C++ : In how many ways compiler optimizes away our code?

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I w开发者_高级运维ould like to know all the possible ways (or atleast the popular ones) in which compilers can/do optimize away our code written in C++? I also would like to know how exactly the optimization is done (in each case)!

So far, I'm aware of two optimizations, viz. Empty Base Optimization (EBO) and Return Value Optimization (RVO). What else are there? I've heard of "const" optimization,"unused variable" optimization. What are they?

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All possible ways? Surely you're joking. For that, look through years of compiler research and practice.

For concrete examples, look up each of the options here: http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

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From Standard docs., Section 1.9,

4) This provision is sometimes called the “as-if” rule, because 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.

So actually the standard compliant compiler can perform any kind of optimizations as long as it produces the desired result.

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Incredibly broad, because there are many optimizations and compiler writers are always thinking up more. There are tons of them, some optimizing for run time, others optimizing for binary size. Many aren't specifically C++ either, the general compiler optimization techniques are implemented for many compilers/interpreters for many different languages.

Just a handful:

• Dead code removal
• Loop unrolling
• Constants folding

More info:

• Optimizing C++ Applications with Intel C++ Compiler
• New Optimizations in VC++ 2010
• Generic Compiler Optimizations Wiki