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. > If just 512 bit, then when next time the vector registers grow in size (will > they?), would we have just low parts of the 1024 bits registers call saved > and upper half call clobbered (I guess that is the case for M$Win 64-bit ABI > now, just with 128 bit vs. more). > I do not think that 1024 bit registers will come in next ten years. If they came tohn call clobbered is better. Full 1024 bits would be used rarely; given that in most cases we will use them just to store 64bit for doubles. > But yeah, it would be nice to have some call saved ones. > > Jakub
