<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
unreachable	O
code	O
is	O
part	O
of	O
the	O
source	O
code	O
of	O
a	O
program	O
which	O
can	O
never	O
be	O
executed	O
because	O
there	O
exists	O
no	O
control	O
flow	O
path	O
to	O
the	O
code	O
from	O
the	O
rest	O
of	O
the	O
program	O
.	O
</s>
<s>
Unreachable	O
code	O
is	O
sometimes	O
also	O
called	O
dead	B-Application
code	I-Application
,	O
although	O
dead	B-Application
code	I-Application
may	O
also	O
refer	O
to	O
code	O
that	O
is	O
executed	O
but	O
has	O
no	O
effect	O
on	O
the	O
output	O
of	O
a	O
program	O
.	O
</s>
<s>
However	O
,	O
unreachable	O
code	O
can	O
have	O
some	O
legitimate	O
uses	O
,	O
like	O
providing	O
a	O
library	O
of	O
functions	O
for	O
calling	O
or	O
jumping	O
to	O
manually	O
via	O
a	O
debugger	B-Application
while	O
the	O
program	O
is	O
halted	O
after	O
a	O
breakpoint	O
.	O
</s>
<s>
It	O
may	O
make	O
sense	O
to	O
have	O
such	O
code	O
in	O
the	O
shipped	O
product	O
,	O
so	O
that	O
a	O
developer	O
can	O
attach	O
a	O
debugger	B-Application
to	O
a	O
client	O
's	O
running	O
instance	O
.	O
</s>
<s>
a	O
consequence	O
of	O
the	O
internal	O
transformations	O
performed	O
by	O
an	O
optimizing	B-Application
compiler	I-Application
;	O
</s>
<s>
Apple	O
's	O
SSL/TLS	B-Protocol
from	O
February	O
2014	O
contained	O
a	O
major	O
security	O
flaw	O
known	O
formally	O
as	O
and	O
informally	O
as	O
the	O
"	O
goto	B-Protocol
fail	I-Protocol
bug	O
"	O
.	O
</s>
<s>
Here	O
,	O
there	O
are	O
two	O
successive	O
calls	O
to	O
goto	B-Protocol
fail	I-Protocol
.	O
</s>
<s>
Applying	O
the	O
Clang	B-Application
compiler	O
with	O
the	O
option	O
-Weverything	O
includes	O
unreachable	O
code	O
analysis	O
,	O
which	O
would	O
trigger	O
an	O
alarm	O
for	O
this	O
code	O
.	O
</s>
<s>
In	O
C++	B-Language
,	O
some	O
constructs	O
are	O
specified	O
to	O
have	O
undefined	B-Language
behavior	I-Language
.	O
</s>
<s>
A	O
compiler	O
is	O
free	O
to	O
implement	O
any	O
behavior	O
or	O
none	O
,	O
and	O
typically	O
an	O
optimizing	B-Application
compiler	I-Application
will	O
assume	O
the	O
code	O
is	O
unreachable	O
.	O
</s>
<s>
Java	B-Language
)	O
some	O
forms	O
of	O
unreachable	O
code	O
are	O
explicitly	O
disallowed	O
.	O
</s>
<s>
The	O
optimization	O
that	O
removes	O
unreachable	O
code	O
is	O
known	O
as	O
dead	B-Application
code	I-Application
elimination	O
.	O
</s>
<s>
Code	O
may	O
become	O
unreachable	O
as	O
a	O
consequence	O
of	O
transformations	O
performed	O
by	O
an	O
optimizing	B-Application
compiler	I-Application
(	O
e.g.	O
,	O
common	O
subexpression	O
elimination	O
)	O
.	O
</s>
<s>
Unreachable	O
code	O
elimination	O
technique	O
is	O
in	O
the	O
same	O
class	O
of	O
optimizations	O
as	O
dead	B-Application
code	I-Application
elimination	O
and	O
redundant	O
code	O
elimination	O
.	O
</s>
<s>
Profiling	O
in	O
general	O
can	O
not	O
prove	O
anything	O
about	O
the	O
unreachability	O
of	O
a	O
piece	O
of	O
code	O
,	O
but	O
may	O
be	O
a	O
good	O
heuristic	B-Algorithm
for	O
finding	O
potentially	O
unreachable	O
code	O
.	O
</s>
