[Bug c/111615] New: NULL check incorrectly skipped at O2 and O3

[gardner.ben at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111615

Bug ID: 111615
   Summary: NULL check incorrectly skipped at O2 and O3
   Product: gcc
   Version: 11.4.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: gardner.ben at gmail dot com
  Target Milestone: ---

Created attachment 56003
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56003&action=edit
Source file that produces the issue.

The attached source code has a function (pr_str()) that prints something
different if the parameter is NULL.
When passed a NULL (const char *) value from a static const structure, the NULL
check is skipped and the first printf() is executed.


static void pr_str(const char *s)
{
   /* BUG: this NULL check is skipped/wrong at O2 and O3 for
* vec->haystack and vec->needle.
*/
   if (s != NULL)
   {
  printf("'%s' %p %d", s, s, (int)(intptr_t)s);
   }
   else
   {
  printf("(nil)");
   }
}


This occurs at O2 and O3, but not at O0, O1, or Os.
If the program prints "h=(nil)" on the 2nd to last line when executed, then it
worked.
The the program prints "h='(null)' (nil) 0" on the 2nd to last line, then if
failed.


Build script:
#!/bin/sh
build_it() {
OP=$1
gcc -g -Wall -O$OP -c -o memmem_test.O$OP.o memmem_test.c
gcc memmem_test.O$OP.o -o memmem_test.O$OP
}
build_it 0
build_it 1
build_it 2
build_it 3
build_it s


GCC detailed info:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/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='Ubuntu
11.4.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --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/gcc-11-XeT9lY/gcc-11-11.4.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-XeT9lY/gcc-11-11.4.0/debian/tmp-gcn/usr
--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=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)

[Bug c/111615] NULL check incorrectly skipped at O2 and O3

[gardner.ben at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111615

--- Comment #1 from Ben Gardner  ---
Created attachment 56004
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56004&action=edit
Build script.

[Bug middle-end/111615] NULL check incorrectly skipped at O2 and O3

[gardner.ben at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111615

--- Comment #3 from Ben Gardner  ---
The issue isn't with memmem(). It is with the value passed into pr_str() from
the structure. I suspect memmem() is a distraction.
I'll try to further reduce the test case to eliminate memmem(), if possible.

[Bug middle-end/111615] NULL check incorrectly skipped at O2 and O3

[gardner.ben at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111615

--- Comment #6 from Ben Gardner  ---
(In reply to Andrew Pinski from comment #5)
> extern void *memmem (const void *__haystack, size_t __haystacklen,
>const void *__needle, size_t __needlelen)
>  __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__pure__))
> __attribute__ ((__nonnull__ (1, 3)));
> 
> 
> memmem is declared with nonnull for the 1st and 3rd argument. If those
> arguments are null, the behavior is undefined and the values of those
> arguments can be assumed as not null afterwards too.
> 
> If you don't want that behavior you can use -fno-delete-null-pointer-checks .
> 
> Otherwise the behavior you are seeing is correct behavior based on well
> defined code.

Thanks for the info. That makes sense. I didn't check the header file, so I
didn't know that memmem() was declared with nonnull.

Also, thanks for the tip about -fno-delete-null-pointer-checks.