB]

Hi Renato,

You're right it's VMLA/VMLS that are generated. Still don't understand what drives generation for Cortex-A9. 
I was using fmac  for floating point MAC not for fused MAC. Than I realized that we spoke about fma instead of fmac.
So back to the original problem why when using -mcpu=cortex-a9 VMLA/VMLS are not generated and when I use -mtriple=armv7-eabi they are ?

Best Regards
Seb

________________________________________
De : JF Bastien [jfb at google.com]
Date d'envoi : vendredi 9 novembre 2012 18:45
À : Sebastien DELDON-GNB
Cc : Renato Golin; llvmdev at cs.uiuc.edu
Objet : Re: [LLVMdev] fmac generation for cortex-a9

cat /proc/cpuinfo ?

Are you sure it's generating VFMA and not VMLA?


On Fri, Nov 9, 2012 at 9:35 AM, Sebastien DELDON-GNB <sebastien.deldon at st.com<mailto:sebastien.deldon at st.com>> wrote:
Hi Renato,

It's definitively not A15. Can this be the case that NEON units for cortex-A9 support it but isn't documented/recommended ?
And as mentioned before code is working !

Seb


> -----Original Message-----
> From: rengolin at gmail.com<mailto:rengolin at gmail.com> [mailto:rengolin at gmail.com<mailto:rengolin at gmail.com>] On Behalf Of
> Renato Golin
> Sent: Friday, November 09, 2012 6:27 PM
> To: Sebastien DELDON-GNB
> Cc: JF Bastien; llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
> Subject: Re: [LLVMdev] fmac generation for cortex-a9
>
> 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<mailto: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<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<mailto: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<mailto: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<mailto:anitha.boyapati at gmail.com>]
> >> Sent: Thursday, November 08, 2012 10:22 AM
> >> To: Sebastien DELDON-GNB
> >> Cc: llvmdev at cs.uiuc.edu<mailto: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<mailto: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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> >
> >
> >
> >
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> >
>
>
>
> --
> cheers,
> --renato
>
> http://systemcall.org/