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:
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.
It would be a separate issue to fixing the VDSO anyway.
Makes totally sense. Thanks for the explanation.