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: unassig...@gcc.gnu.org ReportedBy: yaoc...@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.