Skip to content

Commit c9d475c

Browse files
Lancernphilnik777
Lancern
authored and
philnik777
committed
[libc++abi] Improve performance of __dynamic_cast
The original `__dynamic_cast` implementation does not use the ABI-provided `src2dst_offset` parameter which helps improve performance on the hot paths. This patch improves the performance on the hot paths in `__dynamic_cast` by leveraging hints provided by the `src2dst_offset` parameter. This patch also includes a performance benchmark suite for the `__dynamic_cast` implementation. Reviewed By: philnik, ldionne, #libc, #libc_abi, avogelsgesang Spies: mikhail.ramalho, avogelsgesang, xingxue, libcxx-commits Differential Revision: https://reviews.llvm.org/D138005
1 parent 8510cf9 commit c9d475c

File tree

4 files changed

+300
-62
lines changed

4 files changed

+300
-62
lines changed

‎libcxx/benchmarks/CMakeLists.txt

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -176,6 +176,7 @@ set(BENCHMARK_TESTS
176176
algorithms/sort.bench.cpp
177177
algorithms/sort_heap.bench.cpp
178178
algorithms/stable_sort.bench.cpp
179+
libcxxabi/dynamic_cast.bench.cpp
179180
allocation.bench.cpp
180181
deque.bench.cpp
181182
deque_iterator.bench.cpp

