-----Original Message----- From: Boost-users [mailto:boost-users-bounces@lists.boost.org] On Behalf Of David Roberts Sent: 27 March 2014 15:58 To: boost-users@lists.boost.org Subject: Re: [Boost-users] lexical_cast between double and string slow in Visual Studio 2013
That issue is unknown. I'd really appreciate the investigation.
I have done some more investigation, and there are two factors that only cause
slowness when they both occur together.
Try excluding the lexical_cast from test, I have a feeling that this is only MSVC related issue:
#include <sstream> #include <string>
int main (int, char **) { for (double count = 0.0; count < 1000000.0; count += 1.41) { std::stringstream ss; ss << count; std::string result = std::move(ss.str()); ss.str(std::string());
ss << result; ss >> count; }
return 0; }
Running your test program does not exhibit the problem. It runs in around 3 seconds on my machine when built with either Visual Studio 2010 or Visual Studio 2013.
However, changing it very slightly to match more closely what lexical_cast does internally does recreate the problem:
#include <sstream> #include <string>
int main (int, char **) { for (double count = 0.0; count < 1000000.0; count += 1.41) { std::stringstream ss; ss.unsetf(std::ios::skipws); ss.precision(17);
ss << count; std::string result = std::move(ss.str()); ss.str(std::string());
ss << result; ss >> count; } return 0; }
The effect of setting the precision to 17 is that lots of 9s appear in the string representations. (The number 17 is what boost::detail::lcast_get_precision(double*) chooses.) Without the precision call the contents of the string called result start off like this:
0 1.41 2.82 4.23 5.64 7.05 8.46 9.87 11.28 12.69
With precision set to 17 they start off like this:
0 1.4099999999999999 2.8199999999999998 4.2299999999999995 5.6399999999999997 7.0499999999999998 8.4599999999999991 9.8699999999999992 11.279999999999999 12.69
This happens for both Visual Studio 2010 and Visual Studio 2013.
Then the next difference is that Visual Studio 2013 spends a lot longer handling all the extra 9s. Changing the program so that the double is converted to a string using std::stringstream without a precision call and then back to double using lexical_cast takes about 3 seconds for both Visual Studio 2010 and Visual Studio 2013. It is the combination of having all the extra 9s to parse and using Visual Studio 2013
makes the test using lexical_cast to go both ways slow.
Both Visual Studio 2010 and Visual Studio 2013 do the conversion by calling std::num_get<char,std::istreambuf_iterator<char,std::char_traits<char> >
::do_get() which then calls a function called _Stodx() which is implemented in xstod.c. This function is very different for the two versions. In Visual Studio 2010 it's a relatively thin wrapper around the C function strtod(). In Visual Studio 2013 _Stodx() has got a completely new implementation that's generated by #including xxstod.h with some macros defined.
The original C function strtod() is much faster than the new _Stodx() when
lots of 9s at the end of the strings being parsed. This modification to the
the that there are program:
#include <sstream> #include <string>
#include <stdlib.h>
int main (int, char **) { for (double count = 0.0; count < 1000000.0; count += 1.41) { std::stringstream ss; ss.unsetf(std::ios::skipws); ss.precision(17);
ss << count; std::string result = std::move(ss.str()); ss.str(std::string());
ss << result; char *endptr; count = strtod(ss.str().c_str(), &endptr); } return 0; }
has a runtime of about 3 seconds even though it's got to cope with all the 9s.
I guess only someone from Microsoft or Dinkumware could comment on why _Stodx() was reimplemented.
But the other thing is that by setting precision to 17 lexical_cast is
bloating the string
representations of the doubles with lots of 9s in both Visual Studio 2010 and Visual Studio 2013. Setting precision to 15 instead prevents this, and makes the original test run faster even with Visual Studio 2013 (about 4 seconds rather than 10).
