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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="id1998370"></a>Practical considerations</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="s07.html#id1998375">Performance</a></span></dt> |
| <dt><span class="section"><a href="s07.html#id1998765">About compiling</a></span></dt> |
| <dt><span class="section"><a href="s07.html#id1998815">Portability</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id1998375"></a>Performance</h3></div></div></div> |
| <p>In theory, all overhead of using STL algorithms and lambda functors |
| compared to hand written loops can be optimized away, just as the overhead |
| from standard STL function objects and binders can. |
| |
| Depending on the compiler, this can also be true in practice. |
| We ran two tests with the GCC 3.0.4 compiler on 1.5 GHz Intel Pentium 4. |
| The optimization flag -03 was used. |
| </p> |
| <p> |
| In the first test we compared lambda functors against explicitly written |
| function objects. |
| We used both of these styles to define unary functions which multiply the |
| argument repeatedly by itself. |
| We started with the identity function, going up to |
| x<sup>5</sup>. |
| The expressions were called inside a <code class="literal">std::transform</code> loop, |
| reading the argument from one <code class="literal">std::vector<int></code> |
| and placing the result into another. |
| The length of the vectors was 100 elements. |
| The running times are listed in |
| <a class="xref" href="s07.html#table:increasing_arithmetic_test" title="Table 11.3. Test 1">Table 11.3, “Test 1”</a>. |
| |
| We can observe that there is no significant difference between the |
| two approaches. |
| </p> |
| <p> |
| In the second test we again used <code class="literal">std::transform</code> to |
| perform an operation to each element in a 100-element long vector. |
| This time the element type of the vectors was <code class="literal">double</code> |
| and we started with very simple arithmetic expressions and moved to |
| more complex ones. |
| The running times are listed in <a class="xref" href="s07.html#table:ll_vs_stl_test" title="Table 11.4. Test 2">Table 11.4, “Test 2”</a>. |
| |
| Here, we also included classic STL style unnamed functions into tests. |
| We do not show these expressions, as they get rather complex. |
| For example, the |
| last expression in <a class="xref" href="s07.html#table:ll_vs_stl_test" title="Table 11.4. Test 2">Table 11.4, “Test 2”</a> written with |
| classic STL tools contains 7 calls to <code class="literal">compose2</code>, |
| 8 calls to <code class="literal">bind1st</code> |
| and altogether 14 constructor invocations for creating |
| <code class="literal">multiplies</code>, <code class="literal">minus</code> |
| and <code class="literal">plus</code> objects. |
| |
| In this test the BLL expressions are a little slower (roughly 10% on average, |
| less than 14% in all cases) |
| than the corresponding hand-written function objects. |
| The performance hit is a bit greater with classic STL expressions, |
| up to 27% for the simplest expressios. |
| </p> |
| <p> |
| The tests suggest that the BLL does not introduce a loss of performance |
| compared to STL function objects. |
| With a reasonable optimizing compiler, one should expect the performance characteristics be comparable to using classic STL. |
| Moreover, with simple expressions the performance can be expected to be close |
| to that of explicitly written function objects. |
| |
| |
| |
| Note however, that evaluating a lambda functor consist of a sequence of calls to small functions that are declared inline. |
| If the compiler fails to actually expand these functions inline, |
| the performance can suffer. |
| The running time can more than double if this happens. |
| Although the above tests do not include such an expression, we have experienced |
| this for some seemingly simple expressions. |
| |
| |
| </p> |
| <div class="table"> |
| <a name="table:increasing_arithmetic_test"></a><p class="title"><b>Table 11.3. Test 1</b></p> |
| <div class="table-contents"> |
| <div class="caption">CPU time of expressions with integer multiplication written as a lambda expression and as a traditional hand-coded function object class. |
| The running times are expressed in arbitrary units.</div> |
| <table class="table" summary="Test 1"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th>expression</th> |
| <th>lambda expression</th> |
| <th>hand-coded function object</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td>x</td> |
| <td>240</td> |
| <td>230</td> |
| </tr> |
| <tr> |
| <td>x*x</td> |
| <td>340</td> |
| <td>350</td> |
| </tr> |
| <tr> |
| <td>x*x*x</td> |
| <td>770</td> |
| <td>760</td> |
| </tr> |
| <tr> |
| <td>x*x*x*x</td> |
| <td>1180</td> |
| <td>1210</td> |
| </tr> |
| <tr> |
| <td>x*x*x*x*x</td> |
| <td>1950</td> |
| <td>1910</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <p><br class="table-break"> |
| </p> |
| <p> |
| </p> |
| <div class="table"> |
| <a name="table:ll_vs_stl_test"></a><p class="title"><b>Table 11.4. Test 2</b></p> |
| <div class="table-contents"> |
| <div class="caption">CPU time of arithmetic expressions written as lambda |
| expressions, as classic STL unnamed functions (using <code class="literal">compose2</code>, <code class="literal">bind1st</code> etc.) and as traditional hand-coded function object classes. |
| Using BLL terminology, |
| <code class="literal">a</code> and <code class="literal">b</code> are bound arguments in the expressions, and <code class="literal">x</code> is open. |
| All variables were of types <code class="literal">double</code>. |
| The running times are expressed in arbitrary units.</div> |
| <table class="table" summary="Test 2"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th>expression</th> |
| <th>lambda expression</th> |
| <th>classic STL expression</th> |
| <th>hand-coded function object</th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td>ax</td> |
| <td>330</td> |
| <td>370</td> |
| <td>290</td> |
| </tr> |
| <tr> |
| <td>-ax</td> |
| <td>350</td> |
| <td>370</td> |
| <td>310</td> |
| </tr> |
| <tr> |
| <td>ax-(a+x)</td> |
| <td>470</td> |
| <td>500</td> |
| <td>420</td> |
| </tr> |
| <tr> |
| <td>(ax-(a+x))(a+x)</td> |
| <td>620</td> |
| <td>670</td> |
| <td>600</td> |
| </tr> |
| <tr> |
| <td>((ax) - (a+x))(bx - (b+x))(ax - (b+x))(bx - (a+x))</td> |
| <td>1660</td> |
| <td>1660</td> |
| <td>1460</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <p><br class="table-break"> |
