On Wed, Nov 6, 2024 at 4:53 PM Jakub Jelinek <ja...@redhat.com> wrote: > > On Wed, Nov 06, 2024 at 04:27:51PM +0100, Uros Bizjak wrote: > > I see. While my solution would fit nicely with the above > > ASM_CALL_CONSTRAINT approach, the approach using ASM_CALL_CONSTRAINT > > is wrong by itself. > > > > Oh, well. > > > > Anyway, I guess "redzone" clobber you proposed does not remove the > > need to use ASM_CALL_CONSTRAINT if we want to confine the "asm with > > call inside" between frame setup/teardown insns? Is it possible to > > invent similar clobber (unimaginatively called "stack", perhaps) that > > would prevent scheduler from moving asm to the wrong place? > > I don't understand why would one want to avoid moving such inline asms > there, the profiler can be called from there too and if a function > doesn't use a red zone and the inline asm restores all registers except > those explicitly clobbered in the clobbers, I think such calls can be done > pretty much everywhere. Sure, frame pointers might miss one frame, but > frame pointer reliability for backtraces is abysmal anyway. > But I'm not aware of a testcase where we would actually move inline asm > there either (at least not on x86). Is that some known problem (and then > why?) or just theoretical issue?
Please see [1]: /* * This output constraint should be used for any inline asm which has a "call" * instruction. Otherwise the asm may be inserted before the frame pointer * gets set up by the containing function. If you forget to do this, objtool * may print a "call without frame pointer save/setup" warning. */ register unsigned long current_stack_pointer asm(_ASM_SP); #define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer) Perhaps HPA can say some more? [1] https://elixir.bootlin.com/linux/v6.11.6/source/arch/x86/include/asm/asm.h#L223 > Or do you want a new clobber which will automatically add all call clobbered > registers to the clobbers? That might be useful, although in case where > one compiler can support multiple call ABIs one needs to pick one; would > that stand for the ABI of the current function, or something else? No, functions called from asm are not ABI compliant functions. Uros.
