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Fix MLD level mask generation to support 64-bit masks. (#6123)

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The generation of level masks for compactly storing partition cells
supports sizes that can be stored in 64 bits.

The current implementation fails if the total bit sum is 64 bits
exactly. A bit shift mechanism is used that is undefined when the
shift size is equal to the bit size of the underlying type. This
generates an incorrect mask value.

We fix this by adding a special case for a 64 bit offset. Given this
code is called at most |level| times, there will be no effect on
performance. We also update the assertions to reflect 64 bit masks
are now supported.
This commit is contained in:
Michael Bell
2021-09-21 18:09:38 +01:00
committed by GitHub
parent 8e100ea1a0
commit f1f96166c5
3 changed files with 68 additions and 5 deletions
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include/partitioner/multi_level_partition.hpp
+10 -5
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@@ -186,11 +186,12 @@ template <storage::Ownership Ownership> class MultiLevelPartitionImpl final
auto bits = static_cast<std::uint64_t>(std::ceil(std::log2(num_cells + 1)));
offsets[lidx++] = sum_bits;
sum_bits += bits;
if (sum_bits > 64)
if (sum_bits > NUM_PARTITION_BITS)
{
throw util::exception(
"Can't pack the partition information at level " + std::to_string(lidx) +
" into a 64bit integer. Would require " + std::to_string(sum_bits) + " bits.");
" into a " + std::to_string(NUM_PARTITION_BITS) +
"bit integer. Would require " + std::to_string(sum_bits) + " bits.");
}
}
// sentinel
@@ -211,11 +212,15 @@ template <storage::Ownership Ownership> class MultiLevelPartitionImpl final
[&](const auto offset, const auto next_offset) {
// create mask that has `bits` ones at its LSBs.
// 000011
BOOST_ASSERT(offset < NUM_PARTITION_BITS);
BOOST_ASSERT(offset <= NUM_PARTITION_BITS);
PartitionID mask = (1ULL << offset) - 1ULL;
// 001111
BOOST_ASSERT(next_offset < NUM_PARTITION_BITS);
PartitionID next_mask = (1ULL << next_offset) - 1ULL;
BOOST_ASSERT(next_offset <= NUM_PARTITION_BITS);
// Check offset for shift overflow. Offsets are strictly increasing,
// so we only need the check on the last mask.
PartitionID next_mask = next_offset == NUM_PARTITION_BITS
? -1ULL
: (1ULL << next_offset) - 1ULL;
// 001100
masks[lidx++] = next_mask ^ mask;
});