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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.12: http://docutils.sourceforge.net/" />
<title>Intel® SPMD Program Compiler Performance Guide</title>
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<body>
<div class="document" id="intel-spmd-program-compiler-performance-guide">
<div id="wrap">
<div id="wrap2">
<div id="header">
<h1 id="logo">Intel SPMD Program Compiler</h1>
<div id="slogan">An open-source compiler for high-performance SIMD programming on
the CPU</div>
</div>
<div id="nav">
<div id="nbar">
<ul>
<li><a href="index.html">Overview</a></li>
<li><a href="news.html">News</a></li>
<li><a href="features.html">Features</a></li>
<li><a href="downloads.html">Downloads</a></li>
<li id="selected"><a href="documentation.html">Documentation</a></li>
<li><a href="perf.html">Performance</a></li>
<li><a href="contrib.html">Contributors</a></li>
</ul>
</div>
</div>
<div id="content-wrap">
<div id="sidebar">
<div class="widgetspace">
<h1>Resources</h1>
<ul class="menu">
<li><a href="http://github.com/ispc/ispc/">ispc page on github</a></li>
<li><a href="http://groups.google.com/group/ispc-users/">ispc
users mailing list</a></li>
<li><a href="http://groups.google.com/group/ispc-dev/">ispc
developers mailing list</a></li>
<li><a href="http://github.com/ispc/ispc/wiki/">Wiki</a></li>
<li><a href="http://github.com/ispc/ispc/issues/">Bug tracking</a></li>
<li><a href="doxygen/index.html">Doxygen</a></li>
</ul>
</div>
</div>
<h1 class="title">Intel® SPMD Program Compiler Performance Guide</h1>
<div id="content">
<p>The SPMD programming model provided by <tt class="docutils literal">ispc</tt> naturally delivers
excellent performance for many workloads thanks to efficient use of CPU
SIMD vector hardware. This guide provides more details about how to get
the most out of <tt class="docutils literal">ispc</tt> in practice.</p>
<ul class="simple">
<li><a class="reference internal" href="#key-concepts">Key Concepts</a><ul>
<li><a class="reference internal" href="#efficient-iteration-with-foreach">Efficient Iteration With "foreach"</a></li>
<li><a class="reference internal" href="#improving-control-flow-coherence-with-foreach-tiled">Improving Control Flow Coherence With "foreach_tiled"</a></li>
<li><a class="reference internal" href="#using-coherent-control-flow-constructs">Using Coherent Control Flow Constructs</a></li>
<li><a class="reference internal" href="#use-uniform-whenever-appropriate">Use "uniform" Whenever Appropriate</a></li>
<li><a class="reference internal" href="#use-structure-of-arrays-layout-when-possible">Use "Structure of Arrays" Layout When Possible</a></li>
</ul>
</li>
<li><a class="reference internal" href="#tips-and-techniques">Tips and Techniques</a><ul>
<li><a class="reference internal" href="#understanding-gather-and-scatter">Understanding Gather and Scatter</a></li>
<li><a class="reference internal" href="#avoid-64-bit-addressing-calculations-when-possible">Avoid 64-bit Addressing Calculations When Possible</a></li>
<li><a class="reference internal" href="#avoid-computation-with-8-and-16-bit-integer-types">Avoid Computation With 8 and 16-bit Integer Types</a></li>
<li><a class="reference internal" href="#implementing-reductions-efficiently">Implementing Reductions Efficiently</a></li>
<li><a class="reference internal" href="#using-foreach-active-effectively">Using "foreach_active" Effectively</a></li>
<li><a class="reference internal" href="#using-low-level-vector-tricks">Using Low-level Vector Tricks</a></li>
<li><a class="reference internal" href="#the-fast-math-option">The "Fast math" Option</a></li>
<li><a class="reference internal" href="#inline-aggressively">"inline" Aggressively</a></li>
<li><a class="reference internal" href="#avoid-the-system-math-library">Avoid The System Math Library</a></li>
<li><a class="reference internal" href="#declare-variables-in-the-scope-where-they-re-used">Declare Variables In The Scope Where They're Used</a></li>
<li><a class="reference internal" href="#instrumenting-ispc-programs-to-understand-runtime-behavior">Instrumenting ISPC Programs To Understand Runtime Behavior</a></li>
<li><a class="reference internal" href="#choosing-a-target-vector-width">Choosing A Target Vector Width</a></li>
</ul>
</li>
<li><a class="reference internal" href="#disclaimer-and-legal-information">Disclaimer and Legal Information</a></li>
<li><a class="reference internal" href="#optimization-notice">Optimization Notice</a></li>
</ul>
<div class="section" id="key-concepts">
<h1>Key Concepts</h1>
<p>This section describes the four most important concepts to understand and
keep in mind when writing high-performance <tt class="docutils literal">ispc</tt> programs. It assumes
good familiarity with the topics covered in the <tt class="docutils literal">ispc</tt> <a class="reference external" href="ispc.html">Users Guide</a>.</p>
<div class="section" id="efficient-iteration-with-foreach">
<h2>Efficient Iteration With "foreach"</h2>
<p>The <tt class="docutils literal">foreach</tt> parallel iteration construct is semantically equivalent to
a regular <tt class="docutils literal">for()</tt> loop, though it offers meaningful performance benefits.
