================ @@ -0,0 +1,97 @@ +# 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: +- Functions table + - Address translation tables +- Fragment linkage 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](/bolt/include/bolt/Profile/BoltAddressTranslation.h) +and [BoltAddressTranslation.cpp](/bolt/lib/Profile/BoltAddressTranslation.cpp). + +### Layout +The general layout is as follows: +``` +Functions table header +|------------------| +| Function entry | +| |--------------| | +| | OutOff InOff | | +| |--------------| | +~~~~~~~~~~~~~~~~~~~~ + +Fragment linkage header +|------------------| +| ColdAddr HotAddr | +~~~~~~~~~~~~~~~~~~~~ +``` + +### Functions table +Header: +| Entry | Width | Description | +| ------ | ----- | ----------- | +| `NumFuncs` | 4b | Number of functions in the functions table | ---------------- rafaelauler wrote:
Minor suggestion: 4b -> 4B I usually see Bytes capitalized, but that's up to you https://github.com/llvm/llvm-project/pull/76898 _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
