https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106379
Bug ID: 106379
Summary: DCE depends on order
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
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 the
order of the sub-expressions within the if.
GCC detects that the if expression in the following code snippet evaluates to
false and thus removes the dead code:
#include <stdio.h>
#include <stdbool.h>
static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}
void f(bool c, bool s) {
if (((!c == !s) && !(c) && s)) {
DCEMarker0_();
}
}
In the following snippet the expressions are swapped (the semantics stay the
same). However, GCC cannot eliminate the dead code anymore:
#include <stdio.h>
#include <stdbool.h>
static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}
void f(bool c, bool s) {
if ((!s == !c) && s && !(c)) {
DCEMarker0_();
}
}
This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/vTqhc46qY
