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Bug 1415454 - Replace log2 lookup table with FloorLog2. r=njn

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FloorLog2 expands to, essentially, a compiler builtin/intrinsic, that,
in turn, expands to a single machine instruction on tier 1 and other
platforms. On platforms where that's not the case, we can expect the
compiler to generate fast code anyways. So overall, this is all better
than manually using a log2 lookup table.

Also replace a manual power-of-two check with mozilla::IsPowerOfTwo,
which does the same test.

--HG--
extra : rebase_source : e8164c254723c74ef83e798073327ec6afa6f1fb
This commit is contained in:
Mike Hommey 2017-11-08 16:20:40 +09:00
parent 3a2a081a07
commit e68ddaa1c2
1 changed files with 2 additions and 27 deletions
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29
memory/build/mozjemalloc.cpp
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@ -2359,33 +2359,8 @@ arena_run_reg_dalloc(arena_run_t* run, arena_bin_t* bin, void* ptr, size_t size)
// actual division here can reduce allocator throughput by over 20%!
diff =
(unsigned)((uintptr_t)ptr - (uintptr_t)run - bin->mRunFirstRegionOffset);
if ((size & (size - 1)) == 0) {
// log2_table allows fast division of a power of two in the
// [1..128] range.
//
// (x / divisor) becomes (x >> log2_table[divisor - 1]).
// clang-format off
static const unsigned char log2_table[] = {
0, 1, 0, 2, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 4,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 6,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7
};
// clang-format on
if (size <= 128) {
regind = (diff >> log2_table[size - 1]);
} else if (size <= 32768) {
regind = diff >> (8 + log2_table[(size >> 8) - 1]);
} else {
// The run size is too large for us to use the lookup
// table. Use real division.
regind = diff / size;
}
if (mozilla::IsPowerOfTwo(size)) {
regind = diff >> FloorLog2(size);
} else if (size <= ((sizeof(size_invs) / sizeof(unsigned)) * kQuantum) + 2) {
regind = size_invs[(size / kQuantum) - 3] * diff;
regind >>= SIZE_INV_SHIFT;