In order to be sure of 'round-tripping' one needs to output std::numeric_limits<FPT>::max_digits10 decimal digits. max_digits10 is 17 for double enough to ensure that all *possibly* significant digits are used. digits10 is 15 for double and using this will work for *your* example, but will fail to 'round-trip' exactly for some values of double. The reason for a rewrite *might* be that for VS <=11, there was a slight 'feature' ('feature' according to Microsoft, 'bug' according to many, though the C++ Standard does NOT require round-tripping to be exact. Recent GCC and Clang achieve exact round-tripping.) // The original value causing trouble using serialization was 0.00019075645054089487; // wrote 0.0019075645054089487 // read 0.0019075645054089489 // a increase of just 1 bit. // Although this test uses a std::stringstream, it is possible that // the same behaviour will be found with ALL streams, including cout and cin? // The wrong inputs are only found in a very narrow range of values: // approximately 0.0001 to 0.004, with exponent values of 3f2 to 3f6 // and probably every third value of significand (tested using nextafter). However, a re-test reveals that this 'feature' is still present using VS2013 (version 12.0). (This tests uses random double values to find round-trip or loopback failures).
Description: Autorun "J:\Cpp\Misc\Debug\loopback.exe" 1> 1> failed 78, out of 100000, fraction 0.00077999999999999999 1> 1> wrong min 5.2173006024157652e-310 == 600ac32350ee 1> wrong max 8.7621968418217147e-308 == 2f80e435eb2ef3 1> 1> test min 1.2417072250589532e-311 == 24928faf2f7 1> test max 1.7898906514522990e+308 == 7fefdc71c85a1145 1> 186a0 loopback tests done. 1>FinalizeBuildStatus: 1> Deleting file "Debug\loopback.tlog\unsuccessfulbuild". 1> Touching "Debug\loopback.tlog\loopback.lastbuildstate". 1> 1>Build succeeded.
But this time it only occurs for a *different* and much smaller range :-( 1> Description: Autorun "J:\Cpp\Misc\Debug\loopback.exe" 1> 1> Written : 2.0367658404750995e-308 == ea55b0142dc71 1> Readback : 2.0367658404751000e-308 == ea55b0142dc72 1> Written : 7.2650939912298312e-308 == 2a1eee018d6993 1> Readback : 7.2650939912298322e-308 == 2a1eee018d6994 1> Written : 1.0124608169366832e-308 == 747c6af50194c 1> Readback : 1.0124608169366827e-308 == 747c6af50194b ... 1> failed 77, out of 100000, fraction 0.00076999999999999996 1> 1> wrong min 5.4632820247365795e-310 == 6491f5f0ab91 1> wrong max 8.7543773312713900e-308 == 2f79b1b891b2c1 1> 1> test min 2.1782631694667282e-310 == 2819299bf337 1> test max 1.7974889513081573e+308 == 7fefff11cdbbcb43 1> 186a0 loopback tests done. 1> I've retested using VS 2013 and the failures are now in the narrow range very near to numeric_limits<double>::min() Much better, but still not quite right :-( 1> Readback : 6.1131075857298205e-308 == 25fa9ea293ff26 1> failed 3680, out of 10000000, fraction 0.00036800000000000000 1> 1> wrong min 4.4505959275765217e-308 == 2000699c514815 1> wrong max 8.8998755028746106e-308 == 2fff9d0d8336f1 1> 1> test min 8.9025924527339071e-313 == 29f4307bd7 1> test max 1.7976312864655923e+308 == 7fefffb7d9534507 1> 98bf7a loopback tests done. To work around this 'feature' it was only necessary to use std::scientific format (but of course this means more characters to digest). (But with VS2013 the results are as 'wrong' as not using std::scientific, so go figure ???). This whole process is a minefield and you can find more than you wanted to know from Rich Regan's work, starting (but not ending) with http://www.exploringbinary.com/incorrect-round-trip-conversions-in-visual-c-... -plus/ For me, the bottom line is that, for C++ the whole IO needs to be rewritten *in C++*, perhaps using Fusion. This might be an exercise for a student ;-) Boost must be portable, so I'm not sure about your 'improvement' to speed, but if speed on MSVC matters to you, then use it. Equally, the tiny risk of a small loss of accuracy may not matter to you either, so using just 15 decimal digits may be acceptable. IMO, exact round-tipping is essential (especially for serialization) , speed is just nice. HTH (though I fear not). Paul --- Paul A. Bristow Prizet Farmhouse Kendal UK LA8 8AB +44 01539 561830 07714330204