[Bug c/55075] New: GCC optimizer makes bad assumptions on data range

[julvr at broadcom dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55075

             Bug #: 55075
           Summary: GCC optimizer makes bad assumptions on data range
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: julvr@broadcom.com


Created attachment 28531
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28531
Simple file to demonstrate problem

When compiling a simple program, the compiler makes a bad assumption about the
possible range a value can have.  As a result, in the attached example, it
raises a compile error.  It is unknown if this can cause other, more subtle
errors at this time, however, it is suspected that it may produce bad code in
certain circumstances.

This bug is visible in gcc versions 4.6.2 (cross compiler for mips) and 4.7.1
(native compiler for pc).   This bug does not occur on 4.5.1.

It is also known that the bug only occurs with optimization level -O2.  It does
not occur with -O1.


I have attached test1.c.  To reproduce the bug, compile with:

gcc -O2 -Werror=array-bounds -c test1.c


test1.c:

static int x = 0;
int main(void) {
    while (1) {
        { volatile int somearray[1]; somearray[x] = 0; }
        x--;
        if (x <= 0) {
           return;
        }
        { volatile int somearray[1]; somearray[x] = 0; }

    }
}