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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "StackArena.h"
#include "nsAlgorithm.h"
#include "nsDebug.h"
namespace mozilla {
// A block of memory that the stack will chop up and hand out.
struct StackBlock {
// Subtract sizeof(StackBlock*) to give space for the |mNext| field.
static const size_t MAX_USABLE_SIZE = 4096 - sizeof(StackBlock*);
// A block of memory.
char mBlock[MAX_USABLE_SIZE];
// Another block of memory that would only be created if our stack
// overflowed.
StackBlock* mNext;
StackBlock() : mNext(nullptr) {}
~StackBlock() = default;
};
static_assert(sizeof(StackBlock) == 4096, "StackBlock must be 4096 bytes");
// We hold an array of marks. A push pushes a mark on the stack.
// A pop pops it off.
struct StackMark {
// The block of memory from which we are currently handing out chunks.
StackBlock* mBlock;
// Our current position in the block.
size_t mPos;
};
StackArena* AutoStackArena::gStackArena;
StackArena::StackArena() {
mMarkLength = 0;
mMarks = nullptr;
// Allocate our stack memory.
mBlocks = new StackBlock();
mCurBlock = mBlocks;
mStackTop = 0;
mPos = 0;
}
StackArena::~StackArena() {
// Free up our data.
delete[] mMarks;
while (mBlocks) {
StackBlock* toDelete = mBlocks;
mBlocks = mBlocks->mNext;
delete toDelete;
}
}
size_t StackArena::SizeOfExcludingThis(
mozilla::MallocSizeOf aMallocSizeOf) const {
size_t n = 0;
StackBlock* block = mBlocks;
while (block) {
n += aMallocSizeOf(block);
block = block->mNext;
}
n += aMallocSizeOf(mMarks);
return n;
}
static const int STACK_ARENA_MARK_INCREMENT = 50;
void StackArena::Push() {
// Resize the mark array if we overrun it. Failure to allocate the
// mark array is not fatal; we just won't free to that mark. This
// allows callers not to worry about error checking.
if (mStackTop >= mMarkLength) {
uint32_t newLength = mStackTop + STACK_ARENA_MARK_INCREMENT;
StackMark* newMarks = new StackMark[newLength];
if (newMarks) {
if (mMarkLength) {
memcpy(newMarks, mMarks, sizeof(StackMark) * mMarkLength);
}
// Fill in any marks that we couldn't allocate during a prior call
// to Push().
for (; mMarkLength < mStackTop; ++mMarkLength) {
MOZ_ASSERT_UNREACHABLE("should only hit this on out-of-memory");
newMarks[mMarkLength].mBlock = mCurBlock;
newMarks[mMarkLength].mPos = mPos;
}
delete[] mMarks;
mMarks = newMarks;
mMarkLength = newLength;
}
}
// Set a mark at the top (if we can).
NS_ASSERTION(mStackTop < mMarkLength, "out of memory");
if (mStackTop < mMarkLength) {
mMarks[mStackTop].mBlock = mCurBlock;
mMarks[mStackTop].mPos = mPos;
}
mStackTop++;
}
void* StackArena::Allocate(size_t aSize) {
NS_ASSERTION(mStackTop > 0, "Allocate called without Push");
// Align to a multiple of 8.
aSize = NS_ROUNDUP<size_t>(aSize, 8);
// On stack overflow, grab another block.
if (mPos + aSize >= StackBlock::MAX_USABLE_SIZE) {
NS_ASSERTION(aSize <= StackBlock::MAX_USABLE_SIZE,
"Requested memory is greater that our block size!!");
if (mCurBlock->mNext == nullptr) {
mCurBlock->mNext = new StackBlock();
}
mCurBlock = mCurBlock->mNext;
mPos = 0;
}
// Return the chunk they need.
void* result = mCurBlock->mBlock + mPos;
mPos += aSize;
return result;
}
void StackArena::Pop() {
// Pop off the mark.
NS_ASSERTION(mStackTop > 0, "unmatched pop");
mStackTop--;
if (mStackTop >= mMarkLength) {
// We couldn't allocate the marks array at the time of the push, so
// we don't know where we're freeing to.
MOZ_ASSERT_UNREACHABLE("out of memory");
if (mStackTop == 0) {
// But we do know if we've completely pushed the stack.
mCurBlock = mBlocks;
mPos = 0;
}
return;
}
#ifdef DEBUG
// Mark the "freed" memory with 0xdd to help with debugging of memory
// allocation problems.
{
StackBlock *block = mMarks[mStackTop].mBlock, *block_end = mCurBlock;
size_t pos = mMarks[mStackTop].mPos;
for (; block != block_end; block = block->mNext, pos = 0) {
memset(block->mBlock + pos, 0xdd, sizeof(block->mBlock) - pos);
}
memset(block->mBlock + pos, 0xdd, mPos - pos);
}
#endif
mCurBlock = mMarks[mStackTop].mBlock;
mPos = mMarks[mStackTop].mPos;
}
} // namespace mozilla
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