[Bug c/38961] New: if () block not true but a command in it is still in effect

["jellegeerts at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Complete command line that triggered the bug: gcc -v -save-temps -ansi -Wall
-Wextra -Werror -pedantic -pedantic-errors -g -O2 -o gcc_bug gcc_bug.c

Output of the command line:
**********************
Reading specs from /usr/lib/gcc/i486-slackware-linux/4.2.4/specs
Target: i486-slackware-linux
Configured with: ../gcc-4.2.4/configure --prefix=/usr --enable-shared
--enable-languages=ada,c,c++,fortran,java,objc --enable-threads=posix
--enable-__cxa_atexit --disable-checking --with-gnu-ld --verbose
--with-arch=i486 --target=i486-slackware-linux --host=i486-slackware-linux
Thread model: posix
gcc version 4.2.4
 /usr/libexec/gcc/i486-slackware-linux/4.2.4/cc1 -E -quiet -v gcc_bug.c
-mtune=i486 -march=i486 -ansi -Wall -Wextra -Werror -pedantic -pedantic-errors
-fworking-directory -O2 -fpch-preprocess -o gcc_bug.i
ignoring nonexistent directory
"/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../../i486-slackware-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc/i486-slackware-linux/4.2.4/include
 /usr/include
End of search list.
 /usr/libexec/gcc/i486-slackware-linux/4.2.4/cc1 -fpreprocessed gcc_bug.i
-quiet -dumpbase gcc_bug.c -mtune=i486 -march=i486 -ansi -auxbase gcc_bug -g
-O2 -Wall -Wextra -Werror -pedantic -pedantic-errors -ansi -version -o
gcc_bug.s
GNU C version 4.2.4 (i486-slackware-linux)
        compiled by GNU C version 4.2.4.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: d3491cea3997204b3b932c449c79b80e

/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../../i486-slackware-linux/bin/as
-V -Qy -o gcc_bug.o gcc_bug.s
GNU assembler version 2.18.50.0.9 (i486-slackware-linux) using BFD version
(Linux/GNU Binutils) 2.18.50.0.9.20080822
 /usr/libexec/gcc/i486-slackware-linux/4.2.4/collect2 --eh-frame-hdr -m
elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o gcc_bug
/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../crt1.o
/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../crti.o
/usr/lib/gcc/i486-slackware-linux/4.2.4/crtbegin.o
-L/usr/lib/gcc/i486-slackware-linux/4.2.4
-L/usr/lib/gcc/i486-slackware-linux/4.2.4
-L/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../../i486-slackware-linux/lib
-L/usr/lib/gcc/i486-slackware-linux/4.2.4/../../.. gcc_bug.o -lgcc --as-needed
-lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/i486-slackware-linux/4.2.4/crtend.o
/usr/lib/gcc/i486-slackware-linux/4.2.4/../../../crtn.o
**********************

The source code that triggered the bug:
**********************
#include <stdio.h>
#include <string.h>

const char *logout_cmd = NULL;

static void foo()
{
        const char *logout_cmd = logout_cmd;

        if ( ! logout_cmd )
        {
                /* This line is not executed. */
                printf("line %d, logout_cmd = %s\n", __LINE__, logout_cmd);

                /* This line is not executed when stepping through the code
                 * with gdb, but logout_cmd still is assigned the value
                 * "lxsession-logout", possible optimization bug? */
                logout_cmd = "lxsession-logout";
        }

        printf("line %d, logout_cmd = %s\n", __LINE__, logout_cmd);

        if (strcmp(logout_cmd, "openbox --exit"))
                printf("gcc bug triggered\n");
}

int main(void)
{
        printf("line %d, logout_cmd = %s\n", __LINE__, logout_cmd);

        logout_cmd = "openbox --exit";

        printf("line %d, logout_cmd = %s\n", __LINE__, logout_cmd);

        foo();

        printf("line %d, logout_cmd = %s\n", __LINE__, logout_cmd);

        return 0;
}
**********************


-- 
           Summary: if () block not true but a command in it is still in
                    effect
           Product: gcc
           Version: 4.2.4
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jellegeerts at gmail dot com
  GCC host triplet: i486-slackware-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38961