(See the <a class="reference external" href="ispc.html#parallel-iteration-statements-foreach-and-foreach-tiled">documentation on "foreach" in the Users Guide</a> for a review of
its syntax and semantics.) As an example, consider this simple function
that iterates over some number of elements in an array, doing computation
on each one:</p>
<pre class="literal-block">
export void foo(uniform int a[], uniform int count) {
for (int i = programIndex; i < count; i += programCount) {
// do some computation on a[i]
}
}
</pre>
<p>Depending on the specifics of the computation being performed, the code
generated for this function could likely be improved by modifying the code
so that the loop only goes as far through the data as is possible to pack
an entire gang of program instances with computation each time through the
loop. Doing so enables the <tt class="docutils literal">ispc</tt> compiler to generate more efficient
code for cases where it knows that the execution mask is "all on". Then,
an <tt class="docutils literal">if</tt> statement at the end handles processing the ragged extra bits of
data that didn't fully fill a gang.</p>
<pre class="literal-block">
export void foo(uniform int a[], uniform int count) {
// First, just loop up to the point where all program instances
// in the gang will be active at the loop iteration start
uniform int countBase = count & ~(programCount-1);
for (uniform int i = 0; i < countBase; i += programCount) {
int index = i + programIndex;
// do some computation on a[index]
}
// Now handle the ragged extra bits at the end
if (countBase < count) {
int index = countBase + programIndex;
// do some computation on a[index]
}
}
</pre>
<p>While the performance of the above code will likely be better than the
first version of the function, the loop body code has been duplicated (or
has been forced to move into a separate utility function).</p>
<p>Using the <tt class="docutils literal">foreach</tt> looping construct as below provides all of the
performance benefits of the second version of this function, with the
compactness of the first.</p>
<pre class="literal-block">
export void foo(uniform int a[], uniform int count) {
foreach (i = 0 ... count) {
// do some computation on a[i]
}
}
</pre>
</div>
<div class="section" id="improving-control-flow-coherence-with-foreach-tiled">
<h2>Improving Control Flow Coherence With "foreach_tiled"</h2>
<p>Depending on the computation being performed, <tt class="docutils literal">foreach_tiled</tt> may give
better performance than <tt class="docutils literal">foreach</tt>. (See the <a class="reference external" href="ispc.html#parallel-iteration-statements-foreach-and-foreach-tiled">documentation in the Users
Guide</a> for the syntax and semantics of <tt class="docutils literal">foreach_tiled</tt>.) Given a
multi-dimensional iteration like:</p>
<pre class="literal-block">
foreach (i = 0 ... width, j = 0 ... height) {
// do computation on element (i,j)
}
</pre>
<p>if the <tt class="docutils literal">foreach</tt> statement is used, elements in the gang of program
instances will be mapped to values of <tt class="docutils literal">i</tt> and <tt class="docutils literal">j</tt> by taking spans of
<tt class="docutils literal">programCount</tt> elements across <tt class="docutils literal">i</tt> with a single value of <tt class="docutils literal">j</tt>. For
example, the <tt class="docutils literal">foreach</tt> statement above roughly corresponds to:</p>
<pre class="literal-block">
for (uniform int j = 0; j < height; ++j)
for (int i = 0; i < width; i += programCount) {
// do computation
}
</pre>
<p>When a multi-dimensional domain is being iterated over, <tt class="docutils literal">foreach_tiled</tt>
statement maps program instances to data in a way that tries to select
square n-dimensional segments of the domain. For example, on a compilation
target with 8-wide gangs of program instances, it generates code that
iterates over the domain the same way as the following code (though more
efficiently):</p>
<pre class="literal-block">
for (int j = programIndex/4; j < height; j += 2)
for (int i = programIndex%4; i < width; i += 4) {
// do computation
}
</pre>
<p>Thus, each gang of program instances operates on a 2x4 tile of the domain.
