On Thu, Aug 17, 2017 at 11:25:27AM +0930, Alan Modra wrote: > On Wed, Aug 16, 2017 at 06:23:13PM -0500, Segher Boessenkool wrote: > > Maybe whatever is creating those instructions should set RTX_FRAME_RELATED_P > > by itself? Not sure if that is nicer. > > > > Both this CR2 and R0 handling are pretty nasty hacks. Could you add a > > comment saying that? > > I wouldn't say the R0 handling is a nasty hack at all. You can't save > LR directly, storing to the stack must go via a gpr. I'm 100% sure > you know that, and so would anyone else working on powerpc gcc > support. It so happens that we use r0 in every case we hit this > code. *That* fact is commented. I don't really know what else you > want.
But R0 isn't necessarily used for LR here. That is true currently in all cases I can see, but it can be used for other things. The ABI doesn't force things here. The separate shrink-wrapping code does use R0 for other things, but it manually sets the CFI notes anyway, so no problem there. Maybe I should just stop worrying. > (Incidentally, the dwarf2cfi.c behaviour is described by > > In addition, if a register has previously been saved to a different > register, > > Yup, great comment that one! Dates back to 2004, commit 60ea93bb72.) Perhaps inspired by the REG_CFA_REGISTER name for the RTL note ;-) > The CR2 thing is a long-standing convention for frame info about CR, a > wart fixed by ELFv2. See elsewhere That ELFv2 fix is why I still haven't submitted the separate shrink-wrapping for CR fields code -- it complicates things a lot :-/ > I'l go with: > > /* If we see CR2 then we are here on a Darwin world save. Saves of > CR2 signify the whole CR is being saved. This is a long-standing > ABI wart fixed by ELFv2. As for r0/lr there is no need to check > that CR needs to be saved. */ That is fine, thanks again. Segher
