# LLVM Branch Weight Metadata

1. Introduction
2. Supported Instructions
3. Built-in "expect" Instruction
4. CFG Modifications

Written by Jakub Staszak

## Introduction

Branch Weight Metadata represents branch weights as its likeliness to be taken. Metadata is assigned to the `TerminatorInst` as a `MDNode` of the `MD_prof` kind. The first operator is always a `MDString` node with the string "branch_weights". Number of operators depends on the terminator type.

Branch weights might be fetch from the profiling file, or generated based on `__builtin_expect` instruction.

All weights are represented as an unsigned 32-bit values, where higher value indicates greater chance to be taken.

## Supported Instructions

#### BranchInst

Metadata is only assign to the conditional branches. There are two extra operarands, for the true and the false branch.

    !0 = metadata !{
      metadata !"branch_weights",
      i32 <TRUE_BRANCH_WEIGHT>,
      i32 <FALSE_BRANCH_WEIGHT>
    }

#### SwitchInst

Branch weights are assign to every case (including `default` case which is always case #0).

    !0 = metadata !{
      metadata !"branch_weights",
      i32 <DEFAULT_BRANCH_WEIGHT>
      [ , i32 <CASE_BRANCH_WEIGHT> ... ]
    }

#### IndirectBrInst

Branch weights are assign to every destination.

    !0 = metadata !{
      metadata !"branch_weights",
      i32 <LABEL_BRANCH_WEIGHT>
      [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
    }

#### Other

Other terminator instructions are not allowed to contain Branch Weight Metadata.

## Built-in "expect" Instructions

`__builtin_expect(long exp, long c)` instruction provides branch prediction information. The return value is the value of `exp`.

It is especially useful in conditional statements. Currently Clang supports two conditional statements:

#### `if` statement

The `exp` parameter is the condition. The `c` parameter is the expected comparision value. If it is equal to 1 (true), the condition is likely to be true, in other case condition is likely to be false. For example:

      if (__builtin_expect(x > 0, 1)) {
        // This block is likely to be taken.
      }

#### `switch` statement

The `exp` parameter is the value. The `c` parameter is the expected value. If the expected value doesn't show on the cases list, the `default` case is assumed to be likely taken.

      switch (__builtin_expect(x, 5)) {
      default: break;
      case 0:  // ...
      case 3:  // ...
      case 5:  // This case is likely to be taken.
      }

## CFG Modifications

Branch Weight Metatada is not proof against CFG changes. If terminator operands' are changed some action should be taken. In other case some misoptimizations may occur due to incorrent branch prediction information.