The 05/13/2022 16:35, Richard Sandiford wrote: > Szabolcs Nagy via Gcc-patches <gcc-patches@gcc.gnu.org> writes: > > The RA_SIGN_STATE dwarf pseudo-register is normally only set using the > > DW_CFA_AARCH64_negate_ra_state (== DW_CFA_window_save) operation which > > toggles the return address signedness state (the default state is 0). > > (It may be set by remember/restore_state CFI too, those save/restore > > the state of all registers.) > > > > However RA_SIGN_STATE can be set directly via DW_CFA_val_expression too. > > GCC does not generate such CFI but some other compilers reportedly do. > > > > Note: the toggle operation must not be mixed with other dwarf register > > rule CFI within the same CIE and FDE. > > > > In libgcc we assume REG_UNSAVED means the RA_STATE is set using toggle > > operations, otherwise we assume its value is set by other CFI. > > AFAIK, this is the first time I've looked at the RA_SIGN_STATE code, > so this is probably a naive point/question, but: it seems a bit > underhand for the existing code to be using REG_UNSAVED and > loc.offset to hold the toggle state. Would it make sense to add > a new enum value for known, pre-evaluated constants? _Unwind_GetGR > would then DTRT for both cases. > > That's a comment about the pre-existing code though. I agree this > patch looks like the right fix if we keep to the current approach.
yes, this is a hack. i looked at introducing a generic REG_* enum to deal with RA_SIGN_STATE now, but it's a bit awkward: normally frame state for a reg starts out REG_UNSAVED, which should mean 0 value for the RA_SIGN_STATE pseudo register. when moving up frames the uw context gets copied and updated according to the frame state (where REG_UNSAVED normally means unmodified copy), this is not right for RA_SIGN_STATE which should be reset in the absence of related dwarf ops. we can fix this up in target hooks for update context, but we still have to special case REG_UNSAVED. i think introducing a new REG_CONST does not simplify the aarch64 target code (we might need further changes to get a clean solution).
