Unlikely

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Ezio 2 7 This really isn't specific to the Linux kernel or about macros, but a compiler optimization. Should this be retagged to reflect that? The paper What every Programmer should know about Memory p. There's no portability issue. You can trivially do things like define likely x x and define unlikely x x on platforms that don't support this kind of hinting. These macros mostly were used for error checking. Because error leaves less probably then normal operation. A few people make profiling or calculation to decide most used leaf As regards the fragment "[ In a tight loop, and assuming a branch goes one way most of the time, the branch predictor will likely begin guessing the correct branch very quickly.

What really happens is the compiler arranges the branches so the common case is the not-taken one. This is faster even when branch prediction does work. Taken branches are problematic for instruction-fetch and decode even when they're predicted perfectly. Some CPUs statically predict branches that aren't in their history table, usually with assume not-taken for forward branches. Intel CPUs don't work that way: A hot branch and a cold branch might alias the same entry This answer is mostly obsolete since the main claim is that it helps branch prediction, and as PeterCordes points out, in most modern hardware there is no implicit or explicit static branch prediction.

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In fact the hint is used by the compiler to optimize the code, whether that involves static branch hints, or any other type of optimization. For most architectures today, it is the "any other optimization" that matters, e. BeeOnRope because of cache prefetch and word size, there is still an advantage to running a program linearly. The next memory location will already be fetched and in cache, the branch target maybe or maybe not. With a 64 bit CPU you grab at least 64 bits at a time.

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Depending on DRAM interleave, it may be 2x 3x or more bits that get grabbed. Also, it impacts icache footprint - by keeping unlikely snippets of code out of the hot path. More precisely, it can do it with goto s without repeating the return x: Let's decompile to see what GCC 4. So it is basically the same as: For the record, x86 does take additional space for branch hints. You have to have a one-byte prefix on branches to specify the appropriate hint. Agreed that hinting is a Good Thing TM , though.

All other x86 CPUs simply ignore those prefixes because prefixes are always ignored in contexts where they're meaningless. These macros don't cause gcc to actually emit branch-hint prefixes on x They do help you get gcc to lay out your function with fewer taken branches on the fast-path. In almost all cases will it be used in the context of boolean expressions in which case it is much more convenient to define two helper macros: Ashish Maurya 61 3.

You just won't get the benefit of the optimization if you're on other platforms. You don't use portability - the platforms that don't support them just define them to expand to empty strings.

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I think you two are actually agreeing with each other -- it's just phrased confusingly. From the looks of it, Andrew's comment is saying "you can use them without losing portability" but sharptooth thought that he said "don't use them as they're not portable" and objected. And another opinion, unlikely is more useful rather than likely , because if likely And by the way, if you want to observe the detail behavior of the code, you can do simply as follow: Winter 1, 5 14 It will try to limit code size in unlikely regions by avoiding inlining and loop unrolling, though.

How the branch instructions are built are dependent upon the processor architecture.

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