public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug tree-optimization/96572] New: Failure to optimize out branch when it always results in UB from dereferencing a pointer to an undefined value set in there
@ 2020-08-11 17:07 gabravier at gmail dot com
2020-08-25 8:42 ` [Bug tree-optimization/96572] " rguenth at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: gabravier at gmail dot com @ 2020-08-11 17:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96572
Bug ID: 96572
Summary: Failure to optimize out branch when it always results
in UB from dereferencing a pointer to an undefined
value set in there
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(int *p, bool cond)
{
int i;
int *q;
if (cond)
q = &i;
else
q = p;
return *q + *p;
}
This can be optimized to `return (*p) * 2;`. This transformation is done by
LLVM, but not by GCC.
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug tree-optimization/96572] Failure to optimize out branch when it always results in UB from dereferencing a pointer to an undefined value set in there
2020-08-11 17:07 [Bug tree-optimization/96572] New: Failure to optimize out branch when it always results in UB from dereferencing a pointer to an undefined value set in there gabravier at gmail dot com
@ 2020-08-25 8:42 ` rguenth at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-08-25 8:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96572
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Keywords| |missed-optimization
Last reconfirmed| |2020-08-25
Status|UNCONFIRMED |NEW
CC| |msebor at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
So phiprop sees
<bb 2> [local count: 1073741824]:
if (cond_4(D) != 0)
goto <bb 4>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 536870913]:
<bb 4> [local count: 1073741824]:
# q_3 = PHI <&i(2), p_5(D)(3)>
_1 = *q_3;
_2 = *p_5(D);
_7 = _1 + _2;
i ={v} {CLOBBER};
return _7;
and if we extend it to handle the case of a single address PHI operand
then we possibly would optimize this. After PRE we then have
<bb 2> [local count: 1073741824]:
pretmp_9 = *p_5(D);
if (cond_3(D) != 0)
goto <bb 4>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 536870912]:
<bb 4> [local count: 1073741824]:
# q_2 = PHI <i_7(D)(2), pretmp_9(3)>
_8 = q_2 + pretmp_9;
return _8;
but then we're missing the optimistic copy propagation / value-numbering
we do not perform because it defeats uninit warnings.
So I'm not sure it is desirable to optimize this given the obvious
fallout this will cause for diagnostics.
After-phiprop proposed IL testcase we fail to optimize because of this:
int f(int *p, _Bool cond)
{
int i;
int q;
if (cond)
q = i;
else
q = *p;
return q + *p;
}
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2020-08-25 8:42 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-08-11 17:07 [Bug tree-optimization/96572] New: Failure to optimize out branch when it always results in UB from dereferencing a pointer to an undefined value set in there gabravier at gmail dot com
2020-08-25 8:42 ` [Bug tree-optimization/96572] " rguenth 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).