[LLVMdev] Best way for JIT to query whether llvm.fma.* is fast?

[Robison, Arch]

Thanks for the tip.  Indeed the current plan is to have Julia functions the correspond to both llvm.fmuladd and llvm.fma.  Discussion is at https://github.com/JuliaLang/julia/pull/8112 .

- Arch

From: Lang Hames [mailto:lhames at gmail.com]
Sent: Friday, December 12, 2014 11:16 AM
To: Robison, Arch
Cc: Jingyue Wu; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Best way for JIT to query whether llvm.fma.* is fast?

Hi Arch,

Also, consider just emitting the llvm.fmuladd intrinsic instead of llvm.fma. fmuladd is lowered to llvm.fma in codegen if the optimizer determines that it's beneficial, otherwise it is expanded to a mul and an add.

At the moment I think SelectionDAG is just checking isFMAFasterThanFMulAndFAdd anyway, but that condition could be refined in the future.

Cheers,
Lang.


On Wed, Dec 10, 2014 at 1:17 PM, Robison, Arch <arch.robison at intel.com<mailto:arch.robison at intel.com>> wrote:
Thanks!  That’s probably close enough for practical purposes.  I looked at the overrides on various targets, and they all return true if the FMA hardware exists.

- Arch

From: Jingyue Wu [mailto:jingyue at google.com<mailto:jingyue at google.com>]
Sent: Wednesday, December 10, 2014 2:56 PM
To: Robison, Arch
Cc: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] Best way for JIT to query whether llvm.fma.* is fast?

Does TargetLowering::isFMAFasterThanFMulAndFAdd (http://llvm.org/docs/doxygen/html/TargetLowering_8h_source.html#l01499) help?

Jingyue

On Wed, Dec 10, 2014 at 12:48 PM, Robison, Arch <arch.robison at intel.com<mailto:arch.robison at intel.com>> wrote:
For the Julia language JIT, we’d like be able to tell whether the llvm.fma.* intrinsic has hardware support.  What’s the best way to  query LLVM (JIT) for this information?

The information would be used in situations where the user wants to use different algorithms depending on whether FMA hardware is present or not.

- Arch D. Robison


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