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From: "tmayerl at student dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/106379] New: DCE depends on order
Date: Thu, 21 Jul 2022 09:13:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106379

            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 =3D=3D !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 =3D=3D !c) && s && !(c)) {
        DCEMarker0_();
    }
}


This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/vTqhc46qY=