http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53507
Bug #: 53507
Summary: ia32/amd64: bsf can be used to test null memory, bsf
sets zero flag
Classification: Unclassified
Product: gcc
Version: 4.6.3
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: etienne_lorr...@yahoo.fr
Compiling:
int I;
void ifnull(void);
void testnull(void) {
if (I == 0)
ifnull();
}
void fct (unsigned u) {
if (u != 0)
I = __builtin_ffs(u) - 1;
}
result in for testnull():
movl I, %eax
testl %eax, %eax
je .L4
rep
ret
.L4:
jmp ifnull()
It would be shorter/quicker to replace the two first lines by:
bsfl I, %eax
je ifnull()
i.e. there is no need to load the memory variable into a register with bsf/bsr.
And result for fct():
movl 4(%esp), %eax
testl %eax, %eax
je .L5
bsfl %eax, %eax
movl %eax, I
.L5:
rep
ret
It would be shorter/quicker to test if the parameter u is null by the bsf
instruction:
bsfl 4(%esp), %eax
cmovne %eax, I
.L5:
rep
ret
