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* [Bug tree-optimization/106505] New: DCE depends on whether if or else is used
@ 2022-08-02 11:38 tmayerl at student dot ethz.ch
2022-08-07 20:04 ` [Bug tree-optimization/106505] " pinskia at gcc dot gnu.org
2023-09-17 5:42 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: tmayerl at student dot ethz.ch @ 2022-08-02 11:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106505
Bug ID: 106505
Summary: DCE depends on whether if or else is used
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tmayerl at student dot ethz.ch
Target Milestone: ---
In some cases, the compiler's ability to eliminate dead code depends on whether
the if expression is left as it is or negated and the body moved to the else
block.
GCC detects that the if expressions in the following code snippet evaluate to
false and thus removes the dead code:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(bool s, bool c) {
if (!c == !s) {
if (s && !c) {
DCEMarker0_();
}
}
}
In the following snippet, the inner if has been negated and the body has been
moved to the else block. However, GCC cannot eliminate the dead code anymore:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(bool s, bool c) {
if (!c == !s) {
if (!(s && !c)) {}
else {
DCEMarker0_();
}
}
}
This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/P4a61nsdq
If s and c in the inner if expression are swapped, it is the other way round.
Without the else, the compiler cannot eliminate the dead code:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(bool s, bool c) {
if (!c == !s) {
if (c && !s) {
DCEMarker0_();
}
}
}
However, it suddenly can optimise the code when the if expression is negated
and the body is moved to the else:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(bool s, bool c) {
if (!c == !s) {
if (!(c && !s)) {}
else {
DCEMarker0_();
}
}
}
This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/95jo3Gv45
Due to the order issue this might be related to the following bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106379
However, this example only works with C as a source language. With C++, it is
fine. Thus, this might be related to the following bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106381
Another example is given below. In this case, only one variable is needed.
However, this only works with the datatype unsigned instead of bool. Therefore,
it might be related to the following bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106380
Version with dead code eliminated:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(unsigned s) {
if (!s == !!s) {
DCEMarker0_();
}
}
Version with dead code not eliminated:
#include <stdio.h>
#include <stdbool.h>
void DCEMarker0_();
void f(unsigned s) {
if (!(!s == !!s)) {}
else {
DCEMarker0_();
}
}
This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/zYfoTs3PE
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/106505] DCE depends on whether if or else is used
2022-08-02 11:38 [Bug tree-optimization/106505] New: DCE depends on whether if or else is used tmayerl at student dot ethz.ch
@ 2022-08-07 20:04 ` pinskia at gcc dot gnu.org
2023-09-17 5:42 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-08-07 20:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106505
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/106505] DCE depends on whether if or else is used
2022-08-02 11:38 [Bug tree-optimization/106505] New: DCE depends on whether if or else is used tmayerl at student dot ethz.ch
2022-08-07 20:04 ` [Bug tree-optimization/106505] " pinskia at gcc dot gnu.org
@ 2023-09-17 5:42 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-09-17 5:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106505
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-09-17
Status|UNCONFIRMED |ASSIGNED
Ever confirmed|0 |1
Assignee|unassigned at gcc dot gnu.org |pinskia at gcc dot gnu.org
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-09-17 5:42 UTC | newest]
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2022-08-02 11:38 [Bug tree-optimization/106505] New: DCE depends on whether if or else is used tmayerl at student dot ethz.ch
2022-08-07 20:04 ` [Bug tree-optimization/106505] " pinskia at gcc dot gnu.org
2023-09-17 5:42 ` pinskia at gcc dot gnu.org
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