1b763f230a
Provide secondary entry points for `EntryDiscriminator` call info field in YAML profile. Increases BAT section size to: - large binary: 39655300 bytes (1.03x the original), - medium binary: 3834328 bytes (0.65x), - small binary: 924 bytes (0.64x). Depends on: https://github.com/llvm/llvm-project/pull/76911 Test Plan: - Updated bolt-address-translation{,-yaml}.test - Added openssl test: https://github.com/rafaelauler/bolt-tests/pull/30 Reviewers: dcci, rafaelauler, maksfb, ayermolo Reviewed By: rafaelauler Pull Request: https://github.com/llvm/llvm-project/pull/86218
117 lines
5.0 KiB
Markdown
117 lines
5.0 KiB
Markdown
# BOLT Address Translation (BAT)
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# Purpose
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A regular profile collection for BOLT involves collecting samples from
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unoptimized binary. BOLT Address Translation allows collecting profile
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from BOLT-optimized binary and using it for optimizing the input (pre-BOLT)
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binary.
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# Overview
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BOLT Address Translation is an extra section (`.note.bolt_bat`) inserted by BOLT
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into the output binary containing translation tables and split functions linkage
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information. This information enables mapping the profile back from optimized
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binary onto the original binary.
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# Usage
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`--enable-bat` flag controls the generation of BAT section. Sampled profile
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needs to be passed along with the optimized binary containing BAT section to
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`perf2bolt` which reads BAT section and produces profile for the original
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binary.
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# Internals
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## Section contents
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The section is organized as follows:
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- Hot functions table
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- Address translation tables
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- Cold functions table
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## Construction and parsing
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BAT section is created from `BoltAddressTranslation` class which captures
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address translation information provided by BOLT linker. It is then encoded as a
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note section in the output binary.
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During profile conversion when BAT-enabled binary is passed to perf2bolt,
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`BoltAddressTranslation` class is populated from BAT section. The class is then
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queried by `DataAggregator` during sample processing to reconstruct addresses/
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offsets in the input binary.
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## Encoding format
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The encoding is specified in
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[BoltAddressTranslation.h](/bolt/include/bolt/Profile/BoltAddressTranslation.h)
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and [BoltAddressTranslation.cpp](/bolt/lib/Profile/BoltAddressTranslation.cpp).
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### Layout
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The general layout is as follows:
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```
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Hot functions table
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Cold functions table
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Functions table:
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|------------------|
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| Function entry |
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| Address |
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| translation |
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| table |
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| Secondary entry |
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| points |
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|------------------|
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```
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### Functions table
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Hot and cold functions tables share the encoding except differences marked below.
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Header:
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| Entry | Encoding | Description |
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| ------ | ----- | ----------- |
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| `NumFuncs` | ULEB128 | Number of functions in the functions table |
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The header is followed by Functions table with `NumFuncs` entries.
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Output binary addresses are delta encoded, meaning that only the difference with
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the last previous output address is stored. Addresses implicitly start at zero.
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Output addresses are continuous through function start addresses and function
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internal offsets, and between hot and cold fragments, to better spread deltas
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and save space.
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Hot indices are delta encoded, implicitly starting at zero.
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| Entry | Encoding | Description | Hot/Cold |
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| ------ | ------| ----------- | ------ |
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| `Address` | Continuous, Delta, ULEB128 | Function address in the output binary | Both |
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| `HotIndex` | Delta, ULEB128 | Index of corresponding hot function in hot functions table | Cold |
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| `FuncHash` | 8b | Function hash for input function | Hot |
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| `NumBlocks` | ULEB128 | Number of basic blocks in the original function | Hot |
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| `NumSecEntryPoints` | ULEB128 | Number of secondary entry points in the original function | Hot |
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| `NumEntries` | ULEB128 | Number of address translation entries for a function | Both |
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| `EqualElems` | ULEB128 | Number of equal offsets in the beginning of a function | Hot |
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| `BranchEntries` | Bitmask, `alignTo(EqualElems, 8)` bits | If `EqualElems` is non-zero, bitmask denoting entries with `BRANCHENTRY` bit | Hot |
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Function header is followed by *Address Translation Table* with `NumEntries`
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total entries, and *Secondary Entry Points* table with `NumSecEntryPoints`
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entries (hot functions only).
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### Address translation table
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Delta encoding means that only the difference with the previous corresponding
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entry is encoded. Input offsets implicitly start at zero.
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| Entry | Encoding | Description | Branch/BB |
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| ------ | ------| ----------- | ------ |
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| `OutputOffset` | Continuous, Delta, ULEB128 | Function offset in output binary | Both |
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| `InputOffset` | Optional, Delta, SLEB128 | Function offset in input binary with `BRANCHENTRY` LSB bit | Both |
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| `BBHash` | Optional, 8b | Basic block hash in input binary | BB |
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| `BBIdx` | Optional, Delta, ULEB128 | Basic block index in input binary | BB |
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For hot fragments, the table omits the first `EqualElems` input offsets
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where the input offset equals output offset.
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`BRANCHENTRY` bit denotes whether a given offset pair is a control flow source
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(branch or call instruction). If not set, it signifies a control flow target
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(basic block offset).
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`InputAddr` is omitted for equal offsets in input and output function. In this
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case, `BRANCHENTRY` bits are encoded separately in a `BranchEntries` bitvector.
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### Secondary Entry Points table
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The table is emitted for hot fragments only. It contains `NumSecEntryPoints`
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offsets denoting secondary entry points, delta encoded, implicitly starting at zero.
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| Entry | Encoding | Description |
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| ----- | -------- | ----------- |
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| `SecEntryPoint` | Delta, ULEB128 | Secondary entry point offset |
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