== Your C compiler's optimizer can make your bad programs compile

Every so often I learn something by having something brought to my
awareness. Today's version is that one surprising side effect of
optimization in C compilers can be to make your program compile.
The story starts with [[John Regehr http://blog.regehr.org/]]'s
tweets:

> [[@johnregehr https://twitter.com/johnregehr/status/753091105298714625]]:
> tonight I am annoyed by: a program that both gcc and clang compile
> at -O2 and neither compiles at -O0 \\
>
> [[@johnregehr https://twitter.com/johnregehr/status/753092002208702465]]:
> easier to arrange than you might think \\
>
> void bar(void); \\
> static int x; \\
> int main(void) { if (x) bar(); }

(With the Fedora 23 versions of gcc 5.3.1 and clang 3.7.0, this will
fail to compile at -O0 but compile at -O1 or higher.)

I had to think about this for a bit before the penny dropped and I
realized where the problem was and why it happens this way. John
Regehr means that this is the whole program (there are no other
files), so _bar()_ is undefined. However, _x_ is always 0; it's
initialized to 0 by the rules of C, it's not visible outside this
file since it's _static_, and there's nothing here that creates a
path to change it.  With optimization, the compiler can thus turn
_main()_ into:

.pn prewrap on
>  int main(void) { if (0) bar(); }

This makes the call to _bar()_ unreachable, so the compiler removes
it. This leaves the program with no references to _bar()_, so it
can be linked even though _bar()_ is not defined anywhere.  Without
optimization, an attempt to call _bar()_ remains in the compiled
code (even though it will never be reached in execution) and then
linking fails with an error about 'undefined reference to `bar''.

(The optimized code that both gcc and clang generate boil down to
'_int main(void) { return 0; }_', as you'd expect. You can explore
the actual code through the very handy [[GCC/clang compiler explorer
https://gcc.godbolt.org/]].)

As reported on Twitter by Jed Davis, [[this can apparently happen
accidentally in real code
https://twitter.com/xlerb/status/753250715938586624]] (in this case
Firefox, with a C++ variant).