llvm-project/bolt/docs/BAT.md

BOLT Address Translation (BAT)

Purpose

A regular profile collection for BOLT involves collecting samples from unoptimized binary. BOLT Address Translation allows collecting profile from BOLT-optimized binary and using it for optimizing the input (pre-BOLT) binary.

Overview

BOLT Address Translation is an extra section (.note.bolt_bat) inserted by BOLT into the output binary containing translation tables and split functions linkage information. This information enables mapping the profile back from optimized binary onto the original binary.

Usage

--enable-bat flag controls the generation of BAT section. Sampled profile needs to be passed along with the optimized binary containing BAT section to perf2bolt which reads BAT section and produces fdata profile for the original binary. Note that YAML profile generation is not supported since BAT doesn't contain the metadata for input functions.

Internals

Section contents

The section is organized as follows:

• Hot functions table
  • Address translation tables
• Cold functions table

Construction and parsing

BAT section is created from BoltAddressTranslation class which captures address translation information provided by BOLT linker. It is then encoded as a note section in the output binary.

During profile conversion when BAT-enabled binary is passed to perf2bolt, BoltAddressTranslation class is populated from BAT section. The class is then queried by DataAggregator during sample processing to reconstruct addresses/ offsets in the input binary.

Encoding format

The encoding is specified in BoltAddressTranslation.h and BoltAddressTranslation.cpp.

Layout

The general layout is as follows:

Hot functions table header
|------------------|
|  Function entry  |
| |--------------| |
| | OutOff InOff | |
| |--------------| |
~~~~~~~~~~~~~~~~~~~~

Cold functions table header
|------------------|
|  Function entry  |
| |--------------| |
| | OutOff InOff | |
| |--------------| |
~~~~~~~~~~~~~~~~~~~~

Functions table

Hot and cold functions tables share the encoding except differences marked below. Header:

Entry	Encoding	Description
NumFuncs	ULEB128	Number of functions in the functions table

The header is followed by Functions table with NumFuncs entries. Output binary addresses are delta encoded, meaning that only the difference with the last previous output address is stored. Addresses implicitly start at zero. Output addresses are continuous through function start addresses and function internal offsets, and between hot and cold fragments, to better spread deltas and save space.

Hot indices are delta encoded, implicitly starting at zero.

Entry	Encoding	Description
Address	Continuous, Delta, ULEB128	Function address in the output binary
HotIndex	Delta, ULEB128	Cold functions only: index of corresponding hot function in hot functions table
NumEntries	ULEB128	Number of address translation entries for a function
EqualElems	ULEB128	Hot functions only: number of equal offsets in the beginning of a function
BranchEntries	Bitmask, alignTo(EqualElems, 8) bits	Hot functions only: if EqualElems is non-zero, bitmask denoting entries with BRANCHENTRY bit

Function header is followed by EqualElems offsets (hot functions only) and NumEntries-EqualElems (NumEntries for cold functions) pairs of offsets for current function.

Address translation table

Delta encoding means that only the difference with the previous corresponding entry is encoded. Input offsets implicitly start at zero.

Entry	Encoding	Description
OutputOffset	Continuous, Delta, ULEB128	Function offset in output binary
InputOffset	Optional, Delta, SLEB128	Function offset in input binary with BRANCHENTRY LSB bit

BRANCHENTRY bit denotes whether a given offset pair is a control flow source (branch or call instruction). If not set, it signifies a control flow target (basic block offset). InputAddr is omitted for equal offsets in input and output function. In this case, BRANCHENTRY bits are encoded separately in a BranchEntries bitvector.