I was going through this link Will it optimize and wondered how can we know what optimizations are done by a particular compiler.

Like does VC8.0 convert if-else statements to switch-case? Is such information available on msdn?


First of all, if optimization took place then your program should work faster usually. After that you could inspect disassembly code to find out what kind of optimizations were performed.

Kirill V. Lyadvinsky

I don't know anything about VC8.0, so I'm not sure how you would access that information. However, if you are generally interested in the kinds of optimisations that go on and want to experiment, I recommend you use LLVM. You can look at the unoptimised, disassembled byte code generated from the default C front end, and then run various optimiser passes over it to see what the effect is each time. Because it's a nicer, abstract assembly code, it tends to be a little easier to figure out what is an optimisation derivable from the code and what is a machine-specific optimisation.

+1  A: 

It depends on the level of of optimization you choose for compiler.

you can find a very nice article about it here


Like does VC8.0 convert if-else statements to switch-case?

Compilers do not do magically rewrite your source code. And even if they did, what would that tell you? What you really would want to know is if the compiler compiled it into a jump table or into multiple compare operations. Any dis-assembler will tell you that.

To clarify my point: Writing a switch-case statement does not necesseraly imply that there will be a jump table in the binary. Not needing to worry about this is the whole point of having compilers.


Instead of figuring out which optimizations are done by the compiler in general, it's probably better to NOT have any dependencies on such compiler-specific knowledge.

Instead start out with a good design and algorithm, writing (as much as possible) portable code that's easy to follow. Then profile the code if it's too slow and fix the actual hotspots. Compiler optimizations are useful no doubt, but better is to apply some investigation to what's actually happening in the code. Algorithmic/design improvements at the source level will typically help performance more than the presence or absence of optimizations like transforming if/else into switch-case.

Mark B
+3  A: 

As everyone seems to be bent on telling the OP that he shouldn't worry about it, there is some useful although not as specific as the OP requested) information about compiler optimization (options).

You'll have to figure out what flags you're using, especially for MSVC and Intel (GCC release build should default to -O2), but here are the links:

This is about as close as you'll get before disassembling your binary after compilation.


I'm not sure what "convert if/else to switch/case" means. My processor doesn't have a hardware switch/case instruction.

Typical compilers have several different ways to implement switch/case. A well-known one is using a jump table, but this is only done if appropriate.

For if/else, certainly it is normal for compilers to analyse a digraph of execution flow. I would expect a compiler to notice if each condition references the same variable, and I would expect the compiler to treat equivalent forms of conditionals the same way in general. But this isn't something I'd worry about.

IIRC, the general policy in GCC is that regressions in optimisation are tolerable so long as preferred improvements result. Optimisation is complex and what is "generally" a good optimisation isn't always that great. Plus for perfect optimisation, the compiler would have to know things it can't know (e.g. what inputs it will encounter in real life).

The point is that it really isn't worthwhile knowing that much about specific optimisations unless you happen to be a compiler developer. If you depend on something being optimised by V8, that particular optimisation might not happen in V9 or V10.