[Larceny-users] SCE_BUFFER?

[Felix S Klock II (Larceny-users list proxy)]

Lars (cc'ing larceny-users)-

If I recall correctly, Jesse Tov and I introduced the SCE_BUFFER to
support using the $esp register on Intel as the stack-cache pointer
(rather than the globals pointer, which I think was its role back when
you first developed the Petit/Nasm backend).

The reason to use $esp as the stack-cache pointer is that it enables a
significantly faster call/return instruction sequence on Intel; see:

  https://trac.ccs.neu.edu/trac/larceny/changeset/4000

The reason we need a red-zone between the stack-cache and the nursery
is that you need some space above $esp in order to properly handle
interrupts or asynchronous signals or something along those lines.
(The notes on changeset:4000 don't actually 100% coincide with this
assertion, so I'm sure I'm getting the details wrong in this email.)

The reason its only present on some realizations is that you only need
this red zone on Native Intel backends (Petit/Nasm and Sassy/IA32 aka
IAssassin), because that's the only place where $esp is being used in
this way.

Cheers,
-Felix

On Wed, Jul 11, 2012 at 10:10 AM, Lars T Hansen <lth at acm.org> wrote:
> Can somebody tell me why SCE_BUFFER (aka $sce.buffer), which appears
> to be some kind of red zone for the stack, was introduced into the
> runtime and why it was only done in some realizations?  Neither Sparc
> nor Standard-C make use of it.  I'm looking at code in a recent SVN
> checkout but I've not attempted to find out when it was introduced.
>
> --lars
>
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