[Bug c/50219] New: Combine pass generated wrong code for unsigned char comparison on MIPS target

["yaocang at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50219

             Bug #: 50219
           Summary: Combine pass generated wrong code for unsigned char
                    comparison on MIPS target
    Classification: Unclassified
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: yaocang@gmail.com


Gcc3.4.4 generate wrong MIPS instructions for the codes following when using
-O1 and -O2. 

int test(unsigned char a, int *b)
{
    if(b==NULL)
       return -1;
    if(a>=0x01 && a<=0xaf)
    {
       *b++;
       return 0;
    }
    else if(a<=0xfe)
       return 1;
    else
       return 3;

    return 4;
}

In assemble codes, (a>=0x01 && a<=0xaf) correspond to "add $8,$8,-1; sltui
$8,$8,-81 " .
According to MIPS instruction manual, "sltui $8,$8,-81 " means the value in $8
will be compared with the constant 0xFFFFFFAF. So when the variable "a" has a
value larger than 0xaf, but "sltui $8,$8,-81 " make the condition  "(a>=0x01 &&
a<=0xaf)" hold, and the program flow go into if-body, and get wrong result.

It seems to me that when simplify_comparison combining tow rtl insn about the
if condition, zero_extend and ltu, the constant was signed-extended.

This problem doesn't occur in 2.96 and 4.3, other versions not tested.