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

[Richard Earnshaw]

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.

> 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
>