Hi Paul, On 1/3/20 12:42 AM, Paul Burton wrote: > Using -ffixed-gp wouldn't be correct for the VDSO - the VDSO itself is > position independent code, and will need to use $gp to access the GOT > which is part of how position-independence is achieved (technically you > could access the GOT using another register of course but you'd need > some way to persuade the compiler to break with convention & you'd gain > nothing meaningful since you'd need to use some other register anyway). > If we use -ffixed-gp then we're telling GCC not to use $gp, and that > doesn't make sense. If we consider -ffixed-gp as telling GCC not to use > $gp as a general purpose register then it's meaningless because $gp > already has a specific use & isn't used as a general purpose register. > If we consider -ffixed-gp as telling GCC not to use $gp at all then it > doesn't make sense because it needs to in order to access the GOT. > > In terms of GCC's flags we'd want to use -fcall-saved-gp, but that would > just be telling GCC information it already knows about the n32 & n64 > ABIs & indeed it seems to have no effect at all on the way GCC handles > the global register variable - it doesn't cause gcc to save & restore > $gp with the global register variable present, so you gain nothing. > > We could use -ffixed-gp for the kernel proper (& not the VDSO), but: > > 1) The kernel builds as non-PIC code with no $gp-based optimizations > enabled, and since this has been fine forever it seems safe to expect > the compiler not to start using $gp in new ways. > > 2) It would be a separate issue to fixing the VDSO anyway. Makes totally sense. Thanks for the explanation. -- Regards, Vincenzo
