On Feb 6, 2007, at 5:06 PM, Joe Buck wrote:
On Tue, Feb 06, 2007 at 04:14:30PM -0800, Ian Lance Taylor wrote:
I also think it would be good to have one option affecting it: turn
__builtin_unreachable() into an abort(), or turn it into a "cannot be
reached" marker. I think the former should be the default at -O0,
the
latter at -O1 and above.
Note that a "cannot be reached" marker permits further
optimization to
delete code which leads up to it. Actually implementing that further
optimization may call for adding appropriate warnings, probably
controlled by -Wunreachable-code.
I have experience with this approach, it works well:
http://llvm.org/docs/LangRef.html#i_unreachable
In the case that motivated this discussion, the instruction that would
immediately go before the __builtin_unreachable() is a trap
instruction,
but the compiler does not know that it traps. It would be very bad
if the compiler eliminated the trap, since it is the presence of the
trap that keeps the function from returning.
Nothing with side effects before an 'unreachable' can be removed,
including an asm. Consider if the asm called a no-return function.
-Chris
