[cfe-dev] Optimizations around noexcept()

[Andrew Trick]

On Oct 11, 2013, at 7:50 AM, Nicola Gigante <nicola.gigante at gmail.com> wrote:

> Hello.
> 
> A couple of question regarding how clang can optimize code with the C++11 noexcept specification.
> 
> 1) If I understand correctly, if zero cost exceptions are used, the speed of the call should be the same
> even if the compiler does know that the function doesn't throw. However, does this information enable
> optimizations around the function call? Which kind of optimizations can be enabled with this assumption?

I won’t fully answer your question, but I can tell you that the LLVM IR optimizer can hoist a potentially unsafe load out of a loop if that loop contains readonly, nounwind call. However, if the call is not readonly (may modify memory), then there’s isn’t much the low-level optimizer can do.

If you have a try block or destructors that need to run, then the ability to throw an exception isn’t really zero-cost. I would expect to see a bigger performance difference in that case, although haven’t analyzed this myself.

-Andy

> 2) I suppose the noexcept specification is lowered to the nounwind LLVM attribute, correct me if this is wrong or
> incomplete. However, I've seen llvm correctly adding the nounwind attribute to functions that it can prove don't throw.
> Thus, is noexcept worth something in terms of performance (I'm not talking about maintainability here) if all the function code
> can be seen by the call site and nounwind can be inferred? In the case of a header only template library, where all the code
> can be seen in each compilation unit, this is the common case (but I suppose the same applies to any other kind of code if compiled
> with -O4).
> 
> Thank you very much,
> Nicola
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