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- Dictionaryunlikely/ʌnˈlʌɪkli/
adjective
- 1. not likely to happen, be done, or be true; improbable: "an unlikely explanation" Similar Opposite
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The likely and unlikely macros or C++ [[likely]] / [[unlikely]] annotations can hint the compiler's branch layout to favour I-cache locality for the fast path, and minimize taken branches on the fast path. Also to hint the decision to make branchy vs. branchless asm when that's possible.
Aug 19, 2016 · Here we see one of the context factors: GCC spotted that it could re-use L2 here, so it decided to deem the final conditional unlikely so that it could emit less code. Lets look at the assembly of the example you gave: #define likely(x) __builtin_expect((x),1) #define unlikely(x) __builtin_expect((x),0)
Latest MSVC supports [[likely]] / [[unlikely]] in /std:c++20 and /std:c++latest modes. See demo on Godbolt's compiler explorer that shows the difference. As can be seen from the link above, one visible effect on x86/x64 for if-else statement is that the conditional jump forward will be for unlikely branch.
Apr 29, 2010 · I think likely and unlikely are mostly obsolete. Very cheap CPUs (ARM Cortex A20 in the example) have branch predictors and there is no penalty regardless of jump is taken / jump is not taken. When you introduce likely/unlikely the results will be either the same or worse (because compiler has generated more instructions).
Sep 28, 2016 · 44. Yes they can. In the Linux kernel, they are defined as. #define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0) The __builtin_expect macros are GCC specific macros that use the branch prediction; they tell the processor whether a condition is likely to be true, so that the processor can prefetch ...
Jun 19, 2014 · I know the kernel uses the likely and unlikely macros prodigiously. The docs for the macros are located at Built-in Function: long __builtin_expect (long exp, long c). But they don't really discuss the details. How exactly does a compiler handle likely(x) and __builtin_expect((x),1)?
Apr 7, 2023 · 24. It's meant to let the compiler know which path is the "fast path", as in "more likely to happen". For example, imagine implementing vector::at. This function throws if the index is out of bounds. But you expect this situation to happen very rarely, most of the time you expect the users to access a valid element.
Built-in Function: long __builtin_expect (long exp, long c) You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. However, there are ...
Jun 1, 2019 · In the original link, they have tested it for X86 and the assembly output is different if likely (in the if condition in above code) is replaced with unlikely, which shows the optimisation performed by compiler for branch prediction. But when I compile the above code for ARM (arm-gcc -O2), I don't see any difference in assembly code.
Apr 28, 2023 · The tag does anyway only imply some "arbitrary" preference in likelihood above/below the other paths. The number one guideline for using this kind of annotation is to measure the optimized performance difference. If you don't measure the before and after, you don't know if you've improved things or made them worse.