With higher-dimensional iteration and different gang sizes, a similar
mapping is performed--e.g. for 2D iteration with a 16-wide gang size, 4x4
tiles are iterated over; for 4D iteration with a 8-gang, 1x2x2x2 tiles are
processed, and so forth.</p>
<p>Performance benefit can come from using <tt class="docutils literal">foreach_tiled</tt> in that it
essentially optimizes for the benefit of iterating over <em>compact</em> regions
of the domain (while <tt class="docutils literal">foreach</tt> iterates over the domain in a way that
generally allows linear memory access.) There are two benefits from
processing compact regions of the domain.</p>
<p>First, it's often the case that the control flow coherence of the program
instances in the gang is improved; if data-dependent control flow decisions
are related to the values of the data in the domain being processed, and if
the data values have some coherence, iterating with compact regions will
improve control flow coherence.</p>
<p>Second, processing compact regions may mean that the data accessed by
program instances in the gang is be more coherent, leading to performance
benefits from better cache hit rates.</p>
<p>As a concrete example, for the ray tracer example in the <tt class="docutils literal">ispc</tt>
distribution (in the <tt class="docutils literal">examples/rt</tt> directory), performance is 20% better
when the pixels are iterated over using <tt class="docutils literal">foreach_tiled</tt> than <tt class="docutils literal">foreach</tt>,
because more coherent regions of the scene are accessed by the set of rays
in the gang of program instances.</p>
</div>
<div class="section" id="using-coherent-control-flow-constructs">
<h2>Using Coherent Control Flow Constructs</h2>
<p>Recall from the <tt class="docutils literal">ispc</tt> Users Guide, in the <a class="reference external" href="ispc.html#the-spmd-on-simd-execution-model">SPMD-on-SIMD Execution Model
section</a> that <tt class="docutils literal">if</tt> statements with a <tt class="docutils literal">uniform</tt> test compile to more
efficient code than <tt class="docutils literal">if</tt> tests with varying tests. The coherent <tt class="docutils literal">cif</tt>
statement can provide many benefits of <tt class="docutils literal">if</tt> with a uniform test in the
case where the test is actually varying.</p>
<p>In this case, the code the compiler generates for the <tt class="docutils literal">if</tt>
test is along the lines of the following pseudo-code:</p>
<pre class="literal-block">
bool expr = /* evaluate cif condition */
if (all(expr)) {
// run "true" case of if test only
} else if (!any(expr)) {
// run "false" case of if test only
} else {
// run both true and false cases, updating mask appropriately
}
</pre>
<p>For <tt class="docutils literal">if</tt> statements where the different running SPMD program instances
don't have coherent values for the boolean <tt class="docutils literal">if</tt> test, using <tt class="docutils literal">cif</tt>
introduces some additional overhead from the <tt class="docutils literal">all</tt> and <tt class="docutils literal">any</tt> tests as
well as the corresponding branches. For cases where the program
instances often do compute the same boolean value, this overhead is
worthwhile. If the control flow is in fact usually incoherent, this
overhead only costs performance.</p>
<p>In a similar fashion, <tt class="docutils literal">ispc</tt> provides <tt class="docutils literal">cfor</tt>, <tt class="docutils literal">cwhile</tt>, and <tt class="docutils literal">cdo</tt>
statements. These statements are semantically the same as the
corresponding non-"c"-prefixed functions.</p>
</div>
<div class="section" id="use-uniform-whenever-appropriate">
<h2>Use "uniform" Whenever Appropriate</h2>
<p>For any variable that will always have the same value across all of the
program instances in a gang, declare the variable with the <tt class="docutils literal">uniform</tt>
qualifier. Doing so enables the <tt class="docutils literal">ispc</tt> compiler to emit better code in
many different ways.</p>
<p>As a simple example, consider a <tt class="docutils literal">for</tt> loop that always does the same
number of iterations:</p>
<pre class="literal-block">
for (int i = 0; i < 10; ++i)
// do something ten times
</pre>
<p>If this is written with <tt class="docutils literal">i</tt> as a <tt class="docutils literal">varying</tt> variable, as above, there's
additional overhead in the code generated for the loop as the compiler
emits instructions to handle the possibility of not all program instances
following the same control flow path (as might be the case if the loop
