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BOLT uses MCAsmLayout to calculate the output values of basic blocks. This means output values are calculated based on a pre-linking state and any changes to symbol values during linking will cause incorrect values to be used. This issue was first addressed in D154604 by adding all basic block symbols to the symbol table for the linker to resolve them. However, the runtime overhead of handling this huge symbol table turned out to be prohibitively large. This patch solves the issue in a different way. First, a temporary section containing [input address, output symbol] pairs is emitted to the intermediary object file. The linker will resolve all these references so we end up with a section of [input address, output address] pairs. This section is then parsed and used to: - Replace BinaryBasicBlock::OffsetTranslationTable - Replace BinaryFunction::InputOffsetToAddressMap - Update BinaryBasicBlock::OutputAddressRange Note that the reason this is more performant than the previous attempt is that these symbol references do not cause entries to be added to the symbol table. Instead, section-relative references are used for the relocations. Reviewed By: maksfb Differential Revision: https://reviews.llvm.org/D155604
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