Re: [PATCH] Allow FP to be used as a call-saved registe

[Jeff Law <law@redhat.com>]

On 09/19/2016 03:45 AM, Richard Earnshaw (lists) wrote:
>> So those don't seem to me to imply that the frame pointer needs to be a
>> fixed register.   So the first thing I'd do is fix the aarch64 port to
>> not do that and see what fallout there is and how to fix it.
>>
>> Most ports simply don't mark the frame pointer as fixed.
>>
>
> The AArch64 ABI specification strongly recommends that, when a frame
> record is not created, the frame pointer register is left unused so that
> the frame chain, while not complete, is still valid (a chain of valid
> records but ending in a NULL pointer).  That strongly suggests that FP
> should remain a fixed register.
So essentially you're asking that even with -fomit-frame-pointer that FP 
still not be used and that the user should specify an additional 
-fcall-saved- parameter?

I can see your point -- lots of things use -fomit-frame-pointer and you 
may not necessarily want to make FP available to the register allocator.

But by requiring -fcall-saved-whatever, aren't you forcing folks to 
encode aarch64 specific flags into their build systems?

Neither seems like a good position to be in.
>
> I guess we could push all of this into a new back-end option to permit
> the 'I really want to use FP for general purposes', but it seems to be
> just duplicating the existing use of -fcall-saved; so would be yet
> another flag in the compiler that needs documenting.
Yea, and more aarch64 specific bits in the build system.

>
> It seems much more sensible to me to just make a slight relaxation of
> the fixed-register code and then re-use the existing options.
Maybe.   I understand the situation much better now, but I don't see a 
particularly good solution.
jeff