Re: [PATCH] -Wdangling-pointer: don't mark SSA lhs sets as stores

[Martin Liška]

On 3/3/23 12:12, Richard Biener via Gcc-patches wrote:
> On Fri, Mar 3, 2023 at 9:30 AM Alexandre Oliva <ol...@adacore.com> wrote:
>>
>> On Feb 17, 2023, Richard Biener <richard.guent...@gmail.com> wrote:
>>
>>>> * gimple-ssa-warn-access.cc
>>>> (pass_waccess::check_dangling_stores): Skip non-stores.
>>>>
>>>> for  gcc/testsuite/ChangeLog
>>>>
>>>> * g++.dg/warn/Wdangling-pointer.C (warn_init_ref_member): Add
>>>> two new variants, one fixed, one xfailed.
>>>> * c-c++-common/Wdangling-pointer-5.c
>>>> (nowarn_store_arg_store_arg): Add now-expected warnings.
>>
>> Ping?
>> https://gcc.gnu.org/pipermail/gcc-patches/2023-February/612186.html
> 
> I was hoping Martin would chime in, but he didn't.
> 
> So - OK.
> 
> Thanks,
> Richard.
> 
>>
>> --
>> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>>    Free Software Activist                       GNU Toolchain Engineer
>> Disinformation flourishes because many people care deeply about injustice
>> but very few check the facts.  Ask me about <https://stallmansupport.org>

Hi.

I've just noticed this change triggered one more warning for qemu 7.1.0:

cc -m64 -mcx16 -Ilibqemuutil.a.p -I. -I.. -Isubprojects/libvhost-user 
-I../subprojects/libvhost-user -Iqapi -Itrace -Iui -Iui/shader 
-I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/libmount 
-I/usr/include/blkid -I/usr/include/gio-unix-2.0 -I/usr/include/p11-kit-1 
-I/usr/include/pixman-1 -fdiagnostics-color=auto -Wall -Winvalid-pch -Werror 
-std=gnu11 -O2 -isystem /home/abuild/rpmbuild/BUILD/qemu-7.1.0/linux-headers 
-isystem linux-headers -iquote . -iquote /home/abuild/rpmbuild/BUILD/qemu-7.1.0 
-iquote /home/abuild/rpmbuild/BUILD/qemu-7.1.0/include -iquote 
/home/abuild/rpmbuild/BUILD/qemu-7.1.0/tcg/i386 -pthread -D_GNU_SOURCE 
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes 
-Wredundant-decls -Wundef -Wwrite-strings -Wmissing-prototypes 
-fno-strict-aliasing -fno-common -fwrapv -Wold-style-declaration 
-Wold-style-definition -Wtype-limits -Wformat-security -Wformat-y2k -Winit-self 
-Wignored-qualifiers -Wempty-body -Wnested-externs -Wendif-labels 
-Wexpansion-to-defined -Wimplicit-fallthrough=2 -Wno-missing-include-dirs 
-Wno-shift-negative-value -Wno-psabi -fstack-protector-strong -O2 -Wall 
-fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables 
-fstack-clash-protection -Werror=return-type -U_FORTIFY_SOURCE 
-D_FORTIFY_SOURCE=2 -fPIE -MD -MQ libqemuutil.a.p/util_async.c.o -MF 
libqemuutil.a.p/util_async.c.o.d -o libqemuutil.a.p/util_async.c.o -c 
../util/async.c
In file included from 
/home/abuild/rpmbuild/BUILD/qemu-7.1.0/include/qemu/coroutine.h:18,
                 from 
/home/abuild/rpmbuild/BUILD/qemu-7.1.0/include/block/aio.h:20,
                 from ../util/async.c:28:
../util/async.c: In function 'aio_bh_poll':
/home/abuild/rpmbuild/BUILD/qemu-7.1.0/include/qemu/queue.h:303:22: error: 
storing the address of local variable 'slice' in '*ctx.bh_slice_list.sqh_last' 
[-Werror=dangling-pointer=]
  303 |     (head)->sqh_last = &(elm)->field.sqe_next;                          
\
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
../util/async.c:161:5: note: in expansion of macro 'QSIMPLEQ_INSERT_TAIL'
  161 |     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
      |     ^~~~~~~~~~~~~~~~~~~~
../util/async.c:156:17: note: 'slice' declared here
  156 |     BHListSlice slice;
      |                 ^~~~~
../util/async.c:156:17: note: 'ctx' declared here

which I reduced to:

$ cat util_async.i
typedef struct BHListSlice BHListSlice;
struct BHListSlice {
  struct {
    BHListSlice *sqe_next;
  } next;
} *aio_bh_poll_s;
struct AioContext {
  struct {
    BHListSlice *sqh_first;
    BHListSlice **sqh_last;
  } bh_slice_list;
} aio_bh_dequeue();
int aio_bh_poll_bh;
int aio_bh_poll(struct AioContext *ctx) {
  BHListSlice slice;
  (&ctx->bh_slice_list)->sqh_last = &(slice.next.sqe_next);
  while (aio_bh_poll_s) {
    unsigned flags;
    aio_bh_dequeue(&flags);
    if (aio_bh_poll_bh) {
      (&ctx->bh_slice_list)->sqh_last = &(&ctx->bh_slice_list)->sqh_first;
      continue;
    }
  }
  return 0;
}

$ gcc util_async.i -c -Werror=dangling-pointer
util_async.i: In function ‘aio_bh_poll’:
util_async.i:16:35: error: storing the address of local variable ‘slice’ in 
‘*ctx.bh_slice_list.sqh_last’ [-Werror=dangling-pointer=]
   16 |   (&ctx->bh_slice_list)->sqh_last = &(slice.next.sqe_next);
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
util_async.i:15:15: note: ‘slice’ declared here
   15 |   BHListSlice slice;
      |               ^~~~~
util_async.i:15:15: note: ‘ctx’ declared here
cc1: some warnings being treated as errors

Is the emitted warning correct?

Thank you,
Martin