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closes #486: amp up hashtables M1352889 M1352890 M1379282

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This commit is contained in:
Cameron Kaiser 2018-03-17 15:30:31 -07:00
parent f61c406ba8
commit 5ed131b2ed
1 changed files with 30 additions and 6 deletions
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36
xpcom/glue/PLDHashTable.cpp
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@ -12,6 +12,7 @@
#include "mozilla/HashFunctions.h"
#include "mozilla/MathAlgorithms.h"
#include "nsAlgorithm.h"
#include "mozilla/HashFunctions.h"
#include "mozilla/Likely.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/ChaosMode.h"
@ -71,7 +72,7 @@ PLDHashTable::HashStringKey(PLDHashTable* aTable, const void* aKey)
/* static */ PLDHashNumber
PLDHashTable::HashVoidPtrKeyStub(PLDHashTable* aTable, const void* aKey)
{
return (PLDHashNumber)(ptrdiff_t)aKey >> 2;
return mozilla::HashGeneric(aKey);
}
/* static */ bool
@ -256,15 +257,38 @@ PLDHashTable::Hash1(PLDHashNumber aHash0)
return aHash0 >> mHashShift;
}
// Double hashing needs the second hash code to be relatively prime to table
// size, so we simply make hash2 odd.
void
PLDHashTable::Hash2(PLDHashNumber aHash,
PLDHashTable::Hash2(PLDHashNumber aHash0,
uint32_t& aHash2Out, uint32_t& aSizeMaskOut)
{
uint32_t sizeLog2 = kHashBits - mHashShift;
aHash2Out = ((aHash << sizeLog2) >> mHashShift) | 1;
aSizeMaskOut = (PLDHashNumber(1) << sizeLog2) - 1;
uint32_t sizeMask = (PLDHashNumber(1) << sizeLog2) - 1;
aSizeMaskOut = sizeMask;
// The incoming aHash0 always has the low bit unset (since we leave it
// free for the collision flag), and should have reasonably random
// data in the other 31 bits. We used the high bits of aHash0 for
// Hash1, so we use the low bits here. If the table size is large,
// the bits we use may overlap, but that's still more random than
// filling with 0s.
//
// Since the result of Hash2 controls how far we jump around the table
// to build a chain after starting at a location determined by Hash1,
// we'd like to keep it small, to improve cache behavior.
// Keep the jumps from the second hash small, to improve cache behavior.
const uint32_t kHash2MaskMaxBits = 6;
uint32_t hash2Mask;
if (sizeLog2 >= kHash2MaskMaxBits) {
hash2Mask = (PLDHashNumber(1) << kHash2MaskMaxBits) - 1;
} else {
hash2Mask = sizeMask;
}
// Double hashing needs the second hash code to be relatively prime to table
// size, so we simply make hash2 odd.
//
// This also conveniently covers up the fact that we have the low bit
// unset since aHash0 has the low bit unset.
aHash2Out = (aHash0 & hash2Mask) | 1;
}
// Reserve mKeyHash 0 for free entries and 1 for removed-entry sentinels. Note