‎libcxx/benchmarks/libcxxabi/dynamic_cast.bench.cpp

Lines changed: 172 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,172 @@
1+
//===----------------------------------------------------------------------===//
2+
//
3+
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4+
// See https://llvm.org/LICENSE.txt for license information.
5+
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6+
//
7+
//===----------------------------------------------------------------------===//
8+
9+
#include <cstddef>
10+
11+
#include "benchmark/benchmark.h"
12+
13+
template <std::size_t Depth>
14+
struct Chain : Chain<Depth - 1> {};
15+
16+
template <>
17+
struct Chain<0> {
18+
virtual ~Chain() noexcept = default;
19+
};
20+
21+
template <std::size_t Index, std::size_t Depth>
22+
struct Dag : Dag<Index, Depth - 1>, Dag<Index + 1, Depth - 1> {};
23+
24+
template <std::size_t Index>
25+
struct Dag<Index, 0> {
26+
virtual ~Dag() noexcept = default;
27+
};
28+
29+
template <std::size_t Depth>
30+
struct VChain : virtual VChain<Depth - 1> {};
31+
32+
template <>
33+
struct VChain<0> {
34+
virtual ~VChain() noexcept = default;
35+
};
36+
37+
template <std::size_t Index, std::size_t Depth>
38+
struct VDag : virtual VDag<Index, Depth - 1>, virtual VDag<Index + 1, Depth - 1> {};
39+
40+
template <std::size_t Index>
41+
struct VDag<Index, 0> {
42+
virtual ~VDag() noexcept = default;
43+
};
44+
45+
template <typename Dyn, typename From, typename To = Dyn>
46+
static void DynCast(benchmark::State& state) {
47+
Dyn obj;
48+
From* from_ptr = &obj;
49+
for (auto _ : state) {
50+
To* to_ptr = dynamic_cast<To*>(from_ptr);
51+
benchmark::DoNotOptimize(to_ptr);
52+
}
53+
}
54+
55+
static void StaticCast(benchmark::State& state) {
56+
Chain<9> obj;
57+
Chain<0>* from_ptr = &obj;
58+
for (auto _ : state) {
59+
Chain<9>* to_ptr = static_cast<Chain<9>*>(from_ptr);
60+
benchmark::DoNotOptimize(to_ptr);
61+
}
62+
}
63+
64+
// Downcast along a chain from base to the most derived type
65+
BENCHMARK(DynCast<Chain<1>, Chain<0>>)->Name("Chain, 1 level");
66+
BENCHMARK(DynCast<Chain<2>, Chain<0>>)->Name("Chain, 2 levels");
67+
BENCHMARK(DynCast<Chain<3>, Chain<0>>)->Name("Chain, 3 levels");
68+
BENCHMARK(DynCast<Chain<4>, Chain<0>>)->Name("Chain, 4 levels");
69+
BENCHMARK(DynCast<Chain<5>, Chain<0>>)->Name("Chain, 5 levels");
70+
BENCHMARK(DynCast<Chain<6>, Chain<0>>)->Name("Chain, 6 levels");
71+
BENCHMARK(DynCast<Chain<7>, Chain<0>>)->Name("Chain, 7 levels");
72+
BENCHMARK(DynCast<Chain<8>, Chain<0>>)->Name("Chain, 8 levels");
73+
BENCHMARK(DynCast<Chain<9>, Chain<0>>)->Name("Chain, 9 levels");
74+
75+
// Downcast along a chain from base to the middle of the chain
76+
BENCHMARK(DynCast<Chain<2>, Chain<0>, Chain<1>>)->Name("Chain middle, 1 level");
77+
BENCHMARK(DynCast<Chain<4>, Chain<0>, Chain<2>>)->Name("Chain middle, 2 levels");
78+
BENCHMARK(DynCast<Chain<6>, Chain<0>, Chain<3>>)->Name("Chain middle, 3 levels");
79+
BENCHMARK(DynCast<Chain<8>, Chain<0>, Chain<4>>)->Name("Chain middle, 4 levels");
80+
81+
// Downcast along a chain that fails
82+
BENCHMARK(DynCast<Chain<1>, Chain<0>, Chain<9>>)->Name("Chain fail, 1 level");
83+
BENCHMARK(DynCast<Chain<2>, Chain<0>, Chain<9>>)->Name("Chain fail, 2 levels");
84+
BENCHMARK(DynCast<Chain<3>, Chain<0>, Chain<9>>)->Name("Chain fail, 3 levels");
85+
