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* [Bug tree-optimization/99946] New: fail to exchange if conditions in terms of likely/unlikely probability
@ 2021-04-07 2:01 jiangning.liu at amperecomputing dot com
2021-04-07 2:03 ` [Bug tree-optimization/99946] " jiangning.liu at amperecomputing dot com
2021-04-07 7:33 ` rguenth at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: jiangning.liu at amperecomputing dot com @ 2021-04-07 2:01 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99946
Bug ID: 99946
Summary: fail to exchange if conditions in terms of
likely/unlikely probability
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jiangning.liu at amperecomputing dot com
Target Milestone: ---
For this simple case,
$ cat test_cond.c
#define likely(x) __builtin_expect((x),1)
#define unlikely(x) __builtin_expect((x),0)
extern void g(void);
int a, b;
void f(void)
{
if (likely(a>0))
if (unlikely(b>0))
g();
}
We expect gcc compiler can exchange the if conditions to be like below,
if (unlikely(b>0))
if (likely(a>0))
g();
This way, performance can be improved due to saving the comparison for a>0.
At the moment, gcc generate code as below,
.LFB0:
.cfi_startproc
movl a(%rip), %edx
testl %edx, %edx
jle .L1
movl b(%rip), %eax
testl %eax, %eax
jg .L4
.L1:
ret
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/99946] fail to exchange if conditions in terms of likely/unlikely probability
2021-04-07 2:01 [Bug tree-optimization/99946] New: fail to exchange if conditions in terms of likely/unlikely probability jiangning.liu at amperecomputing dot com
@ 2021-04-07 2:03 ` jiangning.liu at amperecomputing dot com
2021-04-07 7:33 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: jiangning.liu at amperecomputing dot com @ 2021-04-07 2:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99946
--- Comment #1 from Jiangning Liu <jiangning.liu at amperecomputing dot com> ---
Is there any gcc pass that can deal with this simple optimization?
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/99946] fail to exchange if conditions in terms of likely/unlikely probability
2021-04-07 2:01 [Bug tree-optimization/99946] New: fail to exchange if conditions in terms of likely/unlikely probability jiangning.liu at amperecomputing dot com
2021-04-07 2:03 ` [Bug tree-optimization/99946] " jiangning.liu at amperecomputing dot com
@ 2021-04-07 7:33 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-04-07 7:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99946
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Last reconfirmed| |2021-04-07
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
There's no pass doing all of this. There's ifcombine that could end up
turning it into
tem = likely(a>0) & unlikely(b>0);
if (tem)
g ();
and then RTL expansion might expand it to branchy code again.
But of course __builtin_expect is long gone then and we'll lose the
likelyness of the individual tests.
Still ifcombine has analysis logic as to "collecting" a complex predicate,
what's then missing is to "re-associate" the tests according to probability.
Which means implementing it in ifcombine should be feasible.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-04-07 2:01 [Bug tree-optimization/99946] New: fail to exchange if conditions in terms of likely/unlikely probability jiangning.liu at amperecomputing dot com
2021-04-07 2:03 ` [Bug tree-optimization/99946] " jiangning.liu at amperecomputing dot com
2021-04-07 7:33 ` rguenth at gcc dot gnu.org
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