Re: [RFC] New pragma exec_charset

[Martin Sebor <msebor at gmail dot com>]

On 10/19/2017 09:50 AM, Andreas Krebbel wrote:
> The TPF operating system uses the GCC S/390 backend.  They set an
> EBCDIC exec charset for compilation using -fexec-charset.  However,
> certain libraries require ASCII strings instead.  In order to be able
> to put calls to that library into the normal code it is required to
> switch the exec charset within a compilation unit.
>
> This is an attempt to implement it by adding a new pragma which could
> be used like in the following example:
>
> int
> foo ()
> {
>   call_with_utf8("hello world");
>
> #pragma GCC exec_charset("UTF16")
>   call_with_utf16("hello world");
>
> #pragma GCC exec_charset(pop)
>   call_with_utf8("hello world");
> }
>
> Does this look reasonable?

I'm not an expert on this but at a high level it looks reasonable
to me.  But based on some small amount of work I did in this area
I have a couple of questions.

There are a few places in the compiler that already do or that
should but don't yet handle different execution character sets.
The former include built-ins like __bultin_isdigit() and
__builtin_sprintf (in both builtins.c and gimple-ssa-sprintf.c)
The latter is the -Wformat checking done by the C and C++ front
ends.  The missing support for the latter is the subject of bug
38308.  According to bug 81686, LTO is apparently also missing
support for exec-charset.

I'm curious how the pragma might interact with these two areas,
and whether the lack of support for it in the latter is a concern
(and if not, why not).  For the former, I'm also wondering about
the interaction of inlining and other interprocedural optimizations
with the pragma.  Does it propagate through inlined calls as one
would expect?

Thanks
Martin