[Sbcl-devel] Alignment issues with DEREF on Sparc

[<pv...@pv...>]

SBCL/Sparc throws a bus Error when it tried to dereference unaligned
pointers.

From http://docs.sun.com/app/docs/doc/802-5900/6i9kj7ood?a=view
-----8<-----
    All quantities must be aligned on their natural boundaries. Using
standard C data types:

        * short integers are aligned on 16-bit boundaries.
        * int integers are aligned on 32-bit boundaries.

        * long integers are aligned on either 32-bit boundaries or 64-bit
boundaries, depending on whether the data model of the kernel is
64-bit or 32-bit. For information on data models, see Appendix F,
"Making a Device Driver 64-Bit Ready".
        * long long integers are aligned on 64-bit boundaries.
-----8<-----

The problem is that SBCL allows casts to alien-types with stricter
alignment requirements without any warning, and that DEREF does not check
for alignment, even with normal OPTIMIZE settings.

There are two issues here:

1. The programmer shouldn't upcast.
2. SBCL allows this without emitting any warning, trying to emit a more
meaningful error than "Bus error" or defaulting to a slower byte-by-byte
access on unaligned addresses.

The first point is hard to fix, especially after the fact. It wouldn't be
strictly legal in C (it'd probably elicit a warning from the compiler),
but it's perfectly legal on CISCy architectures. Also, it makes some
things incredibly painful (extracting the data byte by byte from CL).
That's why I don't think simply emitting an error on unaligned accesses
(where applicable) is an acceptable solution.

Does anyone else see a better solution than providing finer-grained access
routines, along with a style warning on upcasts?

Thank you,

 Paul Khuong