[Bug c/106381] New: DCE depends on used programming language (C vs C++)

tmayerl at student dot ethz.ch gcc-bugzilla@gcc.gnu.org
Thu Jul 21 09:33:07 GMT 2022


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106381

            Bug ID: 106381
           Summary: DCE depends on used programming language (C vs C++)
           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
used language (C vs C++).

GCC detects that the if expression in the following code snippet evaluates to
false and thus removes the dead code. The code is compiled as C++ code:

#include <stdio.h>
#include <stdbool.h>

static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}

void f(unsigned s, unsigned c) {
    if (((!s == !c) && c && !(s))) {
        DCEMarker0_();
    }
}

However, if the same code is compiled as C code, GCC cannot eliminate the dead
code anymore.

This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/a9ned9Exb

(Might be related to 106379 and 106380)


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