On Thu, 2 Apr 2020 at 16:34, Stefan Agner <stefan@xxxxxxxx> wrote: > > On 2020-04-02 14:05, Ard Biesheuvel wrote: > > On Thu, 2 Apr 2020 at 13:50, Peter Smith <Peter.Smith@xxxxxxx> wrote: > >> > >> > I take it this implies that the LLVM linker does not support the > >> > R_ARM_ALU_PC_Gn relocations? Since otherwise, adrl could simply be > >> > expanded to a pair of adds with the appropriate relocations, letting > >> > the linker fix up the immediates (and the ADD vs SUB bits) > >> > >> Not at the moment. I have a patch in review to add the G0 variants for these in Arm state at reviews.llvm.org/D75349 . As far as I know LLVM MC does not have support for generating the relocations either. This could be added though. I agree that using the G* relocations with a pair of add/sub instructions would be the ideal solution. The adrl psuedo is essentially that but implemented at assembly time. I think it would be possible to implement in LLVM but at the time (4+ years ago) I wasn't confident in finding someone that would think that adrl support was worth the disruption, for example the current Arm assembly backend can only produce 1 instruction as output and adrl requires two. > >> > >> I'd be happy to look at group relocation support in LLD, I haven't got a lot of spare time so progress is likely to be slow though. > >> > > > > For Linux, I have proposed another approach in the past, which is to > > define a (Linux-local) adr_l macro with unlimited range [0], which > > basically comes down to place relative movw/movt pairs for v7+, and > > something along the lines of > > > > ldr <reg>, 222f > > 111: add <reg>, <reg>, pc > > .subsection 1 > > 222: .long <sym> - (111b + 8) > > .previous > > Just to confirm: The instance at hand today seems to be working fine > without adrl, so I guess we are fine here, do you agree? > I agree. Apologies for hijacking the thread :-) > There are a couple more instances of adrl in arch/arm/crypto/, maybe > that is where the adr_l macro could come in. > There are various places in the arch code that could be cleaned up along these lines. But you're right - this is a separate discussion that deserves a thread of its own. I was just satisfying my own curiosity.
