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authortrav90 <travawine@palemoon.org>2018-08-18 15:18:52 -0500
committertrav90 <travawine@palemoon.org>2018-08-18 15:18:52 -0500
commit450eed08c5eb7de7928e185feaf79f0b74b932e5 (patch)
tree7fde5d4c4208c265e6254d8b6b09f7f4801188fd /js
parentf0a708834dfb189058dd90b67765fda7687f5039 (diff)
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Avoid doing a memset on a non-POD structure
|entryCount| tracks -- in fast-to-check manner -- the number of entries in the hashtable. But to actually enumerate entries, we have to loop through all of |table|, checking for entries that are actually live. A live entry is indicated by a zero |hash| in the entry. The |memset| would properly zero that; removing the memset will not. It's not entirely clear whether a memset that overwrites a lot of stuff but is maybe simpler, is faster than compiler-generated likely-SIMD code that zeroes out *just* |hash| fields in all the entries. But I am going to guess that SIMD is good enough. For now, we should just do the simple and thing: don't distinguish POD and non-POD, and know that the compiler is going to recognize that |mem.addr()->~T()| is a no-op when T is trivial. So with POD, the loop should degenerate to just zeroing |hash| at consistent offset, and SIMD will eat that up, and it can't be *that* different from the memset in performance (if it is at all).
Diffstat (limited to 'js')
-rw-r--r--js/public/HashTable.h46
1 files changed, 33 insertions, 13 deletions
diff --git a/js/public/HashTable.h b/js/public/HashTable.h
index 5d4c0665d..8a2493b55 100644
--- a/js/public/HashTable.h
+++ b/js/public/HashTable.h
@@ -12,6 +12,7 @@
#include "mozilla/Attributes.h"
#include "mozilla/Casting.h"
#include "mozilla/HashFunctions.h"
+#include "mozilla/MemoryChecking.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/Move.h"
#include "mozilla/Opaque.h"
@@ -805,17 +806,22 @@ class HashTableEntry
void operator=(const HashTableEntry&) = delete;
~HashTableEntry() = delete;
+ void destroyStoredT() {
+ mem.addr()->~T();
+ MOZ_MAKE_MEM_UNDEFINED(mem.addr(), sizeof(*mem.addr()));
+ }
+
public:
// NB: HashTableEntry is treated as a POD: no constructor or destructor calls.
void destroyIfLive() {
if (isLive())
- mem.addr()->~T();
+ destroyStoredT();
}
void destroy() {
MOZ_ASSERT(isLive());
- mem.addr()->~T();
+ destroyStoredT();
}
void swap(HashTableEntry* other) {
@@ -835,10 +841,28 @@ class HashTableEntry
NonConstT& getMutable() { MOZ_ASSERT(isLive()); return *mem.addr(); }
bool isFree() const { return keyHash == sFreeKey; }
- void clearLive() { MOZ_ASSERT(isLive()); keyHash = sFreeKey; mem.addr()->~T(); }
- void clear() { if (isLive()) mem.addr()->~T(); keyHash = sFreeKey; }
+ void clearLive() {
+ MOZ_ASSERT(isLive());
+ keyHash = sFreeKey;
+ destroyStoredT();
+ }
+
+ void clear() {
+ if (isLive())
+ destroyStoredT();
+
+ MOZ_MAKE_MEM_UNDEFINED(this, sizeof(*this));
+ keyHash = sFreeKey;
+ }
+
bool isRemoved() const { return keyHash == sRemovedKey; }
- void removeLive() { MOZ_ASSERT(isLive()); keyHash = sRemovedKey; mem.addr()->~T(); }
+
+ void removeLive() {
+ MOZ_ASSERT(isLive());
+ keyHash = sRemovedKey;
+ destroyStoredT();
+ }
+
bool isLive() const { return isLiveHash(keyHash); }
void setCollision() { MOZ_ASSERT(isLive()); keyHash |= sCollisionBit; }
void unsetCollision() { keyHash &= ~sCollisionBit; }
@@ -1654,14 +1678,10 @@ class HashTable : private AllocPolicy
public:
void clear()
{
- if (mozilla::IsPod<Entry>::value) {
- memset(table, 0, sizeof(*table) * capacity());
- } else {
- uint32_t tableCapacity = capacity();
- Entry* end = table + tableCapacity;
- for (Entry* e = table; e < end; ++e)
- e->clear();
- }
+ Entry* end = table + capacity();
+ for (Entry* e = table; e < end; ++e)
+ e->clear();
+
removedCount = 0;
entryCount = 0;
#ifdef JS_DEBUG