Re: '#pragma GCC diagnostic' (mis-)use in 'statement' of 'if'

[Martin Sebor <msebor@gmail.com>]

On 7/20/21 2:40 AM, Thomas Schwinge wrote:
> Hi!
> 
> On 2021-07-20T09:23:24+0200, I wrote:
>> On 2021-07-19T10:46:35+0200, I wrote:
>>> | On 7/16/21 11:42 AM, Thomas Schwinge wrote:
>>> |> On 2021-07-09T17:11:25-0600, Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>> |>> The attached tweak avoids the new -Warray-bounds instances when
>>> |>> building libatomic for arm. Christophe confirms it resolves
>>> |>> the problem (thank you!)
>>> |>
>>> |> As Abid has just reported in
>>> |> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101374#c16>, similar
>>> |> problem with GCN target libgomp build:
>>> |>
>>> |>      In function ‘gcn_thrs’,
>>> |>          inlined from ‘gomp_thread’ at [...]/source-gcc/libgomp/libgomp.h:803:10,
>>> |>          inlined from ‘GOMP_barrier’ at [...]/source-gcc/libgomp/barrier.c:34:29:
>>> |>      [...]/source-gcc/libgomp/libgomp.h:792:10: error: array subscript 0 is outside array bounds of ‘__lds struct gomp_thread * __lds[0]’ [-Werror=array-bounds]
>>> |>        792 |   return *thrs;
>>> |>            |          ^~~~~
>>> |>
>>> |>      gcc/config/gcn/gcn.h:  c_register_addr_space ("__lds", ADDR_SPACE_LDS);                   \
>>> |>
>>> |>      libgomp/libgomp.h-static inline struct gomp_thread *gcn_thrs (void)
>>> |>      libgomp/libgomp.h-{
>>> |>      libgomp/libgomp.h-  /* The value is at the bottom of LDS.  */
>>> |>      libgomp/libgomp.h:  struct gomp_thread * __lds *thrs = (struct gomp_thread * __lds *)4;
>>> |>      libgomp/libgomp.h-  return *thrs;
>>> |>      libgomp/libgomp.h-}
>>> |>
>>> |> ..., plus a few more.  Work-around:
>>> |>
>>> |>         struct gomp_thread * __lds *thrs = (struct gomp_thread * __lds *)4;
>>> |>      +# pragma GCC diagnostic push
>>> |>      +# pragma GCC diagnostic ignored "-Warray-bounds"
>>> |>         return *thrs;
>>> |>      +# pragma GCC diagnostic pop
>>> |>
>>> |> ..., but it's a bit tedious to add that in all that the other places,
>>> |> too.
>>>
>>> Wasn't so bad after all; a lot of duplicates due to 'libgomp.h'.  I've
>>> thus pushed "[gcn] Work-around libgomp 'error: array subscript 0 is
>>> outside array bounds of ‘__lds struct gomp_thread * __lds[0]’
>>> [-Werror=array-bounds]' [PR101484]" to master branch in commit
>>> 9f2bc5077debef2b046b6c10d38591ac324ad8b5, see attached.
>>
>> As I should find, these '#pragma GCC diagnostic [...]' directives cause
>> some code generation changes (that seems unexpected, problematic!).
>> (Martin, any idea?  Might be a pre-existing problem, of course.)
> 
> OK, phew.  Martin: your diagnostic changes are *not* to be blamed for
> code generation changes -- it's my '#pragma GCC diagnostic pop'
> placement that triggers:
> 
>> This
>> results in a lot (ten thousands) of 'GCN team arena exhausted' run-time
>> diagnostics, also leading to a few FAILs:
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-11.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-11.c execution test
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-12.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-12.c execution test
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-3.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-3.c execution test
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-5.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-5.c execution test
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-6.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-6.c execution test
>>
>>      PASS: libgomp.c/../libgomp.c-c++-common/for-9.c (test for excess errors)
>>      [-PASS:-]{+FAIL:+} libgomp.c/../libgomp.c-c++-common/for-9.c execution test
>>
>> Same for 'libgomp.c++'.
>>
>> It remains to be analyzed how '#pragma GCC diagnostic [...]' directives
>> can cause code generation changes; for now I'm working around the
>> "unexpected" '-Werror=array-bounds' diagnostics differently:
> 
> In addition to a few in straight-line code, I also had these two:
> 
>> --- a/libgomp/libgomp.h
>> +++ b/libgomp/libgomp.h
>> @@ -128,7 +128,10 @@ team_malloc (size_t size)
>>          : "=v"(result) : "v"(TEAM_ARENA_FREE), "v"(size), "e"(1L) : "memory");
>>
>>     /* Handle OOM.  */
>> +# pragma GCC diagnostic push
>> +# pragma GCC diagnostic ignored "-Warray-bounds" /*TODO PR101484 */
>>     if (result + size > *(void * __lds *)TEAM_ARENA_END)
>> +# pragma GCC diagnostic pop
>>       {
>>         /* While this is experimental, let's make sure we know when OOM
>>         happens.  */
>> @@ -162,8 +159,11 @@ team_free (void *ptr)
>>        However, if we fell back to using heap then we should free it.
>>        It would be better if this function could be a no-op, but at least
>>        LDS loads are cheap.  */
>> +# pragma GCC diagnostic push
>> +# pragma GCC diagnostic ignored "-Warray-bounds" /*TODO PR101484 */
>>     if (ptr < *(void * __lds *)TEAM_ARENA_START
>>         || ptr >= *(void * __lds *)TEAM_ARENA_END)
>> +# pragma GCC diagnostic pop
>>       free (ptr);
>>   }
>>   #else
> 
> ..., and it appears that the '#pragma GCC diagnostic pop' are considered
> here to be the 'statement' of the 'if'!  That's (a) unexpected (to me, at
> least) for this kind of "non-executable" '#pragma', and (b) certainly
> would be worth a dignostic, like we have for OMP pragmas, for example:
> 
>      if (context == pragma_stmt)
>        {
>          error_at (loc, "%<#pragma %s%> may only be used in compound statements",
>                    "[...]");
> 

I agree, that does seem quite surprising.  I opened pr101538 only
to find that the problem is already tracked in pr63326.

> Addressing that is for another day.

David Malcolm (CC'd) has a patch attached to pr63326 to issue
a warning to point out that #pragmas are treated as statements
that would help prevent this type of a bug.  David, do you still
plan to submit it?

Martin