[Bug c/82100] New: gcc cannot detect unreachable code even when -Wunreachable-code is in effect

["dcb314 at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82100

            Bug ID: 82100
           Summary: gcc cannot detect unreachable code even when
                    -Wunreachable-code is in effect
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com
  Target Milestone: ---

Given the following C code

extern void g( int);

void f( int a, int b)
{
        if (a != b)
        {
                if (a == b)
                {
                        g( a);
                }
        }
        if (a < b)
        {
                if (a >= b)
                {
                        g( a);
                }
        }
        if (a > b)
        {
                if (a <= b)
                {
                        g( a);
                }
        }
}

then recent gcc trunk doesn't have much to say, even when provoked:

$ ~/gcc/results/bin/gcc -c -O2 -Wall -Wextra -Wunreachable-code
-Wtautological-compare sep4a.cc
$ 

Here is static analyser cppcheck finding the three bugs:

$ ~/cppcheck/trunk/cppcheck --enable=all sep4a.cc
[sep4a.cc:6] -> [sep4a.cc:8]: (warning) Opposite inner 'if' condition leads to
a dead code block.
[sep4a.cc:13] -> [sep4a.cc:15]: (warning) Opposite inner 'if' condition leads
to a dead code block.
[sep4a.cc:20] -> [sep4a.cc:22]: (warning) Opposite inner 'if' condition leads
to a dead code block.
$ 

Runng cppcheck over the current Linux kernel version 4.13 suggests about
half a dozen bugs would be detected.