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Re: [RFC] New pragma exec_charset
On 10/19/2017 07:13 PM, Martin Sebor wrote:
> 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.
These probably are the areas Richard and Jakub were referring to as well?! These cases did not work
properly with the -fexec-charset cmdline option and this does not change with the pragma. I'll try
to look at what has been proposed in the discussion. Perhaps I can get it working somehow.
> 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?
The pragma does not apply to the callees of a function defined under the pragma regardless of
whether it gets inlined or not. That matches the behavior of other pragmas. If it would apply to
inlined callees the program semantics might change depending on optimization decisions i.e. whether
a certain call got inlined or not.
Callees marked as always_inline might be discussed separately. I remember this being a topic when
looking at function attributes.
Bye,
-Andreas-