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.
