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Summary: it's often the case when the rules in the SpecialCaseList are of the form hel.o*bar. That gives us a chance to build trigram index to quickly discard 99% of inputs without running a full regex. A similar idea was used in Google Code Search as described in the blog post: https://swtch.com/~rsc/regexp/regexp4.html The check is defeated, if there's at least one regex more complicated than that. In this case, all inputs will go through the regex. That said, the real-world rules are often simple or can be simplied. That considerably speeds up compiling Chromium with CFI and UBSan. As measured on Chromium's content_message_generator.cc: before, CFI: 44 s after, CFI: 23 s after, CFI, no blacklist: 23 s (~1% slower, but 3 runs were unable to show the difference) after, regular compilation to bitcode: 23 s Reviewers: pcc Subscribers: mgorny, llvm-commits Differential Revision: https://reviews.llvm.org/D27188 llvm-svn: 288303
71 lines
2.8 KiB
C++
71 lines
2.8 KiB
C++
//===-- TrigramIndex.h - a heuristic for SpecialCaseList --------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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//
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// TrigramIndex implements a heuristic for SpecialCaseList that allows to
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// filter out ~99% incoming queries when all regular expressions in the
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// SpecialCaseList are simple wildcards with '*' and '.'. If rules are more
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// complicated, the check is defeated and it will always pass the queries to a
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// full regex.
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//
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// The basic idea is that in order for a wildcard to match a query, the query
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// needs to have all trigrams which occur in the wildcard. We create a trigram
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// index (trigram -> list of rules with it) and then count trigrams in the query
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// for each rule. If the count for one of the rules reaches the expected value,
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// the check passes the query to a regex. If none of the rules got enough
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// trigrams, the check tells that the query is definitely not matched by any
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// of the rules, and no regex matching is needed.
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// A similar idea was used in Google Code Search as described in the blog post:
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// https://swtch.com/~rsc/regexp/regexp4.html
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_SUPPORT_TRIGRAMINDEX_H
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#define LLVM_SUPPORT_TRIGRAMINDEX_H
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/StringMap.h"
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#include <string>
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#include <unordered_map>
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#include <vector>
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namespace llvm {
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class StringRef;
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class TrigramIndex {
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public:
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/// Inserts a new Regex into the index.
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void insert(std::string Regex);
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/// Returns true, if special case list definitely does not have a line
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/// that matches the query. Returns false, if it's not sure.
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bool isDefinitelyOut(StringRef Query) const;
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/// Returned true, iff the heuristic is defeated and not useful.
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/// In this case isDefinitelyOut always returns false.
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bool isDefeated() { return Defeated; }
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private:
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// If true, the rules are too complicated for the check to work, and full
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// regex matching is needed for every rule.
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bool Defeated = false;
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// The minimum number of trigrams which should match for a rule to have a
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// chance to match the query. The number of elements equals the number of
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// regex rules in the SpecialCaseList.
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std::vector<unsigned> Counts;
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// Index holds a list of rules indices for each trigram. The same indices
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// are used in Counts to store per-rule limits.
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// If a trigram is too common (>4 rules with it), we stop tracking it,
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// which increases the probability for a need to match using regex, but
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// decreases the costs in the regular case.
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std::unordered_map<unsigned, SmallVector<size_t, 4>> Index{256};
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};
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} // namespace llvm
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#endif // LLVM_SUPPORT_TRIGRAMINDEX_H
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