On Thu, Apr 16, 2009 at 09:07:58PM -0700, Shameem Ahamed wrote: > Is there any advantage of using switch-case over if-else. I mean any internal > optimizations, gcc can do on a Linux i386 machine?.
The optimizations in question are architecture-independent, though there would undoubtedly be processor-specific weights. Given a switch statement, gcc will generate either a balanced binary tree or a jump table, depending on the number of branches and their density. It has some freedom to optimize this structure that it might not have for an if-then-else structure. But I think that the difference is only going to be significant for a large switch (with many branches); if there are few branches, the jump table won't be a win (so won't be chosen), and the balanced tree would be about the same as what you would write. I would say that if a switch statement is a natural way to code something, it would be wise to prefer it to if-then-else if there are four or more branches (I admit I have no hard justification for the "four" here); for fewer I'd make the decision based on clarity and maintainability. Switch statements also give compilers more freedom to rearrange based on profile-directed optimization. There was a GCC Summit paper on improving GCC's code generation for switch statements, see http://ols.fedoraproject.org/GCC/Reprints-2006/wienskoski-reprint.pdf I don't know how much of that work got into the compiler, probably it isn't in the 4.2.x version we're using now.
