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//===-- stack_depot.h -------------------------------------------*- 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
//
//===----------------------------------------------------------------------===//
#ifndef SCUDO_STACK_DEPOT_H_
#define SCUDO_STACK_DEPOT_H_
#include "atomic_helpers.h"
#include "mutex.h"
namespace scudo {
class MurMur2HashBuilder {
static const u32 M = 0x5bd1e995;
static const u32 Seed = 0x9747b28c;
static const u32 R = 24;
u32 H;
public:
explicit MurMur2HashBuilder(u32 Init = 0) { H = Seed ^ Init; }
void add(u32 K) {
K *= M;
K ^= K >> R;
K *= M;
H *= M;
H ^= K;
}
u32 get() {
u32 X = H;
X ^= X >> 13;
X *= M;
X ^= X >> 15;
return X;
}
};
class StackDepot {
HybridMutex RingEndMu;
u32 RingEnd;
// This data structure stores a stack trace for each allocation and
// deallocation when stack trace recording is enabled, that may be looked up
// using a hash of the stack trace. The lower bits of the hash are an index
// into the Tab array, which stores an index into the Ring array where the
// stack traces are stored. As the name implies, Ring is a ring buffer, so a
// stack trace may wrap around to the start of the array.
//
// Each stack trace in Ring is prefixed by a stack trace marker consisting of
// a fixed 1 bit in bit 0 (this allows disambiguation between stack frames
// and stack trace markers in the case where instruction pointers are 4-byte
// aligned, as they are on arm64), the stack trace hash in bits 1-32, and the
// size of the stack trace in bits 33-63.
//
// The insert() function is potentially racy in its accesses to the Tab and
// Ring arrays, but find() is resilient to races in the sense that, barring
// hash collisions, it will either return the correct stack trace or no stack
// trace at all, even if two instances of insert() raced with one another.
// This is achieved by re-checking the hash of the stack trace before
// returning the trace.
#ifdef SCUDO_FUZZ
// Use smaller table sizes for fuzzing in order to reduce input size.
static const uptr TabBits = 4;
#else
static const uptr TabBits = 16;
#endif
static const uptr TabSize = 1 << TabBits;
static const uptr TabMask = TabSize - 1;
atomic_u32 Tab[TabSize];
#ifdef SCUDO_FUZZ
static const uptr RingBits = 4;
#else
static const uptr RingBits = 19;
#endif
static const uptr RingSize = 1 << RingBits;
static const uptr RingMask = RingSize - 1;
atomic_u64 Ring[RingSize];
public:
// Insert hash of the stack trace [Begin, End) into the stack depot, and
// return the hash.
u32 insert(uptr *Begin, uptr *End) {
MurMur2HashBuilder B;
for (uptr *I = Begin; I != End; ++I)
B.add(u32(*I) >> 2);
u32 Hash = B.get();
u32 Pos = Hash & TabMask;
u32 RingPos = atomic_load_relaxed(&Tab[Pos]);
u64 Entry = atomic_load_relaxed(&Ring[RingPos]);
u64 Id = (u64(End - Begin) << 33) | (u64(Hash) << 1) | 1;
if (Entry == Id)
return Hash;
ScopedLock Lock(RingEndMu);
RingPos = RingEnd;
atomic_store_relaxed(&Tab[Pos], RingPos);
atomic_store_relaxed(&Ring[RingPos], Id);
for (uptr *I = Begin; I != End; ++I) {
RingPos = (RingPos + 1) & RingMask;
atomic_store_relaxed(&Ring[RingPos], *I);
}
RingEnd = (RingPos + 1) & RingMask;
return Hash;
}
// Look up a stack trace by hash. Returns true if successful. The trace may be
// accessed via operator[] passing indexes between *RingPosPtr and
// *RingPosPtr + *SizePtr.
bool find(u32 Hash, uptr *RingPosPtr, uptr *SizePtr) const {
u32 Pos = Hash & TabMask;
u32 RingPos = atomic_load_relaxed(&Tab[Pos]);
if (RingPos >= RingSize)
return false;
u64 Entry = atomic_load_relaxed(&Ring[RingPos]);
u64 HashWithTagBit = (u64(Hash) << 1) | 1;
if ((Entry & 0x1ffffffff) != HashWithTagBit)
return false;
u32 Size = u32(Entry >> 33);
if (Size >= RingSize)
return false;
*RingPosPtr = (RingPos + 1) & RingMask;
*SizePtr = Size;
MurMur2HashBuilder B;
for (uptr I = 0; I != Size; ++I) {
RingPos = (RingPos + 1) & RingMask;
B.add(u32(atomic_load_relaxed(&Ring[RingPos])) >> 2);
}
return B.get() == Hash;
}
u64 operator[](uptr RingPos) const {
return atomic_load_relaxed(&Ring[RingPos & RingMask]);
}
};
} // namespace scudo
#endif // SCUDO_STACK_DEPOT_H_
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