LLVM Branch Weight Metadata

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 assigned 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 assigned to every case (including the default case which is always case #0).

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

IndirectBrInst

Branch weights are assigned 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 comparison 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.