likely/unlikely

Many people ask a question about the last article: what is the [[unlikely]] attribute? This attribute is taken from a draft C++20 standard: https://en.cppreference.com/w/cpp/language/attributes/likely

It tells the compiler which branch of the condition or case in the switch statement is executed more often.

On x86/x64, the compiler will place the likely branch in assembler immediately after checking the condition, maximally utilizing the processor's instruction cache. There is no static branch prediction here. However, I won’t be surprised if in some PowerPC architecture there will be a special instruction to help the branch predictor.

There is no support for likely/unlikely attributes in compilers yet, but there are extensions:

#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

An example (only Clang and GCC), where we expect IsValid() to return true more often:

bool IsValid();
int Bar();
int Baz();

int Foo() {
  if (likely(IsValid()))
    return Bar();
  else
    return Baz();
}

Assembler:

Foo():  # @Foo()
  push    rax
  call    IsValid()
  test    al, al
  je      .LBB0_2
  pop     rax
  jmp     Bar()  # TAILCALL
 .LBB0_2:
  pop     rax
  jmp     Baz()  # TAILCALL

An example where we expect IsValid() to return false more often:

bool IsValid();
int Bar();
int Baz();

int Foo() {
  if (unlikely(IsValid()))
    return Bar();
  else
    return Baz();
}

Assembler:

 Foo():  # @Foo()
  push    rax
  call    IsValid()
  test    al, al
  jne     .LBB0_1
  pop     rax
  jmp     Baz()  # TAILCALL
 .LBB0_1:
  pop     rax
  jmp     Bar()  # TAILCALL

You may notice that the compiler puts a branch of code corresponding to our help immediately after the checking (test command). This can potentially give good acceleration if everything is marked correctly (notice that you can give an incorrect hint).

You also need to keep in mind that now processors are very well at branch prediction, so the difference can be very small.

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