limit, 10, was itself a <tt class="docutils literal">varying</tt> value.)</p>
<p>If the above loop is instead written with <tt class="docutils literal">i</tt> <tt class="docutils literal">uniform</tt>, as:</p>
<pre class="literal-block">
for (uniform int i = 0; i < 10; ++i)
// do something ten times
</pre>
<p>Then better code can be generated (and the loop possibly unrolled).</p>
<p>In some cases, the compiler may be able to detect simple cases like these,
but it's always best to provide the compiler with as much help as possible
to understand the actual form of your computation.</p>
</div>
<div class="section" id="use-structure-of-arrays-layout-when-possible">
<h2>Use "Structure of Arrays" Layout When Possible</h2>
<p>In general, memory access performance (for both reads and writes) is best
when the running program instances access a contiguous region of memory; in
this case efficient vector load and store instructions can often be used
rather than gathers and scatters. As an example of this issue, consider an
array of a simple point datatype laid out and accessed in conventional
"array of structures" (AOS) layout:</p>
<pre class="literal-block">
struct Point { float x, y, z; };
uniform Point pts[...];
float v = pts[programIndex].x;
</pre>
<p>In the above code, the access to <tt class="docutils literal"><span class="pre">pts[programIndex].x</span></tt> accesses
non-sequential memory locations, due to the <tt class="docutils literal">y</tt> and <tt class="docutils literal">z</tt> values between
the desired <tt class="docutils literal">x</tt> values in memory. A "gather" is required to get the
value of <tt class="docutils literal">v</tt>, with a corresponding decrease in performance.</p>
<p>If <tt class="docutils literal">Point</tt> was defined as a "structure of arrays" (SOA) type, the access
can be much more efficient:</p>
<pre class="literal-block">
struct Point8 { float x[8], y[8], z[8]; };
uniform Point8 pts8[...];
int majorIndex = programIndex / 8;
int minorIndex = programIndex % 8;
float v = pts8[majorIndex].x[minorIndex];
</pre>
<p>In this case, each <tt class="docutils literal">Point8</tt> has 8 <tt class="docutils literal">x</tt> values contiguous in memory
before 8 <tt class="docutils literal">y</tt> values and then 8 <tt class="docutils literal">z</tt> values. If the gang size is 8 or
less, the access for <tt class="docutils literal">v</tt> will have the same value of <tt class="docutils literal">majorIndex</tt> for
all program instances and will access consecutive elements of the <tt class="docutils literal">x[8]</tt>
array with a vector load. (For larger gang sizes, two 8-wide vector loads
would be issues, which is also quite efficient.)</p>
<p>However, the syntax in the above code is messy; accessing SOA data in this
fashion is much less elegant than the corresponding code for accessing the
data with AOS layout. The <tt class="docutils literal">soa</tt> qualifier in <tt class="docutils literal">ispc</tt> can be used to
cause the corresponding transformation to be made to the <tt class="docutils literal">Point</tt> type,
while preserving the clean syntax for data access that comes with AOS
layout:</p>
<pre class="literal-block">
soa<8> Point pts[...];
float v = pts[programIndex].x;
</pre>
<p>Thanks to having SOA layout a first-class concept in the language's type
system, it's easy to write functions that convert data between the
layouts. For example, the <tt class="docutils literal">aos_to_soa</tt> function below converts <tt class="docutils literal">count</tt>
elements of the given <tt class="docutils literal">Point</tt> type from AOS to 8-wide SOA layout. (It
assumes that the caller has pre-allocated sufficient space in the
<tt class="docutils literal">pts_soa</tt> output array.</p>
<pre class="literal-block">
void aos_to_soa(uniform Point pts_aos[], uniform int count,
soa<8> pts_soa[]) {
foreach (i = 0 ... count)
pts_soa[i] = pts_aos[i];
}
</pre>
<p>Analogously, a function could be written to convert back from SOA to AOS if
needed.</p>
</div>
</div>
<div class="section" id="tips-and-techniques">
<h1>Tips and Techniques</h1>
<p>This section introduces a number of additional techniques that are worth
keeping in mind when writing <tt class="docutils literal">ispc</tt> programs.</p>
<div class="section" id="understanding-gather-and-scatter">
<h2>Understanding Gather and Scatter</h2>
<p>Memory reads and writes from the program instances in a gang that access
irregular memory locations (rather than a consecutive set of locations, or
a single location) can be relatively inefficient. As an example, consider
the "simple" array indexing calculation below:</p>