| </p> |
| <p>Some additional performance testing with an earlier version of the |
| library is described |
| <a class="xref" href="../lambda.html#cit:jarvi:00" title="The Lambda Library : Lambda Abstraction in C++">[<abbr class="abbrev">Jär00</abbr>]</a>. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id1998765"></a>About compiling</h3></div></div></div> |
| <p>The BLL uses templates rather heavily, performing numerous recursive instantiations of the same templates. |
| This has (at least) three implications: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p> |
| While it is possible to write incredibly complex lambda expressions, it probably isn't a good idea. |
| Compiling such expressions may end up requiring a lot of memory |
| at compile time, and being slow to compile. |
| </p></li> |
| <li class="listitem"><p> |
| The types of lambda functors that result from even the simplest lambda expressions are cryptic. |
| Usually the programmer doesn't need to deal with the lambda functor types at all, but in the case of an error in a lambda expression, the compiler usually outputs the types of the lambda functors involved. |
| This can make the error messages very long and difficult to interpret, particularly if the compiler outputs the whole chain of template instantiations. |
| </p></li> |
| <li class="listitem"><p> |
| The C++ Standard suggests a template nesting level of 17 to help detect infinite recursion. |
| Complex lambda templates can easily exceed this limit. |
| Most compilers allow a greater number of nested templates, but commonly require the limit explicitly increased with a command line argument. |
| </p></li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id1998815"></a>Portability</h3></div></div></div> |
| <div class="toc"><dl><dt><span class="section"><a href="s07.html#id1998847">Test coverage</a></span></dt></dl></div> |
| <p> |
| The BLL works with the following compilers, that is, the compilers are capable of compiling the test cases that are included with the BLL: |
| |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem">GCC 3.0.4 |
| </li> |
| <li class="listitem">KCC 4.0f with EDG 2.43.1 |
| </li> |
| <li class="listitem">GCC 2.96 (fails with one test case, the <code class="filename">exception_test.cpp</code> results in an internal compiler error. |
| ) |
| |
| </li> |
| </ul></div> |
| <p> |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="id1998847"></a>Test coverage</h4></div></div></div> |
| <p>The following list describes the test files included and the features that each file covers: |
| |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p> |
| <code class="filename">bind_tests_simple.cpp</code> : Bind expressions of different arities and types of target functions: function pointers, function objects and member functions. |
| Function composition with bind expressions.</p></li> |
| <li class="listitem"><p><code class="filename">bind_tests_simple_function_references.cpp</code> : |
| Repeats all tests from <code class="filename">bind_tests_simple.cpp</code> where the target function is a function pointer, but uses function references instead. |
| </p></li> |
| <li class="listitem"><p><code class="filename">bind_tests_advanced.cpp</code> : Contains tests for nested bind expressions, <code class="literal">unlambda</code>, <code class="literal">protect</code>, <code class="literal">const_parameters</code> and <code class="literal">break_const</code>. |
| Tests passing lambda functors as actual arguments to other lambda functors, currying, and using the <code class="literal">sig</code> template to specify the return type of a function object. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">operator_tests_simple.cpp</code> : |
| Tests using all operators that are overloaded for lambda expressions, that is, unary and binary arithmetic, |
| bitwise, |
| comparison, |
| logical, |
| increment and decrement, |
| compound, |
| assignment, |
| subscrict, |
| address of, |
| dereference, and comma operators. |
| The streaming nature of shift operators is tested, as well as pointer arithmetic with plus and minus operators. |
| </p></li> |
| <li class="listitem"><p><code class="filename">member_pointer_test.cpp</code> : The pointer to member operator is complex enough to warrant a separate test file. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">control_structures.cpp</code> : |
| Tests for the looping and if constructs. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">switch_construct.cpp</code> : |
| Includes tests for all supported arities of the switch statement, both with and without the default case. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">exception_test.cpp</code> : |
| Includes tests for throwing exceptions and for try/catch constructs with varying number of catch blocks. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">constructor_tests.cpp</code> : |
| Contains tests for <code class="literal">constructor</code>, <code class="literal">destructor</code>, <code class="literal">new_ptr</code>, <code class="literal">delete_ptr</code>, <code class="literal">new_array</code> and <code class="literal">delete_array</code>. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">cast_test.cpp</code> : Tests for the four cast expressions, as well as <code class="filename">typeid</code> and <code class="literal">sizeof</code>. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">extending_return_type_traits.cpp</code> : Tests extending the return type deduction system for user defined types. |
| Contains several user defined operators and the corresponding specializations for the return type deduction templates. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">is_instance_of_test.cpp</code> : Includes tests for an internally used traits template, which can detect whether a given type is an instance of a certain template or not. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="filename">bll_and_function.cpp</code> : |
| Contains tests for using <code class="literal">boost::function</code> together with lambda functors. |
| </p></li> |
| </ul></div> |
| <p> |
| |
| </p> |
| </div> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 1999-2004 Jaakko Järvi, Gary Powell<p>Use, modification and distribution is subject to the Boost |
| Software License, Version 1.0. (See accompanying file |
| <code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p> |
| </div></td> |
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