[Cfp-interest 2397] Re: WG14 IEEE 754-C binding meeting minutes 2022/03/16

[Steve (Swift) Canon]

> On Mar 17, 2022, at 9:57 AM, Vincent Lefevre <vincent at vinc17.net> wrote:
> 
> On 2022-03-16 16:55:49 +0000, Rajan Bhakta wrote:
>>  Carry-over action items:
>>    Fred: Ask C++ what their issues with *HAS_SUBFORM are and if they are OK with obsoleting it. - Still open
> 
> I suppose that is should be *HAS_SUBNORM (in case this can be
> corrected for the future version on open-std.org).
> 
>>    fma subclause title [Cfp-interest 2388]
>>      David H: We called it fused since there were implementations that had fused and unfused multiply-add's.
>>      Jim: In C it is fused in the sense of only one rounding. The title says "Floating" though and Pavel wants it changed to "Fused".
>>      Fred: The ARM hardware does double rounding.
>>        Rajan: So they are already non-conforming?
>>        Fred: Yes.
> 
> This does not seem to be true. In the Assembler User Guide:
> 
>  https://www.keil.com/support/man/docs/armasm/armasm_pge1415706532461.htm
> 
>  8.3 Fused Multiply-Add extension for VFP
> 
>  The Fused Multiply-Add extension optionally extends the VFPv3
>  architecture.
> 
>  It provides VFP instructions that perform multiply and accumulate
>  operations with a single rounding step, so suffers from less loss
>  of accuracy than performing a multiplication followed by an add.
> 
>  The fused multiply-add instructions are only available on VFP
>  systems that implement the fused multiply-add extension. The VFP
>  system that implements the fused multiply-add extension is VFPv4.

Correct. Also note that:

- VFPv4 was released in 2011
- 64b ARM ISAs _only_ have the fused operation, so the unfused operation will disappear when the 32b ISA is end-of-lifed (2023, IIRC?)


– Steve