[Bug c/98070] New: errno is not re-evaluated after clearing errno and calling realloc(ptr, SIZE_MAX)

["stli at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98070

            Bug ID: 98070
           Summary: errno is not re-evaluated after clearing errno and
                    calling realloc(ptr, SIZE_MAX)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stli at linux dot ibm.com
  Target Milestone: ---

Created attachment 49652
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49652&action=edit
Testcase reproducing the issue with gcc-head

Hi,

After setting errno=0 and calling realloc with a too large size, which sets
errno to ENOMEM, a subsequent "if (errno == ENOMEM)" is not evaluated as true.
Instead gcc assumes that errno has not changed and is directly executing the
else-path without testing errno again.

This happens in the glibc-testcase:
<glibc>/malloc/tst-malloc-too-large.c test
(see
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/tst-malloc-too-large.c;h=b5ad7eb7e7bf764fe57ceff5a810e3c211ca05e0;hb=refs/heads/master)
on at least x86_64 and s390x with gcc-head.

The attached small reproducer fails with gcc-head, but not with gcc 10, 9
(before):
/* Output with gcc 11:
   $ ./tst-errno-realloc (build with >= -O1)
   47: errno == 0 (Cannot allocate memory). We are in the else-part of 'if
(errno == ENOMEM)'. Does errno correspond to %m or the line below or to '(gdb)
p errno'?!
   dump_errno(48, compare to line above!): errno == 12 (Cannot allocate memory)
vs main_errno=0

   On s390x:
   $ gcc -v
   Using built-in specs.
   COLLECT_GCC=./install-s390x-head/bin/gcc
  
COLLECT_LTO_WRAPPER=/home/stli/gccDir/install-s390x-head/libexec/gcc/s390x-ibm-linux-gnu/11.0.0/lto-wrapper
   Target: s390x-ibm-linux-gnu
   Configured with: /home/stli/gccDir/gcc-head/configure
--prefix=/home/stli/gccDir/install-s390x-head/ --enable-shared
--with-system-zlib --enable-threads=posix --enable-__cxa_atexit
--enable-checking --enable-gnu-indirect-function --enable-languages=c,c++
--with-arch=zEC12 --with-tune=z13 --disable-bootstrap --with-long-double-128
--enable-decimal-float
   Thread model: posix
   Supported LTO compression algorithms: zlib
   gcc version 11.0.0 20201127 (experimental) (GCC)
   $ git log --oneline
   5e9f814d754 (HEAD -> master, origin/master, origin/HEAD) rs6000: Change
rs6000_expand_vector_set param

   Also on x86_64:
   $ gcc -v
   Using built-in specs.
   COLLECT_GCC=/home/stli/gccDir/install-x86_64-head/bin/gcc
  
COLLECT_LTO_WRAPPER=/home/stli/gccDir/install-x86_64-head/libexec/gcc/x86_64-pc-linux-gnu/11.0.0/lto-wrapper
   Target: x86_64-pc-linux-gnu
   Configured with: /home/stli/gccDir/gcc-head/configure
--prefix=/home/stli/gccDir/install-x86_64-head/ --enable-shared
--with-system-zlib --enable-threads=posix --enable-__cxa_atexit
--enable-checking --enable-gnu-indirect-function --enable-languages=c,c++
--with-tune=generic --with-arch_32=x86-64 --disable-bootstrap
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --disable-libgcj --disable-multilib
   Thread model: posix
   Supported LTO compression algorithms: zlib zstd
   gcc version 11.0.0 20201130 (experimental) (GCC)
   $ git log --oneline
   a5ad5d5c478 (HEAD -> master, origin/master, origin/HEAD) RISC-V: Always
define MULTILIB_DEFAULTS
*/