Question about -mpreferred-stack-boundary

[Tim Prince]

----- Original Message -----
From: "Andreas Jaeger" <aj@suse.de>
To: <gcc@gcc.gnu.org>
Cc: "Jens Wallner" <wallner@ims.uni-hannover.de>
Sent: Saturday, February 03, 2001 2:37 AM
Subject: Question about -mpreferred-stack-boundary


>
> We (glibc team) got a bug report that the stack is not aligned
> properly - and I'm a bit confused by the documentation of
> -mpreferred-stack-boundary which is:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> @item -mpreferred-stack-boundary=@var{num}
> Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
> byte boundary.  If @samp{-mpreferred-stack-boundary} is not specified,
> the default is 4 (16 bytes or 128 bits).
>
> The stack is required to be aligned on a 4 byte boundary.  On Pentium
> and PentiumPro, @code{double} and @code{long double} values should be
> aligned to an 8 byte boundary (see @samp{-malign-double}) or suffer
> significant run time performance penalties.  On Pentium III, the
> Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
> penalties if it is not 16 byte aligned.
>
> To ensure proper alignment of this values on the stack, the stack boundary
> must be as aligned as that required by any value stored on the stack.
> 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.  It is recommended that
> libraries that use callbacks always use the default setting.
>
> This extra alignment does consume extra stack space.  Code that is sensitive
> to stack space usage, such as embedded systems and operating system kernels,
> may want to reduce the preferred alignment to
> @samp{-mpreferred-stack-boundary=2}.
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Who has to align the stack for calls to a function - the caller or the
> callee?  In other words:  Does this mean that the stack has to be
> aligned before calling a function?  Or does it have to be aligned when
> entering a function?
>
> Andreas
> --
>  Andreas Jaeger
>   SuSE Labs aj@suse.de
>    private aj@arthur.inka.de
>     http://www.suse.de/~aj
I believe the preferred alignment for long double is a 16 byte boundary, and the stack (and instruction) alignments must be so set
before entering a function.  Pentium 4 increases preferred data alignments to 32 bytes in some situations, as well as increasing the
number of situations (SSE2 instructions) where 16 byte alignment is needed.