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Re: Can shrink-wrapping ever move prologue past an ASM statement?





> On Jul 10, 2015, at 7:13 AM, Richard Earnshaw <Richard.Earnshaw@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. 

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
> 


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