forked from OSchip/llvm-project
266 lines
11 KiB
Markdown
266 lines
11 KiB
Markdown
The PE/COFF Linker
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==================
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This directory contains a linker for Windows operating system.
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Because the fundamental design of this port is different from
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the other ports of LLD, this port is separated to this directory.
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The linker is command-line compatible with MSVC linker and is
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generally 2x faster than that. It can be used to link real-world
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programs such as LLD itself or Clang, or even web browsers which
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are probably the largest open-source programs for Windows.
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This document is also applicable to ELF linker because the linker
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shares the same design as this COFF linker.
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Overall Design
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--------------
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This is a list of important data types in this linker.
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* SymbolBody
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SymbolBody is a class for symbols. They may be created for symbols
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in object files or in archive file headers. The linker may create
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them out of nothing.
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There are mainly three types of SymbolBodies: Defined, Undefined, or
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Lazy. Defined symbols are for all symbols that are considered as
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"resolved", including real defined symbols, COMDAT symbols, common
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symbols, absolute symbols, linker-created symbols, etc. Undefined
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symbols are for undefined symbols, which need to be replaced by
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Defined symbols by the resolver. Lazy symbols represent symbols we
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found in archive file headers -- which can turn into Defined symbols
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if we read archieve members, but we haven't done that yet.
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* Symbol
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Symbol is a pointer to a SymbolBody. There's only one Symbol for
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each unique symbol name (this uniqueness is guaranteed by the symbol
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table). Because SymbolBodies are created for each file
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independently, there can be many SymbolBodies for the same
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name. Thus, the relationship between Symbols and SymbolBodies is 1:N.
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The resolver keeps the Symbol's pointer to always point to the "best"
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SymbolBody. Pointer mutation is the resolve operation in this
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linker.
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SymbolBodies have pointers to their Symbols. That means you can
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always find the best SymbolBody from any SymbolBody by following
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pointers twice. This structure makes it very easy to find
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replacements for symbols. For example, if you have an Undefined
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SymbolBody, you can find a Defined SymbolBody for that symbol just
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by going to its Symbol and then to SymbolBody, assuming the resolver
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have successfully resolved all undefined symbols.
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* Chunk
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Chunk represents a chunk of data that will occupy space in an
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output. Each regular section becomes a chunk.
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Chunks created for common or BSS symbols are not backed by sections.
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The linker may create chunks out of nothing to append additional
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data to an output.
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Chunks know about their size, how to copy their data to mmap'ed
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outputs, and how to apply relocations to them. Specifically,
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section-based chunks know how to read relocation tables and how to
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apply them.
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* SymbolTable
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SymbolTable is basically a hash table from strings to Symbols, with
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a logic to resolve symbol conflicts. It resolves conflicts by symbol
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type. For example, if we add Undefined and Defined symbols, the
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symbol table will keep the latter. If we add Defined and Lazy
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symbols, it will keep the former. If we add Lazy and Undefined, it
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will keep the former, but it will also trigger the Lazy symbol to
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load the archive member to actually resolve the symbol.
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* OutputSection
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OutputSection is a container of Chunks. A Chunk belongs to at most
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one OutputSection.
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There are mainly three actors in this linker.
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* InputFile
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InputFile is a superclass of file readers. We have a different
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subclass for each input file type, such as regular object file,
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archive file, etc. They are responsible for creating and owning
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SymbolBodies and Chunks.
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* Writer
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The writer is responsible for writing file headers and Chunks to a
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file. It creates OutputSections, put all Chunks into them, assign
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unique, non-overlapping addresses and file offsets to them, and then
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write them down to a file.
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* Driver
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The linking process is drived by the driver. The driver
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- processes command line options,
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- creates a symbol table,
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- creates an InputFile for each input file and put all symbols in it
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into the symbol table,
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- checks if there's no remaining undefined symbols,
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- creates a writer,
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- and passes the symbol table to the writer to write the result to a
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file.
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Performance
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-----------
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It's generally 2x faster than MSVC link.exe. It takes 3.5 seconds to
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self-host on my Xeon 2580 machine. MSVC linker takes 7.0 seconds to
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link the same executable. The resulting output is 65MB.
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The old LLD is buggy that it produces 120MB executable for some reason,
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and it takes 30 seconds to do that.
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We believe the performance difference comes from simplification and
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optimizations we made to the new port. Notable differences are listed
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below.
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* Reduced number of relocation table reads
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In the old design, relocation tables are read from beginning to
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construct graphs because they consist of graph edges. In the new
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design, they are not read until we actually apply relocations.
