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ea49781571
Summary: The LLVM Bitcode File Format documentation states that all bitstreams begin with the magic number 'BC', and that generic bitstream analyzer tools may check for this number in order to determine whether the stream is a bitstream. However, in practice: * Only LLVM IR bitcode begins with 'BC'. Other bitstreams -- Clang AST files and precompiled headers, Clang serialized diagnostics, Swift modules -- do not start with 'BC'. A tool that actually checked for 'BC' would only be able to recognize LLVM IR. * The `llvm-bcanalyzer`, arguably the most used generic bitstream analyzer tool, does not check for a magic number 'BC' (except to determine whether the file is LLVM IR). Update the bitcode format documentation to make it clear that not all bitstreams begin with 'BC', and that tools should not rely on that particular magic number value. Test Plan: Build the `docs-llvm-html` target and confirm the changes render in a Safari web browser. Reviewers: harlanhaskins, eugenis, mehdi_amini, pcc, angerman Reviewed By: angerman Subscribers: angerman, llvm-commits Differential Revision: https://reviews.llvm.org/D42002 llvm-svn: 322520
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