Why don't modern C++ compilers optimize away simple loops like this? (Clang, MSVC)

Question

When I compile and run this code with Clang (-O3) or MSVC (/O2)...

#include 
#include 

static int con         


        

Answer 1

It's an interesting issue with regards to optimizing. I would expect that in most cases, the compiler would treat each element of the array as an individual variable when doing dead code analysis. Ans 0x8000 make too many individual variables to track, so the compiler doesn't try. The fact that a[j] doesn't always access the the same object could cause problems as well for the optimizer.

Obviously, different compilers use different heuristics; a compiler could treat the array as a single object, and detect that it never affected output (observable behavior). Some compilers may choose not to, however, on the grounds that typically, it's a lot of work for very little gain: how often would such optimizations be applicable in real code?