Re: missing optimization - don't compute return value not used?

["Aaron W. LaFramboise" <aaronavay62 at aaronwl dot com>]

Richard Li wrote:

> Right, page 211 of the C++ standard (2003) explains when copy-ctor and
> dtor are allowed to be optimized away. But the two circumstances are
> both like this:
> A is constructed; A is copy-constructed to B; A is destructed
> Here A is a temporary object in some sense, and the standard allows
> for directly constructing B.
> However, Neal expected the compiler to optimize "A is constructed; A
> is destructed" away. I find nowhere in the standard that allows this.

I think it would fall under the "as-if" rule.  The special rules you 
mention are only necessary if it causes a behavior change.

I think the biggest problem here is that GCC will not elide calls to the 
allocator.  This is a subject of some controversy--even though its 
probably difficult to do such optimization anyway.  It's not quite clear 
that such optimization is legal under the as-if rule.

For example, the code you have written will throw std::bad_alloc if your 
system is out of memory.  The code that you would like it to be 
optimized to would not do that, which is a behavior change.

This has been discussed a few times before.  See, for example, this 
thread <http://gcc.gnu.org/ml/gcc-patches/2003-11/msg00564.html>.

So, you should probably never count on an optimizer removing calls to 
anything that touches memory management.  What's usually done, as 
Richard Li points out, is to write your code in such a way that such 
optimization is not necessary.