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[C++] Holy cow are string streams slow

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I decided to change my parser ( not really a parser more of a line getter and string manipulator ) to using string streams for more elegant and readable code. Elegant and readable code was what I got but the performance hit is massive. Currently it uses getline to get a string and then uses that string to construct a string stream. The string stream is used for various operations like getting the line header / tokens / body / ignoring comments etc. etc..

But just that one operation string -> stringstream takes 6 seconds(!!) looping through a 40MB standford lucy wavefront obj file. 6 seconds!!!!! and that's on a Q6600 (not the best desktop chip nowadays but its not like its some ancient pos either). By comparison just getline without the string stream construction takes 2.3 seconds.

Why on earth does this one constructor triple the time of the code.

The >> operator seems slow as well. The body of the obj loader (consists of getting a header and tokens and then doing some atois and atofs) went from taking 13 seconds to taking 28.

My old parser used manual searching and tokenizing with for loops, but the input stream was still a C++ stl style one, and for comparison it was able to load lucy in ~18 seconds total, which isn't fast but its a lot better than the current 48 (12 of which is just sstream construction ).

So three questions:

1) Is using old school C style file input recommended over the C++ way?
2) alternatively are there some C++ stream speed secrets I am not yet privy to?
3) Is there some way to bypass that slow slow slow slow string stream constructor and getline directly into the string stream? Seriously.

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Unless you're comparing against a build with full optimizations enabled, your claims are unfounded. Do you have full optimizations enabled?

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Unless you're comparing against a build with full optimizations enabled, your claims are unfounded. Do you have full optimizations enabled?



Of course. /Ox in VC++ 10. I also turned off checked iterators and secure scl ( I think I read somewhere that one of these has implications for even release builds ).

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You could also try compiling the text file into a binary format for faster parsing.


I could, and I do this with my own internal format (which I export as a raw chunk of indices, vertices, skeletal weights, material strings etc.). But I am not concerned with that at the moment, I am concerned with why I can't write a fast text file parser using the classes of the C++ standard library.

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Tried running the program through the profiler in VS2010? I would try reusing the same stringstream object if possible to avoid having to reallocate the internal buffer all the time.

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Tried running the program through the profiler in VS2010? I would try reusing the same stringstream object if possible to avoid having to reallocate the internal buffer all the time.


You are onto something. Please explain further. My current code is



string line;
getline( input, line );
stream = stringstream( line ); // member of the class doing this code.



This is code that takes 6 seconds going through the entire file.

By contrast



string line;
getline( input, line );




takes only 2. There must be a better way to get a line into a stream. I feel like such a noob with this

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Can you show us the code?


Why not.



class Parser
{
public:
Parser( std::wstring file );

virtual void Ignore( const std::string& start, const std::string& end );
virtual void Rewind( void );
virtual void Next( void );
virtual void GetLine( std::string& line );
virtual void GetTokens( std::vector<std::string>& tokens );
virtual void GetHeader( std::string& header );
virtual void GetBody( std::string& body );
virtual void GetBodyTokens( std::vector<std::string>& bodyTokens );
virtual bool Good( void );

std::stringstream stream;

protected:
void TrimLine( std::string& line );

int ignoring;
std::vector<std::string> excludeDelims;
std::vector<std::string> includeDelims;
std::ifstream input;
};




void Parser::Ignore( const std::string& start, const std::string& end )
{
excludeDelims.push_back( start );
includeDelims.push_back( end );
}

void Parser::Rewind( void )
{
input.seekg( 0, ios::beg );
input.clear();

ignoring = -1;
stream = stringstream( "" );
}

void Parser::Next( void )
{
string line;
getline( input, line );

if( !input.good() )
return;

if( line.empty() )
{
Next();
return;
}

TrimLine( line );
if( line.empty() )
{
Next();
return;
}

stream = stringstream( line );
}

void Parser::GetLine( std::string& line )
{
line.assign( stream.str() );
}

void Parser::GetTokens( std::vector<std::string>& tokens )
{
tokens.clear();
stream.clear();
stream.seekg( 0, ios::beg );

string token;

while( stream >> token )
tokens.push_back( token );
}

void Parser::GetHeader( std::string& header )
{
header.clear();
stream.clear();
stream.seekg( 0, ios::beg );

stream >> header;
}

void Parser::GetBody( std::string& body )
{
body.clear();
stream.clear();
stream.seekg( 0, ios::beg );

body.assign( stream.str() );

size_t i = 0;

// Ignore any white spaces at the beginning of the line.
while( body == ' ' && body == '\r' && body == '\t' && i < body.length() )
i++;

// Ignore the first word
while( body != ' ' && body != '\r' && body != '\t' && i < body.length() )
i++;

body = body.substr( i, body.length() );
}

void Parser::GetBodyTokens( std::vector<std::string>& bodyTokens )
{
bodyTokens.clear();
stream.clear();
stream.seekg( 0, ios::beg );

string token;

stream >> token;
while( stream >> token )
bodyTokens.push_back( token );
}

bool Parser::Good( void )
{
return input.good();
}

void Parser::TrimLine( string& line )
{
if( ignoring != -1 )
{
size_t incPos = line.find( includeDelims[ignoring] );
if( incPos != string::npos )
{
line = line.substr( incPos, line.length() );
ignoring = -1;
TrimLine( line );
}
else
line.clear();
}
else
{
for( size_t i = 0; i < excludeDelims.size(); i++ )
{
size_t excPos = line.find( excludeDelims );
if( excPos != string::npos )
{
string tail = line.substr( excPos, line.length() );
line = line.substr( 0, excPos );

