Search results
- Dictionaryunlikely/ʌnˈlʌɪkli/
adjective
- 1. not likely to happen, be done, or be true; improbable: "an unlikely explanation" Similar Opposite
Powered by Oxford Dictionaries
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.
Aug 19, 2016 · x(int, int*): testq %rsi, %rsi je .L3 # first branch: assumed unlikely movl $2, %eax testl %edi, %edi jne .L12 # second branch: assumed likely ret .L12: pushq %rbx movq %rsi, %rbx call s(int) movl %eax, %edx movl $3, %eax testl %edx, %edx je .L13 # third branch: assumed likely .L2: popq %rbx ret .L3: movl $1, %eax ret .L13: movq %rbx, %rdi call t(int*) movl %eax, %edx movl $4, %eax testl %edx ...
GCC compiler supports __builtin_expect statement that is used to define likely and unlikely macros. eg ...
Sep 28, 2016 · #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 instructions on the correct "side" of the 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).
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 28, 2023 · What is the general guideline for using the [[likely]] and [[unlikely]] attributes in c++20, for the cases where we have only two possible branches of code execution? Is it advised to put [[likely]] or [[unlikely]] on just one branch of execution path or we should be putting one of it on one code branch and other one on the remaining branch.
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.
Apr 7, 2023 · 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. You can mark the throwing path [[unlikely]] and the compiler will understand your intention and might optimize the program accordingly.
May 9, 2015 · #define likely(x) __builtin_expect (!!(x), 1) #define unlikely(x) __builtin_expect (!!(x), 0) just to ease the task. Mind that: this is non standard; a compiler/cpu branch predictor are likely more skilled than you in deciding such things so this could be a premature micro-optimization
