public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag
@ 2012-05-28 10:03 etienne_lorrain at yahoo dot fr
2021-11-29 9:02 ` [Bug target/53507] " pinskia at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: etienne_lorrain at yahoo dot fr @ 2012-05-28 10:03 UTC (permalink / raw)
To: gcc-bugs
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: unassigned@gcc.gnu.org
ReportedBy: etienne_lorrain@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
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2021-11-29 9:02 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2012-05-28 10:03 [Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag etienne_lorrain at yahoo dot fr
2021-11-29 9:02 ` [Bug target/53507] " pinskia at gcc dot gnu.org
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).