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Bug 1508873 - part 1 - statically assert alignment for hashtable entries; r=luke

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We do this to make our lives easier in later patches; this check
guarantees that we don't need padding between the block of cached hash
values and the block of entries immediately after it.
This commit is contained in:
Nathan Froyd 2018-12-13 12:21:19 -05:00
parent dd49ba8708
commit 1d59bcc108
1 changed files with 13 additions and 0 deletions
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13
mfbt/HashTable.h
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@ -947,6 +947,19 @@ class HashTableEntry {
private:
using NonConstT = typename RemoveConst<T>::Type;
#ifdef HAVE_64BIT_BUILD
static_assert(alignof(NonConstT) <= alignof(void*),
"cannot use over-aligned entries in mozilla::HashTable");
#else
// This assertion is safe for 32-bit builds because on both Windows and Linux
// (including Android), the minimum alignment for allocations larger than 8
// bytes is 8 bytes, and the actual data for entries in our entry store is
// guaranteed to have that alignment as well, thanks to the power-of-two
// number of cached hash values stored prior to the entry data.
static_assert(alignof(NonConstT) <= 2 * alignof(void*),
"cannot use over-aligned entries in mozilla::HashTable");
#endif
static const HashNumber sFreeKey = 0;
static const HashNumber sRemovedKey = 1;
static const HashNumber sCollisionBit = 1;