BENCHMARK(DynCast<Chain<4>, Chain<0>, Chain<9>>)->Name("Chain fail, 4 levels");
86+
BENCHMARK(DynCast<Chain<5>, Chain<0>, Chain<9>>)->Name("Chain fail, 5 levels");
87+
BENCHMARK(DynCast<Chain<6>, Chain<0>, Chain<9>>)->Name("Chain fail, 6 levels");
88+
BENCHMARK(DynCast<Chain<7>, Chain<0>, Chain<9>>)->Name("Chain fail, 7 levels");
89+
BENCHMARK(DynCast<Chain<8>, Chain<0>, Chain<9>>)->Name("Chain fail, 8 levels");
90+
91+
// Downcast along a virtual inheritance chain from base to the most derived type
92+
BENCHMARK(DynCast<VChain<1>, VChain<0>>)->Name("VChain, 1 level");
93+
BENCHMARK(DynCast<VChain<2>, VChain<0>>)->Name("VChain, 2 levels");
94+
BENCHMARK(DynCast<VChain<3>, VChain<0>>)->Name("VChain, 3 levels");
95+
BENCHMARK(DynCast<VChain<4>, VChain<0>>)->Name("VChain, 4 levels");
96+
BENCHMARK(DynCast<VChain<5>, VChain<0>>)->Name("VChain, 5 levels");
97+
98+
// Downcast along a virtual inheritance chain from base to the middle of the chain
99+
BENCHMARK(DynCast<VChain<2>, VChain<0>, VChain<1>>)->Name("VChain middle, 1 level");
100+
BENCHMARK(DynCast<VChain<4>, VChain<0>, VChain<2>>)->Name("VChain middle, 2 levels");
101+
BENCHMARK(DynCast<VChain<6>, VChain<0>, VChain<3>>)->Name("VChain middle, 3 levels");
102+
BENCHMARK(DynCast<VChain<8>, VChain<0>, VChain<4>>)->Name("VChain middle, 4 levels");
103+
104+
// Downcast along a virtual chain that fails
105+
BENCHMARK(DynCast<VChain<1>, VChain<0>, VChain<8>>)->Name("VChain fail, 1 level");
106+
BENCHMARK(DynCast<VChain<2>, VChain<0>, VChain<8>>)->Name("VChain fail, 2 levels");
107+
BENCHMARK(DynCast<VChain<3>, VChain<0>, VChain<8>>)->Name("VChain fail, 3 levels");
108+
BENCHMARK(DynCast<VChain<4>, VChain<0>, VChain<8>>)->Name("VChain fail, 4 levels");
109+
BENCHMARK(DynCast<VChain<5>, VChain<0>, VChain<8>>)->Name("VChain fail, 5 levels");
110+
111+
// Downcast along a DAG from base to the most derived type
112+
BENCHMARK(DynCast<Dag<0, 3>, Dag<3, 0>>)->Name("DAG rightmost, 3 levels");
113+
BENCHMARK(DynCast<Dag<0, 4>, Dag<4, 0>>)->Name("DAG rightmost, 4 levels");
114+
BENCHMARK(DynCast<Dag<0, 5>, Dag<5, 0>>)->Name("DAG rightmost, 5 levels");
115+
BENCHMARK(DynCast<Dag<0, 3>, Dag<0, 0>>)->Name("DAG leftmost, 3 levels");
116+
BENCHMARK(DynCast<Dag<0, 4>, Dag<0, 0>>)->Name("DAG leftmost, 4 levels");
117+
BENCHMARK(DynCast<Dag<0, 5>, Dag<0, 0>>)->Name("DAG leftmost, 5 levels");
118+
119+
// Downcast along a DAG from base to the middle of the DAG
120+
BENCHMARK(DynCast<Dag<0, 4>, Dag<4, 0>, Dag<3, 1>>)->Name("DAG rightmost middle, 1 level");
121+
BENCHMARK(DynCast<Dag<0, 4>, Dag<4, 0>, Dag<2, 2>>)->Name("DAG rightmost middle, 2 levels");
122+
BENCHMARK(DynCast<Dag<0, 4>, Dag<4, 0>, Dag<1, 3>>)->Name("DAG rightmost middle, 3 levels");
123+
BENCHMARK(DynCast<Dag<0, 4>, Dag<0, 0>, Dag<0, 1>>)->Name("DAG leftmost middle, 1 level");
124+
BENCHMARK(DynCast<Dag<0, 4>, Dag<0, 0>, Dag<0, 2>>)->Name("DAG leftmost middle, 2 levels");
125+
BENCHMARK(DynCast<Dag<0, 4>, Dag<0, 0>, Dag<0, 3>>)->Name("DAG leftmost middle, 3 levels");
126+
127+
// Sidecast along a DAG
128+
BENCHMARK(DynCast<Dag<0, 3>, Dag<3, 0>, Dag<0, 0>>)->Name("DAG sidecast, 3 levels");
129+
BENCHMARK(DynCast<Dag<0, 3>, Dag<2, 1>, Dag<0, 1>>)->Name("DAG sidecast, 2 levels");
130+
BENCHMARK(DynCast<Dag<0, 3>, Dag<1, 2>, Dag<0, 2>>)->Name("DAG sidecast, 1 level");
131+
132+
// Sidecast along a DAG that fails
