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Optimize filesystem::path by providing weaker exception guarantees.

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path uses string::append to construct, append, and concatenate paths. Unfortunatly
string::append has a strong exception safety guaranteed and if it can't prove
that the iterator operations don't throw then it will allocate a temporary
string copy to append to. However this extra allocation and copy is very
undesirable for path which doesn't have the same exception guarantees.

To work around this this patch adds string::__append_forward_unsafe which exposes
the std::string::append interface for forward iterators without enforcing
that the iterator is noexcept.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285532 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier
2016-10-31 02:46:25 +00:00
parent eb6b13f578
commit 026d38e8fb
4 changed files with 65 additions and 26 deletions
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34
benchmarks/filesystem.bench.cpp
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@@ -2,6 +2,7 @@
#include "benchmark/benchmark_api.h"
#include "GenerateInput.hpp"
#include "test_iterators.h"
namespace fs = std::experimental::filesystem;
@@ -41,6 +42,39 @@ void BM_PathConstructCStr(benchmark::State &st, GenInputs gen) {
BENCHMARK_CAPTURE(BM_PathConstructCStr, large_string,
getRandomStringInputs)->Arg(TestNumInputs);
template <template <class...> class ItType, class GenInputs>
void BM_PathConstructIter(benchmark::State &st, GenInputs gen) {
using namespace fs;
using Iter = ItType<std::string::const_iterator>;
const auto in = gen(st.range(0));
path PP;
for (auto& Part : in)
PP /= Part;
auto Start = Iter(PP.native().begin());
auto End = Iter(PP.native().end());
benchmark::DoNotOptimize(PP.native().data());
benchmark::DoNotOptimize(Start);
benchmark::DoNotOptimize(End);
while (st.KeepRunning()) {
const path P(Start, End);
benchmark::DoNotOptimize(P.native().data());
}
}
template <class GenInputs>
void BM_PathConstructInputIter(benchmark::State &st, GenInputs gen) {
BM_PathConstructIter<input_iterator>(st, gen);
}
template <class GenInputs>
void BM_PathConstructForwardIter(benchmark::State &st, GenInputs gen) {
BM_PathConstructIter<forward_iterator>(st, gen);
}
BENCHMARK_CAPTURE(BM_PathConstructInputIter, large_string,
getRandomStringInputs)->Arg(TestNumInputs);
BENCHMARK_CAPTURE(BM_PathConstructForwardIter, large_string,
getRandomStringInputs)->Arg(TestNumInputs);
template <class GenInputs>
void BM_PathIterateMultipleTimes(benchmark::State &st, GenInputs gen) {
using namespace fs;