<pre class="literal-block">
int i = ....;
uniform float x[10] = { ... };
float f = x[i];
</pre>
<p>Since the index <tt class="docutils literal">i</tt> is a varying value, the program instances in the gang
will in general be reading different locations in the array <tt class="docutils literal">x</tt>. Because
current CPUs have a "gather" instruction, the <tt class="docutils literal">ispc</tt> compiler has to
serialize these memory reads, performing a separate memory load for each
running program instance, packing the result into <tt class="docutils literal">f</tt>. (The analogous
case happens for a write into <tt class="docutils literal">x[i]</tt>.)</p>
<p>In many cases, gathers like these are unavoidable; the program instances
just need to access incoherent memory locations. However, if the array
index <tt class="docutils literal">i</tt> actually has the same value for all of the program instances or
if it represents an access to a consecutive set of array locations, much
more efficient load and store instructions can be generated instead of
gathers and scatters, respectively.</p>
<p>In many cases, the <tt class="docutils literal">ispc</tt> compiler is able to deduce that the memory
locations accessed by a varying index are either all the same or are
uniform. For example, given:</p>
<pre class="literal-block">
uniform int x = ...;
int y = x;
return array[y];
</pre>
<p>The compiler is able to determine that all of the program instances are
loading from the same location, even though <tt class="docutils literal">y</tt> is not a <tt class="docutils literal">uniform</tt>
variable. In this case, the compiler will transform this load to a regular
vector load, rather than a general gather.</p>
<p>Sometimes the running program instances will access a linear sequence of
memory locations; this happens most frequently when array indexing is done
based on the built-in <tt class="docutils literal">programIndex</tt> variable. In many of these cases,
the compiler is also able to detect this case and then do a vector load.
For example, given:</p>
<pre class="literal-block">
for (int i = programIndex; i < count; i += programCount)
// process array[i];
</pre>
<p>Regular vector loads and stores are issued for accesses to <tt class="docutils literal">array[i]</tt>.</p>
<p>Both of these cases have been ones where the compiler is able to determine
statically that the index has the same value at compile-time. It's
often the case that this determination can't be made at compile time, but
this is often the case at run time. The <tt class="docutils literal">reduce_equal()</tt> function from
the standard library can be used in this case; it checks to see if the
given value is the same across over all of the running program instances,
returning true and its <tt class="docutils literal">uniform</tt> value if so.</p>
<p>The following function shows the use of <tt class="docutils literal">reduce_equal()</tt> to check for an
equal index at execution time and then either do a scalar load and
broadcast or a general gather.</p>
<pre class="literal-block">
uniform float array[..] = { ... };
float value;
int i = ...;
uniform int ui;
if (reduce_equal(i, &ui) == true)
value = array[ui]; // scalar load + broadcast
else
value = array[i]; // gather
</pre>
<p>For a simple case like the one above, the overhead of doing the
<tt class="docutils literal">reduce_equal()</tt> check is likely not worthwhile compared to just always
doing a gather. In more complex cases, where a number of accesses are done
based on the index, it can be worth doing. See the example
<tt class="docutils literal">examples/volume_rendering</tt> in the <tt class="docutils literal">ispc</tt> distribution for the use of
this technique in an instance where it is beneficial to performance.</p>
</div>
<div class="section" id="understanding-memory-read-coalescing">
<h2>Understanding Memory Read Coalescing</h2>
<p>XXXX todo</p>
</div>
<div class="section" id="avoid-64-bit-addressing-calculations-when-possible">
<h2>Avoid 64-bit Addressing Calculations When Possible</h2>
<p>Even when compiling to a 64-bit architecture target, <tt class="docutils literal">ispc</tt> does many of
the addressing calculations in 32-bit precision by default--this behavior
can be overridden with the <tt class="docutils literal"><span class="pre">--addressing=64</span></tt> command-line argument. This
option should only be used if it's necessary to be able to address over 4GB
of memory in the <tt class="docutils literal">ispc</tt> code, as it essentially doubles the cost of
memory addressing calculations in the generated code.</p>
</div>
