|
|
|
|
|
|
by DSrcl
3185 days ago
|
|
|
more sensical approach to analysing this program, e.g. as employed by a human, would be to see that NeverCalled() is the only function that can write Do
It is not only non-trivial but also impossible (even) for a human to see if NeverCalled is ever called. Since NeverCalled is visible to other translation units, it can be called, for instance, by a global constructor before main is called.Adapting thinkings such as "I can't figure out how to optimise this, so I'll do the simplest thing that works."
will disable the compiler from many non-trivial optimizations. Consider eliminating a branch predicated on `w < w + c`: this expression can be, a check for signed-integer-overflow, coming from a macro-expansion and expected by the programmer to be eliminated, a result of constant-progagation, or a result of other optimizations.Saying you have ideas but don't have time to make a better compiler only makes you an armchair compiler-writer. |
|
|
The compiler is also doing the linking in this case, being given the complete command line, and it very much knows all the "translation units" which will be present, and none of them call that function.
Consider eliminating a branch predicated on `w < w + c`: this expression can be, a check for signed-integer-overflow, coming from a macro-expansion and expected by the programmer to be eliminated, a result of constant-progagation, or a result of other optimizations.
A bit of range analysis, like what an intelligent human would do, suffices to determine whether that expression will be a constant. If all the terms of the expression are constant, then so will the result be; and the sane thing to do, in the face of incomplete information, is to assume that a variable can take on any value.
https://en.wikipedia.org/wiki/Value_range_analysis