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The implementation of GeneralHash::addBits broke C++ aliasing rules; fix

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it with memcpy. This also fixes a problem on big-endian hosts, where
addUnaligned would return different results depending on the alignment
of the data.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151247 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jay Foad 2012-02-23 09:16:04 +00:00
parent 9d91c5d31c
commit 6592eacf90
2 changed files with 20 additions and 72 deletions
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46
include/llvm/ADT/Hashing.h
View File

@ -19,6 +19,7 @@
#include "llvm/Support/AlignOf.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/DataTypes.h"
#include <cstring>
namespace llvm {
@ -140,39 +141,32 @@ private:
mix(uint32_t(Val));
}
template<typename T, bool isAligned>
struct addBitsImpl {
static void add(GeneralHash &Hash, const T *I, const T *E) {
Hash.addUnaligned(
reinterpret_cast<const uint8_t *>(I),
reinterpret_cast<const uint8_t *>(E));
// Add a range of bytes from I to E.
void addBytes(const char *I, const char *E) {
uint32_t Data;
// Note that aliasing rules forbid us from dereferencing
// reinterpret_cast<uint32_t *>(I) even if I happens to be suitably
// aligned, so we use memcpy instead.
for (; E - I >= ptrdiff_t(sizeof Data); I += sizeof Data) {
// A clever compiler should be able to turn this memcpy into a single
// aligned or unaligned load (depending on the alignment of the type T
// that was used in the call to addBits).
std::memcpy(&Data, I, sizeof Data);
mix(Data);
}
};
template<typename T>
struct addBitsImpl<T, true> {
static void add(GeneralHash &Hash, const T *I, const T *E) {
Hash.addAligned(
reinterpret_cast<const uint32_t *>(I),
reinterpret_cast<const uint32_t *>(E));
if (I != E) {
Data = 0;
std::memcpy(&Data, I, E - I);
mix(Data);
}
};
}
// Add a range of bits from I to E.
template<typename T>
void addBits(const T *I, const T *E) {
addBitsImpl<T, AlignOf<T>::Alignment_GreaterEqual_4Bytes>::add(*this, I, E);
addBytes(reinterpret_cast<const char *>(I),
reinterpret_cast<const char *>(E));
}
// Add a range of uint32s
void addAligned(const uint32_t *I, const uint32_t *E) {
while (I < E) {
mix(*I++);
}
}
// Add a possibly unaligned sequence of bytes.
void addUnaligned(const uint8_t *I, const uint8_t *E);
};
} // end namespace llvm

46
lib/Support/Hashing.cpp
View File

@ -1,46 +0,0 @@
//===-- llvm/ADT/Hashing.cpp - Utilities for hashing ------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/Hashing.h"
namespace llvm {
// Add a possibly unaligned sequence of bytes.
void GeneralHash::addUnaligned(const uint8_t *I, const uint8_t *E) {
ptrdiff_t Length = E - I;
if ((uintptr_t(I) & 3) == 0) {
while (Length > 3) {
mix(*reinterpret_cast<const uint32_t *>(I));
I += 4;
Length -= 4;
}
} else {
while (Length > 3) {
mix(
uint32_t(I[0]) +
(uint32_t(I[1]) << 8) +
(uint32_t(I[2]) << 16) +
(uint32_t(I[3]) << 24));
I += 4;
Length -= 4;
}
}
if (Length & 3) {
uint32_t Data = 0;
switch (Length & 3) {
case 3: Data |= uint32_t(I[2]) << 16; // fall through
case 2: Data |= uint32_t(I[1]) << 8; // fall through
case 1: Data |= uint32_t(I[0]); break;
}
mix(Data);
}
}
}