Re: [PATCH]middle-end Use subregs to expand COMPLEX_EXPR to set the lowpart.

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/13/2022 5:54 AM, Richard Biener wrote:
> On Sun, Jun 12, 2022 at 7:27 PM Jeff Law via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> [...]
>> On a related topic, any thoughts on keeping complex objects as complex
>> types/modes through gimple and into at least parts of the RTL pipeline?
>>
>> The way complex arithmetic instructions work on our chip is going to be
>> extremely tough to utilize in GCC -- we really need to the complex
>> types/arithmetic up through RTL generation at the least. Ideally we'd
>> even expose complex modes all the way to final.    Is that something
>> y'all could benefit from as well?  Have y'all poked at this problem at all?
> Since you are going to need to "recover" complex operations from people
> open-coding them (both fortran and C and also C++ with std::complex) it
> should be less work to just do that ;)  I think that complex modes and types
> exist solely for ABI purposes.
I don't see any reasonable way to do that.  Without going into all the 
details, our complex ops work on low elements within a vector 
register.   Trying to recover them after gimple->rtl expansion would be 
similar to trying to SLP vectorize on RTL, something I'm not keen to chase.

It would be a hell of a lot easier to leave the complex ops as complex 
ops to the expanders, then make the decision to use the complex 
instructions or decompose into components.

jeff