Hi Segher > On 29 Sep 2022, at 18:18, Segher Boessenkool <seg...@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 <seg...@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
