Messages in this thread | | | Date | Sat, 12 Sep 2009 23:51:40 +0100 (BST) | From | "Maciej W. Rozycki" <> | Subject | Re: [PATCH] x86: apic: convert BUG() to BUG_ON() |
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On Sat, 12 Sep 2009, Daniel Walker wrote:
> For one it condenses duplicate code (i.e. the if()). If the BUG_ON() > macro gets updated with something new, all the users get the updates > automatically. The other thing is your re-using potentially more > advanced code that's inside the macro. In this case it's fairly trivial, > > #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0) > > So we're getting the benefit on the new "unlikely" in the apic code. > unlikely/likely calls will usually allow the compiler to create smaller, > and or, more optimized code.
For non-x86 platforms the use of the BUG_ON() macro may result in more efficient code GCC may not be able to optimise to with if (...) BUG();. For example the macro may expand to inline assembly with a conditional trap instruction GCC would not emit for an if () clause. While GCC does have a __builtin_trap() intrinsic that could be optimised if alone in a conditional block, such usage may not be frequent enough for a dedicated optimisation to be provided and build-time efficiency of the compiler does matter too, so such an optimisation might be of too questionable a value to incur an additional performance hit for the compiler.
Just a general note on patches of this kind, or to put it short, yes I agree it's a good idea.
Maciej
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