Question

When the same piece of code is compiled with optimizations fully disabled (g++ -O0) and then again with the optimizations fully enabled (g++ -O3), how can the logic of the source code itself be changed?

For example, compilers can

• unwind loops and
• do constant folding.

These two optimizations make the code perform faster without affecting the integrity of the original source code. Any code that runs without these optimizations will run with them enabled.

But, compiler optimizations can also affect code logic. Here are two examples that I know of:

• Removing copy constructors and assignment operators from temporaries may remove possible side-effects.
• Rearranging arithmetic containing floating point values may affect floating point error (hopefully requires -ffast-math argument).

I was very surprised, and lucky, to learn about these because they could become huge potential gotchas in the wrong situation.

So I want to know, are there any other cases where c++ compiler optimizations will affect code logic? I’m specifically looking for information about c++11 (without any undefined behaviour) under the g++ compiler, but tips for other compilers are welcome.

Answer 1

The "as-if" Rule:

An implementation is free to disregard any requirement of this
International Standard as long as the result is as if the requirement
had been obeyed, as far as can be determined from the observable
behavior of the program. For instance, an actual implementation need
not evaluate part of an expression if it can deduce that its value is
not used and that no side effects affecting the observable behavior of
the program are produced.

However, the standard mentions one optimization which is allowed, and which breaks the "as-if" rule:

When certain criteria are met, an implementation is allowed to omit
the copy/move construction of a class object, even if the copy/move
constructor and/or destructor for the object have side effects. In
such cases, the implementation treats the source and target of the
omitted copy/move operation as simply two different ways of referring
to the same object, and the destruction of that object occurs at the
later of the times when the two objects would have been destroyed
without the optimization.123 This elision of copy/move operations,
called copy elision, is permitted in the following circumstances
(which may be combined to eliminate multiple copies):

— in a return
statement in a function with a class return type, when the expression
is the name of a non-volatile automatic object (other than a function
or catch-clause parameter) with the same cvunqualified type as the
function return type, the copy/move operation can be omitted by
constructing the automatic object directly into the function’s return
value

— in a throw-expression, when the operand is the name of a
non-volatile automatic object (other than a function or catch-clause
parameter) whose scope does not extend beyond the end of the innermost
enclosing try-block (if there is one), the copy/move operation from
the operand to the exception object (15.1) can be omitted by
constructing the automatic object directly into the exception object

—
when a temporary class object that has not been bound to a reference
(12.2) would be copied/moved to a class object with the same
cv-unqualified type, the copy/move operation can be omitted by
constructing the temporary object directly into the target of the
omitted copy/move

— when the exception-declaration of an exception
handler (Clause 15) declares an object of the same type (except for
cv-qualification) as the exception object (15.1), the copy/move
operation can be omitted by treating the exception-declaration as an
alias for the exception object if the meaning of the program will be
unchanged except for the execution of constructors and destructors for
the object declared by the exception-declaration.