Re: optimization/7799: [3.2/3.3 regression] Loop bug with optimization flag -Os in gcc

["Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7799

Hi,

this PR is about the following code snippet that is miscompiled with -Os

void fill (int* p, int* q[10])
{
        int i;
        for (i = 0; i < 10; i++)
                *q++ = &p[i];
}

The asm-Code is this:

        pushl   %ebp
        movl    %esp, %ebp
        movl    8(%ebp), %eax
        movl    12(%ebp), %edx
        leal    36(%eax), %ecx
.L6:
        movl    %eax, (%edx)
        addl    $4, %eax
        addl    $4, %edx
        cmpl    %ecx, %eax
        jle     .L6
        popl    %ebp
        ret

This code roughly corresponds to the following C-Code:

void fill (int * p, int *q[10])
{
	int ecx = (int)p + 9;
	do {
		*q = p; p++; q++;
	} while ((int)p <= ecx);
}

This transformation is IMHO illegal because there is no way to make the
comparison in general equivialent to that in the original for loop.
If p is initially 0x7ffffffc the comparison must be treated as unsigned,
however, if p is initially 0xfffffffc the comparison must be treated as
signed.

    regards   Christian

-- 
THAT'S ALL FOLKS!