On Thu, Jul 25, 2013 at 08:55:38AM +0200, Ondřej Bílka wrote: > On Thu, Jul 25, 2013 at 05:06:55AM +0200, Jakub Jelinek wrote: > > On Wed, Jul 24, 2013 at 07:36:31PM +0200, Richard Biener wrote: > > > >Make them callee saved means we need to change ld.so to > > > >preserve them and we need to change unwind library to > > > >support them. It is certainly doable. > > > > > > IMHO it was a mistake to not have any callee saved xmm register in the > > > original abi - we should fix this at this opportunity. Loops with > > > function calls are not that uncommon. > > > > I've raised that earlier already. One issue with that beyond having to > > teach unwinders about this (dynamic linker if you mean only for the lazy PLT > > resolving is only a matter of whether the dynamic linker itself has been > > built with a compiler that would clobber those registers anywhere) is that > > as history shows, the vector registers keep growing over time. > > So if we reserve now either 8 or all 16 zmm16 to zmm31 registers as call > > saved, do we save them as 512 bit registers, or say 1024 bit already? > > We shouldn't save them all as we would often need to unnecessarily save > register in leaf function. I am fine with 8. In practice 4 should be > enough for most use cases.
You can't add call-saved registers without breaking the ABI, because they need to be saved in the jmp_buf, which does not have space for them. Also, unless you add them at the same time the registers are added to the machine (so there's no existing code using those registers), you'll have ABI problems like this: function using the new call-saved registers calls qsort, which calls application code, which assumes the registers are call-clobbered and clobbers them; after qsort returns, the original caller's state is gone. Adding call-saved registers to an existing psABI is just fundamentally misguided. Rich
