On 10/07/15 16:16, Richard Earnshaw wrote: > On 10/07/15 16:00, pins...@gmail.com wrote: >> >> >> >> >>> On Jul 10, 2015, at 7:13 AM, Richard Earnshaw >>> <richard.earns...@foss.arm.com> wrote: >>> >>>> On 10/07/15 13:18, Segher Boessenkool wrote: >>>>> On Fri, Jul 10, 2015 at 10:02:06AM +0100, Richard Earnshaw wrote: >>>>> This isn't going to reliably work for ARM or AArch64. If the only call >>>>> within a leaf function is via the ASM the compiler doesn't guarantee to >>>>> ensure the stack is aligned to the ABI requirements. >>>> >>>> Those archs have a link register, which is clobbered by the call, so >>>> asm doing a call should have the link register in its clobber list, >>>> which is enough to prevent shrink-wrapping the asm. Does that also >>>> help aligning the stack (as a side effect?) >>> >>> No. Currently there's no safe way to guarantee that the stack will be >>> correctly aligned for a call from within an ASM block since the compiler >>> has no way of detecting that this is necessary. >> >> Note aarch64 is not really affected as the architecture also prevents the >> unaligned stack from happening in that you will get an interrupt if it is >> unaligned. >> > > Not quite. You'll get an interrupt if SP is used for a *memory > dereference* while it is unaligned. >
Oh, and I believe this is a configurable option on at least some cores... R. > R. > >> Thanks, >> Andrew >> >> >>> >>>> The problem you mention is not target-specific, and would also happen >>>> without shrink-wrapping. Maybe forcing a frame pointer (which they >>>> do in the reported case) helps? But there certainly could exist ABIs >>>> where it does not. This is arch-dependent code by nature, but still. >>>> >>>> Nastiness :-) >>>> >>>> >>>> Segher >>> >
