array subscript is below array bounds : false positive?

["Peter A. Felvegi" <petschy@praire-chicken.com>]

Hello,

I've run into this strange warning when compiling w/ optimization:
gcc-4.3 -O2 -Werror -Wall -c -o t.o t.c
cc1: warnings being treated as errors
t.c: In function ‘foo’:
t.c:25: error: array subscript is below array bounds

gcc-4.4 gives the same warning/error, however, gcc 4.1 and 4.2 compiles 
the source.

gcc-4.1 -v says:
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++ 
--prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib 
--without-included-gettext --enable-threads=posix --enable-nls 
--with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 
--enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug 
--with-tune=generic --enable-checking=release x86_64-linux-gnu
Thread model: posix
gcc version 4.1.3 20080704 (prerelease) (Debian 4.1.2-27)

gcc-4.2 -v says:
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v 
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr 
--enable-shared --with-system-zlib --libexecdir=/usr/lib 
--without-included-gettext --enable-threads=posix --enable-nls 
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2 
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc 
--enable-mpfr --with-tune=generic --enable-checking=release 
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.2.4 (Debian 4.2.4-6)

gcc-4.3 -v says:
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.4-2' 
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs 
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr 
--enable-shared --enable-multiarch --enable-linker-build-id 
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext 
--enable-threads=posix --enable-nls 
--with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3 
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc 
--enable-mpfr --with-tune=generic --enable-checking=release 
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.4 (Debian 4.3.4-2)

gcc-4.4 -v says:
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.1-1' 
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs 
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr 
--enable-shared --enable-multiarch --enable-linker-build-id 
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext 
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 
--program-suffix=-4.4 --enable-nls --enable-clocale=gnu 
--enable-libstdcxx-debug --enable-mpfr --enable-objc-gc 
--with-arch-32=i486 --with-tune=generic --enable-checking=release 
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.1 (Debian 4.4.1-1)

t.c is :
---->8---->8---->8---->8---->8---->8---->8---->8---->8---->8----
#define ASSERT(x)	if (x) { } else { __asm__("int $0x03"); }
#define SIZE		5

char hnd[SIZE];
char flg[SIZE];

char crd();
int  idx(char);
void set(int i, char v);

#if 1
void set(int i, char v)
{
	ASSERT(i >=0 && i < SIZE);
	flg[i] = v;
}
#endif


void foo()
{
	char c = crd();
	int  i = idx(0);
	ASSERT(i != -1);
	hnd[i] = c; // array subscript is below array bounds
	set(i, 1);
}
---->8---->8---->8---->8---->8---->8---->8---->8---->8---->8----

Suppose that idx(c) returns the position of c in an array, an the return 
value of -1 means that c is not in the array. The assertion checks that.

The funny thing is, if I change the source a bit, the warning goes away:
1) set '#if 1' to '#if 0' so that only the prototype of set() is visible
2) comment out the ASSERT() int set()
3) comment out ASSERT() just before the marked line
4) comment out set(i, 1) just after the marked line

The warning is not present under -O2.

Is this warning legal?

Cheers, Peter