133+
BENCHMARK(DynCast<Dag<0, 3>, Dag<3, 0>, Dag<0, 4>>)->Name("DAG sidecast fail, 3 levels");
134+
BENCHMARK(DynCast<Dag<0, 3>, Dag<2, 1>, Dag<0, 4>>)->Name("DAG sidecast fail, 2 levels");
135+
BENCHMARK(DynCast<Dag<0, 3>, Dag<1, 2>, Dag<0, 4>>)->Name("DAG sidecast fail, 1 level");
136+
137+
// Downcast along a virtual inheritance DAG from base to the most derived type
138+
BENCHMARK(DynCast<VDag<0, 3>, VDag<3, 0>>)->Name("VDAG rightmost, 3 levels");
139+
BENCHMARK(DynCast<VDag<0, 4>, VDag<4, 0>>)->Name("VDAG rightmost, 4 levels");
140+
BENCHMARK(DynCast<VDag<0, 5>, VDag<5, 0>>)->Name("VDAG rightmost, 5 levels");
141+
BENCHMARK(DynCast<VDag<0, 3>, VDag<0, 0>>)->Name("VDAG leftmost, 3 levels");
142+
BENCHMARK(DynCast<VDag<0, 4>, VDag<0, 0>>)->Name("VDAG leftmost, 4 levels");
143+
BENCHMARK(DynCast<VDag<0, 5>, VDag<0, 0>>)->Name("VDAG leftmost, 5 levels");
144+
145+
// Downcast along a virtual inheritance DAG from base to the middle of the DAG
146+
BENCHMARK(DynCast<VDag<0, 3>, VDag<3, 0>, VDag<2, 1>>)->Name("VDAG rightmost middle, 1 level");
147+
BENCHMARK(DynCast<VDag<0, 4>, VDag<4, 0>, VDag<2, 2>>)->Name("VDAG rightmost middle, 2 levels");
148+
BENCHMARK(DynCast<VDag<0, 5>, VDag<5, 0>, VDag<2, 3>>)->Name("VDAG rightmost middle, 3 levels");
149+
BENCHMARK(DynCast<VDag<0, 3>, VDag<0, 0>, VDag<0, 1>>)->Name("VDAG leftmost middle, 1 level");
150+
BENCHMARK(DynCast<VDag<0, 4>, VDag<0, 0>, VDag<0, 2>>)->Name("VDAG leftmost middle, 2 levels");
151+
BENCHMARK(DynCast<VDag<0, 5>, VDag<0, 0>, VDag<0, 3>>)->Name("VDAG leftmost middle, 3 levels");
152+
153+
// Sidecast along a virtual inheritance DAG
154+
BENCHMARK(DynCast<VDag<0, 3>, VDag<3, 0>, VDag<0, 0>>)->Name("VDAG sidecast, 3 levels");
155+
BENCHMARK(DynCast<VDag<0, 3>, VDag<2, 1>, VDag<0, 1>>)->Name("VDAG sidecast, 2 levels");
156+
BENCHMARK(DynCast<VDag<0, 3>, VDag<1, 2>, VDag<0, 2>>)->Name("VDAG sidecast, 1 level");
157+
158+
// Sidecast along a virtual inheritance DAG that fails
159+
BENCHMARK(DynCast<VDag<0, 3>, VDag<3, 0>, VDag<0, 4>>)->Name("VDAG sidecast fail, 3 levels");
160+
BENCHMARK(DynCast<VDag<0, 3>, VDag<2, 1>, VDag<0, 4>>)->Name("VDAG sidecast fail, 2 levels");
161+
BENCHMARK(DynCast<VDag<0, 3>, VDag<1, 2>, VDag<0, 4>>)->Name("VDAG sidecast fail, 1 level");
162+
163+
// Cast to complete object pointer
164+
BENCHMARK(DynCast<Chain<8>, Chain<0>, void>)->Name("Chain to complete");
165+
BENCHMARK(DynCast<VChain<5>, VChain<0>, void>)->Name("VChain to complete");
166+
BENCHMARK(DynCast<Dag<0, 3>, Dag<3, 0>, void>)->Name("DAG to complete");
167+
BENCHMARK(DynCast<VDag<0, 3>, VDag<3, 0>, void>)->Name("VDAG to complete");
168+
169+
// Static cast as the baseline.
170+
BENCHMARK(StaticCast)->Name("Static");
171+
172+
BENCHMARK_MAIN();

‎libcxx/docs/ReleaseNotes.rst

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -48,6 +48,9 @@ Improvements and New Features
4848
- ``std::string_view`` now provides iterators that check for out-of-bounds accesses when the safe
4949
libc++ mode is enabled.
5050

51+
- The performance of ``dynamic_cast`` on its hot paths is greatly improved and is as efficient as the
52+
``libsupc++`` implementation. Note that the performance improvements are shipped in ``libcxxabi``.
53+
5154
Deprecations and Removals
5255
-------------------------
5356

0 commit comments

Comments
 (0)