Re: [PATCH] rs6000: Rework option -mpowerpc64 handling [PR106680]

[Iain Sandoe <iain@sandoe.co.uk>]

Hi Segher

> On 29 Sep 2022, at 18:18, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> On Thu, Sep 29, 2022 at 12:04:05AM +0100, Iain Sandoe wrote:
>>> On 28 Sep 2022, at 22:30, Segher Boessenkool <segher@kernel.crashing.org> wrote:
>>> That works on Linux as well.  What still does not work is user-mode
>>> context switches in 32-bit processes (so setjmp and getcontext stuff).
>> 
>> AFAIU the Darwin impl. it is the same - the user context only contains 32b
>> register images.
> 
> Huh, I thought Darwin did this properly.
> 
>> Since one can only use the feature between function calls,
> 
> You still have to preserve the non-volatile GPRs.  All 64 bits of it.

The OS does do that - e.g. on an interrupt .. but AFAIR, the user-visible mcontext
in a 32b process only shows the lower 32 bits.

( i’d better stop making too many assertions here from memory, ;) )

>> I guess that the
>> setjmp/longjmp stuff is not so critical on Darwin***. However, even being able
>> to use 64b insns between calls could give a massive win in allowing, for
>> example, lock-free 64b atomics.
> 
> But that is not how GCC with -mpowerpc64 works: the calling convention
> is the usual 32-bit one, but the functions are 64-bit otherwise; it uses
> all 64 bits of GPRs everywhere except in function calls.

I think we said the same thing with different words.

The CC is unchanged (so that we can only use 64b insns between calls, since
the upper 32b of callee-saved regs are not preserved).

cheers
Iain