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//===--- SourceLocationEncoding.h - Small serialized locations --*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// We wish to encode the SourceLocation from other module file not dependent
// on the other module file. So that the source location changes from other
// module file may not affect the contents of the current module file. Then the
// users don't need to recompile the whole project due to a new line in a module
// unit in the root of the dependency graph.
//
// To achieve this, we need to encode the index of the module file into the
// encoding of the source location. The encoding of the source location may be:
//
// |-----------------------|-----------------------|
// | A | B | C |
//
// * A: 32 bit. The index of the module file in the module manager + 1. The +1
// here is necessary since we wish 0 stands for the current module file.
// * B: 31 bit. The offset of the source location to the module file containing
// it.
// * C: The macro bit. We rotate it to the lowest bit so that we can save some
// space in case the index of the module file is 0.
//
// Specially, if the index of the module file is 0, we allow to encode a
// sequence of locations we store only differences between successive elements.
//
//===----------------------------------------------------------------------===//
#include "clang/Basic/SourceLocation.h"
#include "llvm/Support/MathExtras.h"
#include <climits>
#ifndef LLVM_CLANG_SERIALIZATION_SOURCELOCATIONENCODING_H
#define LLVM_CLANG_SERIALIZATION_SOURCELOCATIONENCODING_H
namespace clang {
class SourceLocationSequence;
/// Serialized encoding of SourceLocations without context.
/// Optimized to have small unsigned values (=> small after VBR encoding).
///
// Macro locations have the top bit set, we rotate by one so it is the low bit.
class SourceLocationEncoding {
using UIntTy = SourceLocation::UIntTy;
constexpr static unsigned UIntBits = CHAR_BIT * sizeof(UIntTy);
static UIntTy encodeRaw(UIntTy Raw) {
return (Raw << 1) | (Raw >> (UIntBits - 1));
}
static UIntTy decodeRaw(UIntTy Raw) {
return (Raw >> 1) | (Raw << (UIntBits - 1));
}
friend SourceLocationSequence;
public:
using RawLocEncoding = uint64_t;
static RawLocEncoding encode(SourceLocation Loc, UIntTy BaseOffset,
unsigned BaseModuleFileIndex,
SourceLocationSequence * = nullptr);
static std::pair<SourceLocation, unsigned>
decode(RawLocEncoding, SourceLocationSequence * = nullptr);
};
/// Serialized encoding of a sequence of SourceLocations.
///
/// Optimized to produce small values when locations with the sequence are
/// similar. Each element can be delta-encoded against the last nonzero element.
///
/// Sequences should be started by creating a SourceLocationSequence::State,
/// and then passed around as SourceLocationSequence*. Example:
///
/// // establishes a sequence
/// void EmitTopLevelThing() {
/// SourceLocationSequence::State Seq;
/// EmitContainedThing(Seq);
/// EmitRecursiveThing(Seq);
/// }
///
/// // optionally part of a sequence
/// void EmitContainedThing(SourceLocationSequence *Seq = nullptr) {
/// Record.push_back(SourceLocationEncoding::encode(SomeLoc, Seq));
/// }
///
/// // establishes a sequence if there isn't one already
/// void EmitRecursiveThing(SourceLocationSequence *ParentSeq = nullptr) {
/// SourceLocationSequence::State Seq(ParentSeq);
/// Record.push_back(SourceLocationEncoding::encode(SomeLoc, Seq));
/// EmitRecursiveThing(Seq);
/// }
///
class SourceLocationSequence {
using UIntTy = SourceLocation::UIntTy;
using EncodedTy = uint64_t;
constexpr static auto UIntBits = SourceLocationEncoding::UIntBits;
static_assert(sizeof(EncodedTy) > sizeof(UIntTy), "Need one extra bit!");
// Prev stores the rotated last nonzero location.
UIntTy &Prev;
// Zig-zag encoding turns small signed integers into small unsigned integers.
// 0 => 0, -1 => 1, 1 => 2, -2 => 3, ...
static UIntTy zigZag(UIntTy V) {
UIntTy Sign = (V & (1 << (UIntBits - 1))) ? UIntTy(-1) : UIntTy(0);
return Sign ^ (V << 1);
}
static UIntTy zagZig(UIntTy V) { return (V >> 1) ^ -(V & 1); }
SourceLocationSequence(UIntTy &Prev) : Prev(Prev) {}
EncodedTy encodeRaw(UIntTy Raw) {
if (Raw == 0)
return 0;
UIntTy Rotated = SourceLocationEncoding::encodeRaw(Raw);
if (Prev == 0)
return Prev = Rotated;
UIntTy Delta = Rotated - Prev;
Prev = Rotated;
// Exactly one 33 bit value is possible! (1 << 32).
// This is because we have two representations of zero: trivial & relative.
return 1 + EncodedTy{zigZag(Delta)};
}
UIntTy decodeRaw(EncodedTy Encoded) {
if (Encoded == 0)
return 0;
if (Prev == 0)
return SourceLocationEncoding::decodeRaw(Prev = Encoded);
return SourceLocationEncoding::decodeRaw(Prev += zagZig(Encoded - 1));
}
public:
SourceLocation decode(EncodedTy Encoded) {
return SourceLocation::getFromRawEncoding(decodeRaw(Encoded));
}
EncodedTy encode(SourceLocation Loc) {
return encodeRaw(Loc.getRawEncoding());
}
class State;
};
/// This object establishes a SourceLocationSequence.
class SourceLocationSequence::State {
UIntTy Prev = 0;
SourceLocationSequence Seq;
public:
// If Parent is provided and non-null, then this root becomes part of that
// enclosing sequence instead of establishing a new one.
State(SourceLocationSequence *Parent = nullptr)
: Seq(Parent ? Parent->Prev : Prev) {}
// Implicit conversion for uniform use of roots vs propagated sequences.
operator SourceLocationSequence *() { return &Seq; }
};
inline SourceLocationEncoding::RawLocEncoding
SourceLocationEncoding::encode(SourceLocation Loc, UIntTy BaseOffset,
unsigned BaseModuleFileIndex,
SourceLocationSequence *Seq) {
// If the source location is a local source location, we can try to optimize
// the similar sequences to only record the differences.
if (!BaseOffset)
return Seq ? Seq->encode(Loc) : encodeRaw(Loc.getRawEncoding());
if (Loc.isInvalid())
return 0;
// Otherwise, the higher bits are used to store the module file index,
// so it is meaningless to optimize the source locations into small
// integers. Let's try to always use the raw encodings.
assert(Loc.getOffset() >= BaseOffset);
Loc = Loc.getLocWithOffset(-BaseOffset);
RawLocEncoding Encoded = encodeRaw(Loc.getRawEncoding());
// 16 bits should be sufficient to store the module file index.
assert(BaseModuleFileIndex < (1 << 16));
Encoded |= (RawLocEncoding)BaseModuleFileIndex << 32;
return Encoded;
}
inline std::pair<SourceLocation, unsigned>
SourceLocationEncoding::decode(RawLocEncoding Encoded,
SourceLocationSequence *Seq) {
unsigned ModuleFileIndex = Encoded >> 32;
if (!ModuleFileIndex)
return {Seq ? Seq->decode(Encoded)
: SourceLocation::getFromRawEncoding(decodeRaw(Encoded)),
ModuleFileIndex};
Encoded &= llvm::maskTrailingOnes<RawLocEncoding>(32);
SourceLocation Loc = SourceLocation::getFromRawEncoding(decodeRaw(Encoded));
return {Loc, ModuleFileIndex};
}
} // namespace clang
#endif
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