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This simplification has two benefits. One is that we don't create
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additional objects for relocations but instead consume relocation
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tables directly. The other is that it reduces number of relocation
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entries we have to read, because we won't read relocations for
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dead-stripped COMDAT sections. Large C++ programs tend to consist of
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lots of COMDAT sections. In the old design, the time to process
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relocation table is linear to size of input. In this new model, it's
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linear to size of output.
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* Reduced number of symbol table lookup
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Symbol table lookup can be a heavy operation because number of
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symbols can be very large and each symbol name can be very long
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(think of C++ mangled symbols -- time to compute a hash value for a
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string is linear to the length.)
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We look up the symbol table exactly only once for each symbol in the
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new design. This is I believe the minimum possible number. This is
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achieved by the separation of Symbol and SymbolBody. Once you get a
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pointer to a Symbol by looking up the symbol table, you can always
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get the latest symbol resolution result by just dereferencing a
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pointer. (I'm not sure if the idea is new to the linker. At least,
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all other linkers I've investigated so far seem to look up hash
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tables or sets more than once for each new symbol, but I may be
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wrong.)
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* Reduced number of file visits
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The symbol table implements the Windows linker semantics. We treat
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the symbol table as a bucket of all known symbols, including symbols
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in archive file headers. We put all symbols into one bucket as we
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visit new files. That means we visit each file only once.
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This is different from the Unix linker semantics, in which we only
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keep undefined symbols and visit each file one by one until we
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resolve all undefined symbols. In the Unix model, we have to visit
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archive files many times if there are circular dependencies between
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archives.
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* Avoiding creating additional objects or copying data
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The data structures described in the previous section are all thin
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wrappers for classes that LLVM libObject provides. We avoid copying
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data from libObject's objects to our objects. We read much less data
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than before. For example, we don't read symbol values until we apply
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relocations because these values are not relevant to symbol
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resolution. Again, COMDAT symbols may be discarded during symbol
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resolution, so reading their attributes too early could result in a
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waste. We use underlying objects directly where doing so makes
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sense.
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Parallelism
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-----------
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The abovementioned data structures are also chosen with
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multi-threading in mind. It should relatively be easy to make the
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symbol table a concurrent hash map, so that we let multiple workers
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work on symbol table concurrently. Symbol resolution in this design is
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a single pointer mutation, which allows the resolver work concurrently
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in a lock-free manner using atomic pointer compare-and-swap.
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It should also be easy to apply relocations and write chunks concurrently.
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We created an experimental multi-threaded linker using the Microsoft
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ConcRT concurrency library, and it was able to link itself in 0.5
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seconds, so we think the design is promising.
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Link-Time Optimization
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----------------------
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LTO is implemented by handling LLVM bitcode files as object files.
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The linker resolves symbols in bitcode files normally. If all symbols
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are successfully resolved, it then calls an LLVM libLTO function
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with all bitcode files to convert them to one big regular COFF file.
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Finally, the linker replaces bitcode symbols with COFF symbols,
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so that we can link the input files as if they were in the native
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format from the beginning.
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The details are described in this document.
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http://llvm.org/docs/LinkTimeOptimization.html
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Glossary
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--------
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* RVA
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Short for Relative Virtual Address.
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Windows executables or DLLs are not position-independent; they are
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linked against a fixed address called an image base. RVAs are
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offsets from an image base.
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Default image bases are 0x140000000 for executables and 0x18000000
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for DLLs. For example, when we are creating an executable, we assume
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that the executable will be loaded at address 0x140000000 by the
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loader, so we apply relocations accordingly. Result texts and data
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will contain raw absolute addresses.
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* VA
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Short for Virtual Address. Equivalent to RVA + image base. It is
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rarely used. We almost always use RVAs instead.
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* Base relocations
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Relocation information for the loader. If the loader decides to map
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an executable or a DLL to a different address than their image
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bases, it fixes up binaries using information contained in the base
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relocation table. A base relocation table consists of a list of
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locations containing addresses. The loader adds a difference between
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RVA and actual load address to all locations listed there.
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Note that this run-time relocation mechanism is much simpler than ELF.
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There's no PLT or GOT. Images are relocated as a whole just
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by shifting entire images in memory by some offsets. Although doing
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this breaks text sharing, I think this mechanism is not actually bad
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on today's computers.
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* ICF
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Short for Identical COMDAT Folding.
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ICF is an optimization to reduce output size by merging COMDAT sections
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by not only their names but by their contents. If two COMDAT sections
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happen to have the same metadata, actual contents and relocations,
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they are merged by ICF. It is known as an effective technique,
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and it usually reduces C++ program's size by a few percent or more.
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Note that this is not entirely sound optimization. C/C++ require
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different functions have different addresses. If a program depends on
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that property, it would fail at runtime. However, that's not really an
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issue on Windows because MSVC link.exe enabled the optimization by
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default. As long as your program works with the linker's default
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settings, your program should be safe with ICF.
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