"gcc -E" does not honor #pragma

"gcc -E" does not honor #pragma

[Chelsea Meyers]

Hi,

I'm new to contributing to GCC, so I looked for issues in Bugzilla with the
keyword "easyhack" that looked comprehensible to me, and found bug 53920: "'gcc
-E' does not honor #pragma GCC diagnostic ignored '-Wunused-macro'". I
verified using a fresh build of gcc from the master branch that when "gcc
-E -Wunused-macros" is used to invoke the preprocessor on the following two
lines, a warning is given for the unused FOO macro despite the #pragma
directive:

#pragma GCC diagnostic ignored "-Wunused-macros"
#define FOO

My impression is that the preprocessor is ignoring the content of pragma
diagnostics when invoked with -E, and that the proposed change is to add
support for handling pragma diagnostics for this use case. The Bugzilla
entry for this issue hasn't been commented on since 2013, so I want to make
sure this is actually a good thing to work on. Might there be a reason why
support for handling pragma diagnostics is missing from c-ppoutput.c?

Thanks,
Chelsea Meyers

Re: "gcc -E" does not honor #pragma

[Nathan Sidwell]

On 9/13/20 2:18 PM, Chelsea Meyers via Gcc wrote:
> Hi,
> 
> I'm new to contributing to GCC, so I looked for issues in Bugzilla with the
> keyword "easyhack" that looked comprehensible to me, and found bug 53920: "'gcc
> -E' does not honor #pragma GCC diagnostic ignored '-Wunused-macro'". I
> verified using a fresh build of gcc from the master branch that when "gcc
> -E -Wunused-macros" is used to invoke the preprocessor on the following two
> lines, a warning is given for the unused FOO macro despite the #pragma
> directive:
> 
> #pragma GCC diagnostic ignored "-Wunused-macros"
> #define FOO
> 
> My impression is that the preprocessor is ignoring the content of pragma
> diagnostics when invoked with -E, and that the proposed change is to add
> support for handling pragma diagnostics for this use case. The Bugzilla
> entry for this issue hasn't been commented on since 2013, so I want to make
> sure this is actually a good thing to work on. Might there be a reason why
> support for handling pragma diagnostics is missing from c-ppoutput.c?

I'm not aware of a reason.  IIUC in -E pragmas go through to the cpp 
output, as they're needed by the compiler proper.  However, this is a 
case of a preprocessor warning, which I guess got forgotten about.  I 
suppose diagnostic options should both be propagated into the output 
/and/ acted upon?

Don't forget that '-E -fdirectives-only' will want to defer macro 
checking until proper compilation, and in general would be difficult to 
get right, considering

#define bob 1
#if bob
....
#else
....
#endif

where 'bob' is only used during the preprocessing.

nathan

-- 
Nathan Sidwell