[Bug preprocessor/69126] [6 regression] _Pragma does not apply if part of a macro

["dmalcolm at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69126

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #25 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Stephan provided me with a tarball of his reproducer (thanks!), and I'm able to
reproduce this locally.

The root cause is a bug in linemap_compare_locations.

(gdb) call inform (pre, "pre")
test.cc:8:16: note: pre
 #define IGNORE _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
                ^
test.cc:8:16: note: in definition of macro âIGNOREâ
 #define IGNORE _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
                ^~~~~~~
(gdb) call inform (post, "post")
test.cc:12:5: note: post
     f();
     ^
After macro expansion, we have (at the end of linemap_compare_locations):
(gdb) p /x l0
$13 = 0x800101ec
(gdb) p /x l1
$14 = 0x50684c05

and hence:

(gdb) p /x l1 - l0
$23 = 0xd0674a19

it's effectively negative, and so before_p is false.

But this is wrong: l0 is an ad-hoc loc, whereas l1 is a non-ad-hoc loc.

It's clearly insane to treat all ad-hoc locations as being later than all
non-ad-hoc locations.  The fix is simple: resolve ads-hoc locations at the end
of linemap_compare_locations.

For this bug to occur, we need a location for the macro name that's an ad-hoc
location, and a location for the diagnostic that's *not* an ad-hoc location.

The reason it triggered for Stephan is that the sheer quantity of code in his
reproducer meant that both locations in question were above
LINE_MAP_MAX_LOCATION_WITH_PACKED_RANGES (but below
LINE_MAP_MAX_LOCATION_WITH_COLS), so range-packing was disabled, in particular
for the "IGNORE" macro.

I can reproduce it trivially by using a macro name that's >=32 characters (thus
forcing the use of an ad-hoc location):

/* { dg-options "-Wdeprecated-declarations" } */

#define IGNORE_WHERE_MACRO_IS_LONGER_THAN_31_CHARS  _Pragma("GCC diagnostic
ignored \"-Wdeprecated-declarations\"")
__attribute__((deprecated)) void f();
int main() {
    IGNORE_WHERE_MACRO_IS_LONGER_THAN_31_CHARS
    f();
}

I have a patch that seems to work for this test case; am testing it more
thoroughly now.