[Bug c/86768] New: gcc wrongly decides that variable is <=1 after "while (x>1 && blah)" construct.

[Emmanuel.Thome at inria dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86768

            Bug ID: 86768
           Summary: gcc wrongly decides that variable is <=1 after "while
                    (x>1 && blah)" construct.
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Emmanuel.Thome at inria dot fr
  Target Milestone: ---

Created attachment 44477
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44477&action=edit
test program (wrong diagnostic emitted with -m32 -O3 only)

I have a construct of the following form (comes from actual code, test case
attached).

// check argc >= 3
while (argc > 1 && argv[1][0] == '-') {
  // do something and then argc--, or maybe call exit()
}
// do code that relies on argc >= 3

As presented, it looks kinda weird, but I could as well make the condition be:

while (argc > 1 && argv[1][0] == '-' && foo(argc))

with foo(argc) being outside the compilation unit. So we could be in a
situation where because of the way foo() acts, the post-condition argc>=3 holds
anyway (and the test argc>1 is pointless).

However, I get the following with -m32 -O3: (note: nr_B2 is argc-2)

localhost $ gcc -m32 -O3  /tmp/t.c -W -Wall -c
/tmp/t.c: In function ‘blah’:
/tmp/t.c:32:10: warning: argument 1 value ‘4294967292’ exceeds maximum object
size 2147483647 [-Walloc-size-larger-than=]
     B2 = malloc(nr_B2 * sizeof(unsigned long));
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /tmp/t.c:2:
/usr/include/stdlib.h:539:14: note: in a call to allocation function ‘malloc’
declared here
 extern void *malloc (size_t __size) __THROW __attribute_malloc__ __wur;
              ^~~~~~

Meaning that gcc has decided that the post-condition argc==1 holds after the
while loop. I think that this is an incorrect guess.