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.
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.
--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.
The section is organized as follows:
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.
The encoding is specified in BoltAddressTranslation.h and BoltAddressTranslation.cpp.
The general layout is as follows:
Hot functions table header |------------------| | Function entry | | |--------------| | | | OutOff InOff | | | |--------------| | ~~~~~~~~~~~~~~~~~~~~ Cold functions table header |------------------| | Function entry | | |--------------| | | | OutOff InOff | | | |--------------| | ~~~~~~~~~~~~~~~~~~~~
Hot and cold functions tables share the encoding except difference 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 |
Function header is followed by NumEntries pairs of offsets for current function.
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 | 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).