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C++20: Add TrivialInitAtomic

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C++20 does away with trivial initializers for std::atomic<T>, which means our
global pagemaps always get zeroed, sometimes after other static ctors have run
(fun fun!).  Use the new std::atomic_ref<T> when available.  Abstract all this
behind an #ifdef-ful wrapper.
This commit is contained in:
Nathaniel Filardo
2021-04-20 19:06:39 +01:00
committed by Nathaniel Wesley Filardo
parent c132c86cf9
commit 80c6e95210
2 changed files with 89 additions and 10 deletions
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32
src/mem/pagemap.h
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@@ -107,12 +107,20 @@ namespace snmalloc
struct Leaf
{
std::atomic<T> values[ENTRIES_PER_LEAF];
TrivialInitAtomic<T> values[ENTRIES_PER_LEAF];
static_assert(sizeof(TrivialInitAtomic<T>) == sizeof(T));
static_assert(alignof(TrivialInitAtomic<T>) == alignof(T));
};
struct PagemapEntry
{
std::atomic<PagemapEntry*> entries[ENTRIES_PER_INDEX_LEVEL];
TrivialInitAtomic<PagemapEntry*> entries[ENTRIES_PER_INDEX_LEVEL];
static_assert(
sizeof(TrivialInitAtomic<PagemapEntry*>) == sizeof(PagemapEntry*));
static_assert(
alignof(TrivialInitAtomic<PagemapEntry*>) == alignof(PagemapEntry*));
};
static_assert(
@@ -127,11 +135,11 @@ namespace snmalloc
// allocation.
// TODO: This is fragile that it is not being memset, and we should review
// to ensure we don't get bitten by this in the future.
std::atomic<PagemapEntry*> top[TOPLEVEL_ENTRIES]; // = {nullptr};
TrivialInitAtomic<PagemapEntry*> top[TOPLEVEL_ENTRIES];
template<bool create_addr>
SNMALLOC_FAST_PATH PagemapEntry*
get_node(std::atomic<PagemapEntry*>* e, bool& result)
get_node(TrivialInitAtomic<PagemapEntry*>* e, bool& result)
{
// The page map nodes are all allocated directly from the OS zero
// initialised with a system call. We don't need any ordered to guarantee
@@ -156,7 +164,7 @@ namespace snmalloc
}
SNMALLOC_SLOW_PATH PagemapEntry*
get_node_slow(std::atomic<PagemapEntry*>* e, bool& result)
get_node_slow(TrivialInitAtomic<PagemapEntry*>* e, bool& result)
{
// The page map nodes are all allocated directly from the OS zero
// initialised with a system call. We don't need any ordered to guarantee
@@ -198,7 +206,7 @@ namespace snmalloc
#endif
size_t ix = addr >> TOPLEVEL_SHIFT;
size_t shift = TOPLEVEL_SHIFT;
std::atomic<PagemapEntry*>* e = &top[ix];
TrivialInitAtomic<PagemapEntry*>* e = &top[ix];
// This is effectively a
// for (size_t i = 0; i < INDEX_LEVELS; i++)
@@ -242,13 +250,14 @@ namespace snmalloc
}
template<bool create_addr>
SNMALLOC_FAST_PATH std::atomic<T>* get_addr(uintptr_t p, bool& success)
SNMALLOC_FAST_PATH TrivialInitAtomic<T>*
get_addr(uintptr_t p, bool& success)
{
auto leaf_ix = get_leaf_index<create_addr>(p, success);
return &(leaf_ix.first->values[leaf_ix.second]);
}
std::atomic<T>* get_ptr(uintptr_t p)
TrivialInitAtomic<T>* get_ptr(uintptr_t p)
{
bool success;
return get_addr<true>(p, success);
@@ -333,7 +342,7 @@ namespace snmalloc
for (; ix < last; ix++)
{
SNMALLOC_ASSUME(leaf_ix.first != nullptr);
leaf_ix.first->values[ix] = x;
leaf_ix.first->values[ix].store(x);
}
length = length - diff;
@@ -355,7 +364,10 @@ namespace snmalloc
static constexpr size_t ENTRIES = 1ULL << COVERED_BITS;
static constexpr size_t SHIFT = GRANULARITY_BITS;
std::atomic<T> top[ENTRIES];
TrivialInitAtomic<T> top[ENTRIES];
static_assert(sizeof(TrivialInitAtomic<T>) == sizeof(T));
static_assert(alignof(TrivialInitAtomic<T>) == alignof(T));
public:
/**