On 19 November 2014 17:32, Konstantin Khlebnikov <koc...@gmail.com> wrote: > On Wed, Nov 19, 2014 at 7:25 PM, Ard Biesheuvel > <ard.biesheu...@linaro.org> wrote: >> On 19 November 2014 17:07, Russell King - ARM Linux >> <li...@arm.linux.org.uk> wrote: >>> On Wed, Nov 19, 2014 at 05:02:40PM +0100, Ard Biesheuvel wrote: >>>> On 19 November 2014 16:52, Konstantin Khlebnikov <koc...@gmail.com> wrote: >>>> > Do you mean ldr pc, =symbol ? >>>> > >>>> > In this case I get this error: >>>> > >>>> > /tmp/ccAHtONU.s: Assembler messages: >>>> > /tmp/ccAHtONU.s:220: Error: invalid literal constant: pool needs to be >>>> > closer >>>> > >>>> > Probably constant pool doesn't work well in inline assembly. >>>> > >>>> > >>>> > Something like this seems work: >>>> > >>>> > add lr, pc, #4 >>>> > ldr pc, [pc, #-4] >>>> > .long symbol >>>> > >>>> >>>> You can add a '.ltorg' instruction which tells the assembler to dump >>>> the literal pool, but you still need to jump over it, i.e., >>>> >>>> adr lr, 0f >>>> ldr pc, =symbol >>>> .ltorg >>>> 0: >>> >>> Which is not a good idea either, because the compiler needs to know how >>> far away its own manually generated literal pool is from the instructions >>> which reference it. The .ltorg statement can end up emitting any number >>> of literals at that point, which makes it indeterminant how many words >>> are contained within the asm() statement. >>> >> >> That applies to any inline asm statement in general: the compiler >> assumes that the expanded size will not interfere with its ability to >> emit literals after the function's return instruction. >> Sometimes it will put a literal pool in the middle of the function if >> it is very large, and I am not sure if an inline asm by itself would >> ever trigger that heuristic to kick in. >> >> But by the same logic, i.e., due to the fact that GCC manages its own >> literals, the literal pool at the assembly level is unlikely to be so >> large that you will actually hit this condition. >> >>> Yes, it isn't desirable to waste an entire data cache line per indirect >>> call like the original quote above, but I don't see a practical >>> alternative. >>> >> >> We could at least add some labels instead of doing explicit pc arithmetic, >> i.e., >> >> adr lr, 1f >> ldr pc, 0f >> 0: .long symbol >> 1: > > I think we need some unique prefix here, this macro is used inside > bigger inline assembly constructions and probably another macro.
Numbers are disambiguated by the f and b suffixes, so they can be reused in the same .s file. So as long as you use a strictly numerical prefix, you can deal correctly with the case where, for instance, do_div() is called twice in the same compilation unit, and still not clash with other inline asm -- Ard. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
