Re: [Patch] libcpp: Fix _Pragma expansion [PR102409]

[Tobias Burnus <tobias@codesourcery.com>]

Hi Martin,

On 28.10.21 18:28, Martin Sebor wrote:
> There are a number of bug reports of _Pragma not working right
> in macros, including (and especially) to control diagnostics:
> https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=_Pragma%20macro&list_id=328003
>
>
> Just by the description this change seems like it could also
> fix some of them.

I think it does not help with them – or only partially.

I believe there currently still two issues:

* _Pragma("GCC foo") – when "foo" or "GCC foo" are
   not registered is immediately processed, leading
   to wrong placement in the output with "-E".

A probably not fully correct draft patch is attached to
https://gcc.gnu.org/PR90400 which fixes the issue
(misses a location before the #pragma).


* With _Pragma("GCC diagnostic") in macros the problem is:
The macro is replaced by all the macro code including the
#pragma and all other code in there.

By construction, all those have the same line. But if
   else { b--;
#pragma GCC diagnostic push
;
#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
; a--;
#pragma GCC diagnostic pop
; }

the location is the input_location of the expanded macro,
i.e. all code is in the same line. As the 'pop' check checks
whether the loc is before the pragma, it might pop too early
and the 'ignored' is already ignored for the 'a--' in this
example.  Cf. https://gcc.gnu.org/PR91669

@Martin: As you seem to have spare cycles, how about spending
some time fixing either issue?

Tobias

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