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dd647e3e60
Apologies for the large change, I looked for ways to break this up and all of the ones I saw added real complexity. This change focuses on the option's prefixed names and the array of prefixes. These are present in every option and the dominant source of dynamic relocations for PIE or PIC users of LLVM and Clang tooling. In some cases, 100s or 1000s of them for the Clang driver which has a huge number of options. This PR addresses this by building a string table and a prefixes table that can be referenced with indices rather than pointers that require dynamic relocations. This removes almost 7k dynmaic relocations from the `clang` binary, roughly 8% of the remaining dynmaic relocations outside of vtables. For busy-boxing use cases where many different option tables are linked into the same binary, the savings add up a bit more. The string table is a straightforward mechanism, but the prefixes required some subtlety. They are encoded in a Pascal-string fashion with a size followed by a sequence of offsets. This works relatively well for the small realistic prefixes arrays in use. Lots of code has to change in order to land this though: both all the option library code has to be updated to use the string table and prefixes table, and all the users of the options library have to be updated to correctly instantiate the objects. Some follow-up patches in the works to provide an abstraction for this style of code, and to start using the same technique for some of the other strings here now that the infrastructure is in place.
53 lines
1.4 KiB
C++
53 lines
1.4 KiB
C++
//===--- DriverOptions.cpp - Driver Options Table -------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "clang/Driver/Options.h"
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#include "llvm/Option/OptTable.h"
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#include <cassert>
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using namespace clang::driver;
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using namespace clang::driver::options;
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using namespace llvm::opt;
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#define OPTTABLE_STR_TABLE_CODE
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#include "clang/Driver/Options.inc"
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#undef OPTTABLE_STR_TABLE_CODE
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#define OPTTABLE_VALUES_CODE
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#include "clang/Driver/Options.inc"
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#undef OPTTABLE_VALUES_CODE
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#define OPTTABLE_PREFIXES_TABLE_CODE
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#include "clang/Driver/Options.inc"
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#undef OPTTABLE_PREFIXES_TABLE_CODE
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#define OPTTABLE_PREFIXES_UNION_CODE
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#include "clang/Driver/Options.inc"
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#undef OPTTABLE_PREFIXES_UNION_CODE
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static constexpr OptTable::Info InfoTable[] = {
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#define OPTION(...) LLVM_CONSTRUCT_OPT_INFO(__VA_ARGS__),
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#include "clang/Driver/Options.inc"
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#undef OPTION
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};
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namespace {
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class DriverOptTable : public PrecomputedOptTable {
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public:
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DriverOptTable()
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: PrecomputedOptTable(OptionStrTable, OptionPrefixesTable, InfoTable,
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OptionPrefixesUnion) {}
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};
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}
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const llvm::opt::OptTable &clang::driver::getDriverOptTable() {
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static DriverOptTable Table;
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return Table;
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}
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