[Bug c/112947] New: Contradictory indications of extern __thread address NULL or not with UBSAN

[Bug c/112947] New: Contradictory indications of extern __thread address NULL or not with UBSAN

[resence at parsoma dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112947

            Bug ID: 112947
           Summary: Contradictory indications of extern __thread address
                    NULL or not with UBSAN
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: resence at parsoma dot net
  Target Milestone: ---

Created attachment 56848
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56848&action=edit
The main code which reproduces this issue

I have not managed to reduce this below 2 files.

File 1, m.c:
__thread int nimInErrorMode__system_u4299;

File 2, g.c, attached.

Then run:
gcc -c  -Wall -Wextra -pedantic -pthread -fsanitize=undefined -O0
-fno-strict-aliasing -o m.o m.c
gcc -c  -Wall -Wextra -pedantic -pthread -fsanitize=undefined -O2
-fno-strict-aliasing -o g.o g.c
gcc -o repro m.o g.o  -fsanitize=undefined  -lpthread

With the output:
g.c: In function ‘nimErrorFlag’:
g.c:25:40: warning: the comparison will always evaluate as ‘false’ for the
address of ‘nimInErrorMode__system_u4299’ will never be NULL [-Waddress]
   25 |   if (((&nimInErrorMode__system_u4299) == (void *)0)) {
      |                                        ^~
g.c:16:21: note: ‘nimInErrorMode__system_u4299’ declared here
   16 | extern __thread int nimInErrorMode__system_u4299;
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
Always-false comparison is true

gcc -v output, running from Debian unstable on amd64:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 13.2.0-8'
--with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/reproducible-path/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/reproducible-path/gcc-13-13.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=3
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Debian 13.2.0-8)

[Bug c/112947] Contradictory indications of extern __thread address NULL or not with UBSAN

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112947

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is by design.

-fsanitize=undefined turns off -fdelete-null-pointer-checks .

[Bug c/112947] Contradictory indications of extern __thread address NULL or not with UBSAN

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112947

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
See PR 95656 also.

*** This bug has been marked as a duplicate of bug 95656 ***

[Bug c/112947] Contradictory indications of extern __thread address NULL or not with UBSAN

[resence at parsoma dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112947

--- Comment #3 from resence at parsoma dot net ---
It's not only the null pointer check per se though?

Re-add a stdio.h import for convenience, and it still vacillates whether or not
that address is NULL or not:

BAR2: 0x7f25d843173c
BAR3: 0x7f25d843173c
BAR4: 5
BAR5: 5
BAR2: 0x7f25d843173c
BAR3: 0x7f25d843173c
BAR4: 5
BAR5: 5
BAR2: 0x7f25d843173c
BAR3: 0x7f25d843173c
BAR4: 5
BAR5: 5

when using:
static int *nimErrorFlag(void) {
  /* With this, no always-false warning */
  /* printf("BAR1: %p\n", (void*)&nimInErrorMode__system_u4299); */

  if (((&nimInErrorMode__system_u4299) == (void *)0)) {
    write(1, "Always-false comparison is true\n", 32);
    printf("FOO: %p\n", (void*)&nimInErrorMode__system_u4299);
  }

  int *result = (&nimInErrorMode__system_u4299);
  printf("BAR2: %p\n", (void*)&nimInErrorMode__system_u4299);
  printf("BAR3: %p\n", (void*)result);
  printf("BAR4: %d\n", nimInErrorMode__system_u4299);
  printf("BAR5: %d\n", *result);

  return result;
}

Which still holds if one ensures that nimInErrorMode__system_u4299 is defined
and initialized in m.c:
__thread int nimInErrorMode__system_u4299 = 5;

[Bug c/112947] Contradictory indications of extern __thread address NULL or not with UBSAN

[resence at parsoma dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112947

--- Comment #4 from resence at parsoma dot net ---
Or rather,

static int *nimErrorFlag(void) {
  /* With this, no always-false warning */
  /* printf("BAR1: %p\n", (void*)&nimInErrorMode__system_u4299); */

  if (((&nimInErrorMode__system_u4299) == (void *)0)) {
    write(1, "Always-false comparison is true\n", 32);
    printf("FOO: %p\n", (void*)&nimInErrorMode__system_u4299);
  }

  int *result = (&nimInErrorMode__system_u4299);
  printf("BAR2: %p\n", (void*)&nimInErrorMode__system_u4299);
  printf("BAR3: %p\n", (void*)result);

  return result;
}

resulting in
BAR2: 0x7f9ee746773c
BAR3: 0x7f9ee746773c
BAR2: 0x7f9ee746773c
BAR3: 0x7f9ee746773c
Always-false comparison is true
FOO: (nil)
BAR2: 0x7f9ee746773c
BAR3: 0x7f9ee746773c

(adding BAR4/BAR5 change behavior again).