Re: [boost] Boost.Convert. Take 2.

21 Feb 2014

      alex  writes:
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-----Original Message-----
From: Boost [mailto:boost-bounces <at> lists.boost.org] On Behalf Of Rob
Stewart
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Sent: 21 February 2014 15:35
To: boost <at> lists.boost.org
Subject: Re: [boost] Boost.Convert. Take 2.
On February 21, 2014 5:48:32 AM EST, alex 
wrote:
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All loops are actually like
for (int k = 0; k < local::num_cycles; ++k)
{
   int k = boost::convert<int>::from("12345", ccnv).value();
   BOOST_ASSERT(k == 12345);
}
Do you think there still might be something I missed?
The assertion won't help in an optimized build, of course. Save the
values and
print them at the end,  after you've captured the elapsed times.
Isn't it also a problem that the inside of the loop doesn't vary? A
smart
compiler may decide to evaluate boost::convert<int>::from("12345",
ccnv).value() only once and re-use the result?
Of course,  yes. I should have noticed that. Incrementing a value each
iteration
and printing that value at the end should work. Overflow isn't important.
Just
using the final value in an opaque way will prevent optimizing away the
conversions in the loops.
That seems easier said than done, because it is the value of a string that
needs to be unpredictable?
int sum  = 0;
for (int k = 0; k < local::num_cycles; ++k)
{
    char str[] = "12345";
    str[4 - k % 5] = 49 + k % 9; //because the char '1' has value 49
    sum += boost::convert<int>::from(str, ccnv).value();
}
std::cout << sum;
Rob, Alex, thank you for the input. Humiliated by the magnitude of my own
ignorance. Learned something today. Optimized and tested as suggested.
Optimization gains seem to vary a lot depending on the compiler:

MS Visual C++ 2010 Express. Not optimized:

for user-defined type: lcast/convert=18.95/14.28 seconds.
for int type: scanf/lcast/convert=3.05/1.20/24.17 seconds.

MS Visual C++ 2010 Express. Optimized for speed:

for user-defined type: lcast/convert=6.36/2.95 seconds.
for int type: scanf/lcast/convert=0.70/0.86/5.59 seconds.

$ g++ --version
g++ (GCC) 4.8.2 Cygwin
$ g++ -O3 test_convert.cpp -I../..
for user-defined type: lcast/convert=5.42/4.14 seconds.
for int type: scanf/lcast/convert=1.08/1.16/2.38 seconds.

$ g++ -O0 test_convert.cpp -I../..
for user-defined type: lcast/convert=6.16/4.59 seconds.
for int type: scanf/lcast/convert=1.08/1.80/2.69 seconds.

Still what I am puzzled by is that lexical_cast performance table in the
docs claims to be far ahead of scanf... and I cannot reproduce their
numbers. :-(

Re: [boost] Boost.Convert. Take 2.

Vladimir Batov