| // Copyright David Abrahams, Matthias Troyer, Michael Gauckler |
| // 2005. Distributed under the Boost Software License, Version |
| // 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
| // http://www.boost.org/LICENSE_1_0.txt) |
| |
| #if !defined(LIVE_CODE_TYPE) |
| # define LIVE_CODE_TYPE int |
| #endif |
| |
| #include <boost/timer.hpp> |
| |
| namespace test |
| { |
| // This value is required to ensure that a smart compiler's dead |
| // code elimination doesn't optimize away anything we're testing. |
| // We'll use it to compute the return code of the executable to make |
| // sure it's needed. |
| LIVE_CODE_TYPE live_code; |
| |
| // Call objects of the given Accumulator type repeatedly with x as |
| // an argument. |
| template <class Accumulator, class Arg> |
| void hammer(Arg const& x, long const repeats) |
| { |
| // Strategy: because the sum in an accumulator after each call |
| // depends on the previous value of the sum, the CPU's pipeline |
| // might be stalled while waiting for the previous addition to |
| // complete. Therefore, we allocate an array of accumulators, |
| // and update them in sequence, so that there's no dependency |
| // between adjacent addition operations. |
| // |
| // Additionally, if there were only one accumulator, the |
| // compiler or CPU might decide to update the value in a |
| // register rather that writing it back to memory. we want each |
| // operation to at least update the L1 cache. *** Note: This |
| // concern is specific to the particular application at which |
| // we're targeting the test. *** |
| |
| // This has to be at least as large as the number of |
| // simultaneous accumulations that can be executing in the |
| // compiler pipeline. A safe number here is larger than the |
| // machine's maximum pipeline depth. If you want to test the L2 |
| // or L3 cache, or main memory, you can increase the size of |
| // this array. 1024 is an upper limit on the pipeline depth of |
| // current vector machines. |
| const std::size_t number_of_accumulators = 1024; |
| live_code = 0; // reset to zero |
| |
| Accumulator a[number_of_accumulators]; |
| |
| for (long iteration = 0; iteration < repeats; ++iteration) |
| { |
| for (Accumulator* ap = a; ap < a + number_of_accumulators; ++ap) |
| { |
| (*ap)(x); |
| } |
| } |
| |
| // Accumulate all the partial sums to avoid dead code |
| // elimination. |
| for (Accumulator* ap = a; ap < a + number_of_accumulators; ++ap) |
| { |
| live_code += ap->sum; |
| } |
| } |
| |
| // Measure the time required to hammer accumulators of the given |
| // type with the argument x. |
| template <class Accumulator, class T> |
| double measure(T const& x, long const repeats) |
| { |
| // Hammer accumulators a couple of times to ensure the |
| // instruction cache is full of our test code, and that we don't |
| // measure the cost of a page fault for accessing the data page |
| // containing the memory where the accumulators will be |
| // allocated |
| hammer<Accumulator>(x, repeats); |
| hammer<Accumulator>(x, repeats); |
| |
| // Now start a timer |
| boost::timer time; |
| hammer<Accumulator>(x, repeats); // This time, we'll measure |
| return time.elapsed() / repeats; // return the time of one iteration |
| } |
| } |