Re: GCN: Add -mlow-precision-sqrt for double-precision sqrt [PR105246] (was: Re: [PATCH] amdgcn: Add support for additional natively supported floating-point operations)

[Andrew Stubbs <ams@codesourcery.com>]

On 09/09/2022 13:20, Tobias Burnus wrote:
> However, the pre-existing 'sqrt' problem still is real. It also applies 
> to reverse sqrt ("v_rsq"), but that's for whatever reason not used for GCN.
> 
> This patch now adds a commandline flag - off by default - to choose 
> whether this behavior is wanted. I did use the same name as aarch64, 
> https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html#index-mlow-precision-sqrt (the latter also has -mlow-precision-recip-sqrt, which is not (yet) sensible for GCN.)
> 
> This patch was manually tested for all combinations and I also looked at 
> insn-recog.cc, given that it is my first .md patch – it it seems to work 
> fine.
> 
> OK for mainline – or are there comments or more suggestions? I also 
> included some word for the release notes.

No, thank you.

I don't see any value in adding an option no one cares about (but we 
still have to maintain and test).

I think it will make sense to drop the double-precision insn definition 
and fall back to libm in that case.

Kwok is currently reviewing all the libm functions and can probably 
include this one.

Andrew