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Improve performance of constructing filesystem::path from strings.

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This patch fixes a performance bug when constructing or appending to a path
from a string or c-string. Previously we called 'push_back' to append every
single character. This caused multiple re-allocation and copies when at most
one reallocation is necessary. The new behavior is to simply call
`string::append` so it can correctly handle reallocation.

For large strings this change is a ~4x improvement. This also makes our path
faster to construct than libstdc++'s.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285530 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier 2016-10-30 23:53:50 +00:00
parent 271a19ec19
commit ad1a12c312
3 changed files with 22 additions and 14 deletions
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20
benchmarks/filesystem.bench.cpp
View File

@ -21,10 +21,26 @@ void BM_PathConstructString(benchmark::State &st, GenInputs gen) {
benchmark::DoNotOptimize(P.native().data());
}
}
BENCHMARK_CAPTURE(BM_PathConstructString, iterate_elements,
BENCHMARK_CAPTURE(BM_PathConstructString, large_string,
getRandomStringInputs)->Arg(TestNumInputs);
template <class GenInputs>
void BM_PathConstructCStr(benchmark::State &st, GenInputs gen) {
using namespace fs;
const auto in = gen(st.range(0));
path PP;
for (auto& Part : in)
PP /= Part;
benchmark::DoNotOptimize(PP.native().data());
while (st.KeepRunning()) {
const path P(PP.native().c_str());
benchmark::DoNotOptimize(P.native().data());
}
}
BENCHMARK_CAPTURE(BM_PathConstructCStr, large_string,
getRandomStringInputs)->Arg(TestNumInputs);
template <class GenInputs>
void BM_PathIterateMultipleTimes(benchmark::State &st, GenInputs gen) {
using namespace fs;
@ -85,6 +101,4 @@ void BM_PathIterateOnceBackwards(benchmark::State &st, GenInputs gen) {
BENCHMARK_CAPTURE(BM_PathIterateOnceBackwards, iterate_elements,
getRandomStringInputs)->Arg(TestNumInputs);
BENCHMARK_MAIN()

7
include/experimental/filesystem
View File

@ -623,10 +623,10 @@ struct _PathCVT {
template <>
struct _PathCVT<char> {
template <class _Iter>
static void __append_range(string& __dest, _Iter __b, _Iter __e) {
for (; __b != __e; ++__b)
__dest.push_back(*__b);
__dest.append(__b, __e);
}
template <class _Iter>
@ -640,7 +640,8 @@ struct _PathCVT<char> {
static void __append_source(string& __dest, _Source const& __s)
{
using _Traits = __is_pathable<_Source>;
__append_range(__dest, _Traits::__range_begin(__s), _Traits::__range_end(__s));
__append_range(__dest, _Traits::__range_begin(__s),
_Traits::__range_end(__s));
}
};

9
src/experimental/filesystem/path.cpp
View File

@ -42,8 +42,7 @@ private:
public:
PathParser(string_view_t P, string_view_t E, unsigned char S)
: Path(P), RawEntry(E), State(static_cast<ParserState>(S)) {
assert(S != 0);
assert(S != PS_BeforeBegin);
// S cannot be '0' or PS_BeforeBegin.
}
static PathParser CreateBegin(string_view_t P) noexcept {
@ -94,7 +93,6 @@ public:
case PS_InFilenames: {
PosPtr SepEnd = consumeSeparator(Start, End);
assert(SepEnd);
if (SepEnd != End) {
PosPtr TkEnd = consumeName(SepEnd, End);
if (TkEnd)
@ -131,7 +129,6 @@ public:
SepEnd + 1, RStart + 1);
} else {
PosPtr TkStart = consumeName(RStart, REnd);
assert(TkStart);
if (TkStart == REnd + 2 && consumeSeparator(TkStart, REnd) == REnd)
return makeState(PS_InRootName, Path.data(), RStart + 1);
else
@ -192,14 +189,10 @@ public:
private:
void makeState(ParserState NewState, PosPtr Start, PosPtr End) noexcept {
assert(NewState != PS_BeforeBegin && NewState != PS_AtEnd);
State = NewState;
assert(Start < End);
assert(Start >= &Path.front() && End <= &Path.back() + 1);
RawEntry = string_view_t(Start, End - Start);
}
void makeState(ParserState NewState) noexcept {
assert(NewState == PS_BeforeBegin || NewState == PS_AtEnd);
State = NewState;
RawEntry = {};
}