http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50217

             Bug #: 50217
           Summary: combine pass generated wrong code for unsigned char
                    comparison on MIPS
    Classification: Unclassified
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: yaoc...@gmail.com
            Target: mips


Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Target|                            |mips
          Component|c                           |rtl-optimization

I found 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.

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