<div class="section" id="avoid-computation-with-8-and-16-bit-integer-types">
<h2>Avoid Computation With 8 and 16-bit Integer Types</h2>
<p>The code generated for 8 and 16-bit integer types is generally not as
efficient as the code generated for 32-bit integer types. It is generally
worthwhile to use 32-bit integer types for intermediate computations, even
if the final result will be stored in a smaller integer type.</p>
</div>
<div class="section" id="implementing-reductions-efficiently">
<h2>Implementing Reductions Efficiently</h2>
<p>It's often necessary to compute a reduction over a data set--for example,
one might want to add all of the values in an array, compute their minimum,
etc. <tt class="docutils literal">ispc</tt> provides a few capabilities that make it easy to efficiently
compute reductions like these. However, it's important to use these
capabilities appropriately for best results.</p>
<p>As an example, consider the task of computing the sum of all of the values
in an array. In C code, we might have:</p>
<pre class="literal-block">
/* C implementation of a sum reduction */
float sum(const float array[], int count) {
float sum = 0;
for (int i = 0; i < count; ++i)
sum += array[i];
return sum;
}
</pre>
<p>Exactly this computation could also be expressed as a purely uniform
computation in <tt class="docutils literal">ispc</tt>, though without any benefit from vectorization:</p>
<pre class="literal-block">
/* inefficient ispc implementation of a sum reduction */
uniform float sum(const uniform float array[], uniform int count) {
uniform float sum = 0;
for (uniform int i = 0; i < count; ++i)
sum += array[i];
return sum;
}
</pre>
<p>As a first try, one might try using the <tt class="docutils literal">reduce_add()</tt> function from the
<tt class="docutils literal">ispc</tt> standard library; it takes a <tt class="docutils literal">varying</tt> value and returns the sum
of that value across all of the active program instances.</p>
<pre class="literal-block">
/* inefficient ispc implementation of a sum reduction */
uniform float sum(const uniform float array[], uniform int count) {
uniform float sum = 0;
foreach (i = 0 ... count)
sum += reduce_add(array[i]);
return sum;
}
</pre>
<p>This implementation loads a gang's worth of values from the array, one for
each of the program instances, and then uses <tt class="docutils literal">reduce_add()</tt> to reduce
across the program instances and then update the sum. Unfortunately this
approach loses most benefit from vectorization, as it does more work on the
cross-program instance <tt class="docutils literal">reduce_add()</tt> call than it saves from the vector
load of values.</p>
<p>The most efficient approach is to do the reduction in two phases: rather
than using a <tt class="docutils literal">uniform</tt> variable to store the sum, we maintain a varying
value, such that each program instance is effectively computing a local
partial sum on the subset of array values that it has loaded from the
array. When the loop over array elements concludes, a single call to
<tt class="docutils literal">reduce_add()</tt> computes the final reduction across each of the program
instances' elements of <tt class="docutils literal">sum</tt>. This approach effectively compiles to a
single vector load and a single vector add for each loop iteration's of
values--very efficient code in the end.</p>
<pre class="literal-block">
/* good ispc implementation of a sum reduction */
uniform float sum(const uniform float array[], uniform int count) {
float sum = 0;
foreach (i = 0 ... count)
sum += array[i];
return reduce_add(sum);
}
</pre>
</div>
<div class="section" id="using-foreach-active-effectively">
<h2>Using "foreach_active" Effectively</h2>
<p>For high-performance code,</p>
<p>For example, consider this segment of code, from the introduction of
<tt class="docutils literal">foreach_active</tt> in the ispc User's Guide:</p>
<pre class="literal-block">
uniform float array[...] = { ... };
int index = ...;
foreach_active (i) {
++array[index];
}
</pre>
<p>Here, <tt class="docutils literal">index</tt> was assumed to possibly have the same value for multiple
program instances, so the updates to <tt class="docutils literal">array[index]</tt> are serialized by the
<tt class="docutils literal">foreach_active</tt> statement in order to not have undefined results when
<tt class="docutils literal">index</tt> values do collide.</p>
<p>The code generated by the compiler can be improved in this case by making
it clear that only a single element of the array is accessed by
<tt class="docutils literal">array[index]</tt> and that thus a general gather or scatter isn't required.
