Re: A proposal to align GCC stack

["H.J. Lu" <hjl at lucon dot org>]

On Tue, Dec 18, 2007 at 03:39:42AM -0500, Ross Ridge wrote:
> >> changes the ABI.  According your defintions, I would think
> >> that INCOMING should be ABI_STACK_BOUNDARY in the first case,
> >> and MAX(ABI_STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY) in the second.
> >
> > That isn't true since some .o files may not be compiled with
> > -fpreferred-stack-boundary or with a different value of
> > -fpreferred-stack-boundary.
> 
> Like with any ABI changing flag, that's not supported:
> 
> 	... Further, every function must be generated such that it keeps
> 	the stack aligned.  Thus calling a function compiled with a higher
> 	preferred stack boundary from a function compiled with a lower
> 	preferred stack boundary will most likely misalign the stack.
> 
> The -fpreferrred-stack-boundary flag currently generates code that
> assumes the stack aligned to the preferred alignment on function entry.
> If you assume a worse incoming alignment you'll be aligning the stack
> unnecessarily and generating code that this flag doesn't require.

That is how we get into trouble in the first place. The only place
I think of where you can guarantee everything is compiled with the
same -fpreferrred-stack-boundary is kernel. Our proposal will
align stack only when needed. PREFERRED_STACK_BOUNDARY >
ABI_STACK_BOUNDARY will generate a largr stack unnecessarily.

We have considered adding a new option, -fincoming-stack-boundary.
But we need to consider local/global functions as well as function
pointers. If a function can only be called locally, its incoming
stack boundary will be PREFERRED_STACK_BOUNDARY. Otherwise, its
incoming stack boundary will be MIN(INCOMING_STACK_BOUNDARY,
INCOMING_STACK_BOUNDARY,PREFERRED_STACK_BOUNDARY). We aren't sure
if its benefit will be worth its complexity.

> 
> > On x86-64, ABI_STACK_BOUNDARY is 16byte, but the Linux kernel may
> > want to use 8 byte for PREFERRED_STACK_BOUNDARY.
> 
> Ok, if people are using this flag to change the alignment to something
> smaller than used by the standard ABI, then INCOMING should be
> MAX(STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY).

On x86-64, ABI_STACK_BOUNDARY is 16byte, but the Linux kernel may
want to use 8 byte for PREFERRED_STACK_BOUNDARY. INCOMING will
be MIN(STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY) == 8 byte.


H.J.