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Re: Proposal for merging scalar-storage-order branch into mainline


On Mon, Jun 8, 2015 at 10:05 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
> Hi,
>
> I'd like to propose merging the scalar-storage-order branch that I have been
> maintaining for a couple of years into mainline.  Original announcement at:
>   https://gcc.gnu.org/ml/gcc/2013-05/msg00249.html
>
> It implements an attribute (C/C++/Ada only) that makes it possible to specify
> the storage order (aka endianness) of scalar components of aggregate types;
> for example, you can declare a structure with big-endian SSO containing only
> scalar fields and it will have the same representation in memory on x86 and on
> PowerPC or SPARC.  Nesting of structures with different SSO is also supported.
>
> The feature has been present in the GCC-based compilers released by AdaCore
> for a few more years and the users generally find it very useful (some of them
> even asked why we hadn't implemented it earlier).
>
> As the initial plan was to maintain it in AdaCore's tree until it reached a
> sufficient level of maturity, the implementation was designed to be relatively
> light and maintainable, with the following basic principle: specifying the
> same SSO as that of the target machine is equivalent to specifying no SSO.
> This principle holds for the entire implementation, which means that only the
> reverse SSO is tracked, which in turn means that the target machine must be
> uniform wrt endianness (e.g. PDP endianness is not supported).
>
> Only GENERIC is extended (one flag on aggregate types and one flag on some
> _REF nodes) by using the following guidelines:
>
>    The overall strategy is to preserve the invariant that every scalar in
>    memory is associated with a single storage order, i.e. all accesses to
>    this scalar are done with the same storage order.  This invariant makes
>    it possible to factor out the storage order in most transformations, as
>    only the address and/or the value (in target order) matter for them.
>    But, of course, the storage order must be preserved when the accesses
>    themselves are rewritten or transformed.
>
> GIMPLE proper and RTL are not changed.  The byte swapping operations are made
> explicit during RTL expansion and use the bswap patterns of the target machine
> if present.

What's the reason to not expose the byte swapping operations earlier, like on
GIMPLE?  (or even on GENERIC?)

> The bulk of the implementation is in the FEs (sanity checks, propagation, etc)

What frontends are affected?

Thanks,
Richard.

> and the RTL expander (+ varasm.c for aggregate literals).  RTL optimizers are
> not changed.  GIMPLE optimizers are minimally changed: they can either punt if
> they see _REF nodes with reverse SSO or choose to locally maintain the SSO.
>
> Again this was designed with maintainability and simplicity in mind, so no
> attempt was made at generating optimal code.  At the same time the support is
> transparent for most GIMPLE optimizers so there is no definitive blocker
> towards this goal if it is deemed worth pursuing for this kind of feature.
>
> --
> Eric Botcazou


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