// If the includeDelim is the end of the line just return the head.
if( includeDelims == "\n" )
return;

ignoring = i;
TrimLine( tail );
line += tail;
return;
}
}
}
}


Here is the obj loader code, although this hasn't changed since my 18 second lucy bench mark, just the above posted backend has.


shared_ptr<Mesh> ImportImpl::LoadObjMesh( wstring file )
{
shared_ptr<Mesh> mesh = LookupMesh( file );
if( mesh )
return mesh;

mesh = shared_ptr<Mesh>( new Mesh );

wstring path = FindFullPath( file );
ObjParser parser( path );

int numPositions = 0;
int numTexcoords = 0;
int numNormals = 0;
int numGroups = 0;
int numFaces = 0;

parser.Ignore( "#", "\n" );

// Preliminary run through to gather information.
while( parser.Good() )
{
parser.Next();
string line;

parser.GetLine( line );

switch( line[0] )
{
case 'v':
switch( line[1] )
{
case ' ':
numPositions++;
break;
case 't':
numTexcoords++;
break;
case 'n':
numNormals++;
break;
}
break;
case 'f':
numFaces++;
break;
case 'g':
numGroups++;
break;
}
}

if( !numPositions )
throw ExcFailed( L"[ImportImpl::LoadObjMesh] " + file + L" does not contain vertex positions.\n" );
if( numPositions < 0 || numFaces < 0 || numGroups < 0 )
throw ExcFailed( L"[ImportImpl::LoadObjMesh] " + file + L" holds way too much attribute data.\n" );
parser.Rewind();

vector<Position> positions;
vector<Normal> normals;
vector<Texcoord> texcoords;

positions.reserve( numPositions );
normals.reserve( numNormals );
texcoords.reserve( numTexcoords );
mesh->subMeshes.reserve( numGroups );
mesh->triangles.reserve( numFaces );

wstring_convert<std::codecvt_utf8<wchar_t>> converter;
Hash hasher;
forward_list<int> hashGrid[65536];

while( parser.Good() )
{
parser.Next();

string header;
vector<string> tokens;

parser.GetHeader( header );
parser.GetBodyTokens( tokens );

if( header == "v" )
{
Position p;

p.x = float( ( atof( tokens[0].c_str() ) ) );
p.y = float( ( atof( tokens[1].c_str() ) ) );
p.z = float( ( atof( tokens[2].c_str() ) ) );

if( tokens.size() == 4 )
p.w = float( ( atof( tokens[3].c_str() ) ) );
else
p.w = 1.0f;

positions.push_back( p );
}
else if( header == "vt" )
{
Texcoord o;

o.s = float( atof( tokens[0].c_str() ) );
o.t = float( atof( tokens[1].c_str() ) );

texcoords.push_back( o );
}
else if( header == "vn" )
{
Normal n;

n.x = float( atof( tokens[0].c_str() ) );
n.y = float( atof( tokens[1].c_str() ) );
n.z = float( atof( tokens[2].c_str() ) );

normals.push_back( n );
}
else if( header == "f" )
{
vector<Vertex> faceVertices = parser.GetFaceVertices( positions, normals, texcoords );

for( unsigned int i = 0; i < tokens.size() - 2; i++ )
{
Vertex v[3];

v[0] = faceVertices[0];
v[1] = faceVertices[i + 1];
v[2] = faceVertices[i + 2];

// Fill out the vertex indices of the triangle by either pushing vertices into
// the mesh vector, or finding the index of an already existant equivalent.
Triangle tri;
for( int j = 0; j < 3; j++ )
{
unsigned int hash = hasher.GenerateHash16( v[j] );
bool found = false;
int index;

forward_list<int>::iterator it = hashGrid[hash].begin();
while( it != hashGrid[hash].end() )
{
if( mesh->vertices[*it] == v[j] )
{
index = *it;
found = true;
break;
}
it++;
}

if( !found )
{
index = mesh->vertices.size();
mesh->vertices.push_back( v[j] );
hashGrid[hash].push_front( index );
}

// Vertices are even indices in the t array.
tri.t[j * 2] = index;
}

mesh->triangles.push_back( tri );

if( !mesh->subMeshes.empty() )
mesh->subMeshes.back().triangleIndices.push_back( mesh->triangles.size() );
}
}
else if( header == "g" )
{
mesh->subMeshes.push_back( SubMesh() );
}
else if( header == "usemtl" )
{
wstring mtl = converter.from_bytes( tokens[0] );

mesh->subMeshes.back().materialIndex = mesh->materials.size();
mesh->materials.push_back( LoadMtlMaterial( mtl ) );
}
}

mesh->FindTriangleNeighbors();

if( normals.empty() )
mesh->FindVertexNormals();

mesh->Trim();

meshCache.push_back( Record<Mesh>( file, mesh ) );
return mesh;
}


Before you mention it, TrimLine has an overhead of an extra 0.3 seconds on lucy, and that is an overhead I am willing to pay for something that can get rid of /* */ and // style commented text on the fly.The GetBody function has an assignment that I can probably cut out but I don't use it in the obj loader ( I do in the md5 loader but I am just looking at the obj loader for profiling the backend for now ). The part of the obj loader that hashes vertices is welding them on the fly as my internal format requires them that way: that algorithm is not super fast (incurs at least a 6 second overhead) but fast enough considering what it does.

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In your next function:
stream = stringstream( line );

should you not use..

stream << line ;

I haven't read every line of the code but I'm not sure you need to be creating a new stringstream on each read.

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