Specifically, by using the <tt class="docutils literal">extract()</tt> function from the standard library
to extract the current program instance's value of <tt class="docutils literal">index</tt> into a
<tt class="docutils literal">uniform</tt> variable and then using that to index into <tt class="docutils literal">array</tt>, as below,
more efficient code is generated.</p>
<pre class="literal-block">
foreach_active (instanceNum) {
uniform int unifIndex = extract(index, instanceNum);
++array[unifIndex];
}
</pre>
</div>
<div class="section" id="using-low-level-vector-tricks">
<h2>Using Low-level Vector Tricks</h2>
<p>Many low-level Intel® SSE and AVX coding constructs can be implemented in
<tt class="docutils literal">ispc</tt> code. The <tt class="docutils literal">ispc</tt> standard library functions <tt class="docutils literal">intbits()</tt> and
<tt class="docutils literal">floatbits()</tt> are often useful in this context. Recall that
<tt class="docutils literal">intbits()</tt> takes a <tt class="docutils literal">float</tt> value and returns it as an integer where
the bits of the integer are the same as the bit representation in memory of
the <tt class="docutils literal">float</tt>. (In other words, it does <em>not</em> perform an integer to
floating-point conversion.) <tt class="docutils literal">floatbits()</tt>, then, performs the inverse
computation.</p>
<p>As an example of the use of these functions, the following code efficiently
reverses the sign of the given values.</p>
<pre class="literal-block">
float flipsign(float a) {
unsigned int i = intbits(a);
i ^= 0x80000000;
return floatbits(i);
}
</pre>
<p>This code compiles down to a single XOR instruction.</p>
</div>
<div class="section" id="the-fast-math-option">
<h2>The "Fast math" Option</h2>
<p><tt class="docutils literal">ispc</tt> has a <tt class="docutils literal"><span class="pre">--opt=fast-math</span></tt> command-line flag that enables a number of
optimizations that may be undesirable in code where numerical precision is
critically important. For many graphics applications, for example, the
approximations introduced may be acceptable, however. The following two
optimizations are performed when <tt class="docutils literal"><span class="pre">--opt=fast-math</span></tt> is used. By default, the
<tt class="docutils literal"><span class="pre">--opt=fast-math</span></tt> flag is off.</p>
<ul class="simple">
<li>Expressions like <tt class="docutils literal">x / y</tt>, where <tt class="docutils literal">y</tt> is a compile-time constant, are
transformed to <tt class="docutils literal">x * <span class="pre">(1./y)</span></tt>, where the inverse value of <tt class="docutils literal">y</tt> is
precomputed at compile time.</li>
<li>Expressions like <tt class="docutils literal">x / y</tt>, where <tt class="docutils literal">y</tt> is not a compile-time constant,
are transformed to <tt class="docutils literal">x * rcp(y)</tt>, where <tt class="docutils literal">rcp()</tt> maps to the
approximate reciprocal instruction from the <tt class="docutils literal">ispc</tt> standard library.</li>
</ul>
</div>
<div class="section" id="inline-aggressively">
<h2>"inline" Aggressively</h2>
<p>Inlining functions aggressively is generally beneficial for performance
with <tt class="docutils literal">ispc</tt>. Definitely use the <tt class="docutils literal">inline</tt> qualifier for any short
functions (a few lines long), and experiment with it for longer functions.</p>
</div>
<div class="section" id="avoid-the-system-math-library">
<h2>Avoid The System Math Library</h2>
<p>The default math library for transcendentals and the like that <tt class="docutils literal">ispc</tt> has
higher error than the system's math library, though is much more efficient
due to being vectorized across the program instances and due to the fact
that the functions can be inlined in the final code. (It generally has
errors in the range of 10ulps, while the system math library generally has
no more than 1ulp of error for transcendentals.)</p>
<p>If the <tt class="docutils literal"><span class="pre">--math-lib=system</span></tt> command-line option is used when compiling an
<tt class="docutils literal">ispc</tt> program, then calls to the system math library will be generated
instead. This option should only be used if the higher precision is
absolutely required as the performance impact of using it can be
significant.</p>
</div>
<div class="section" id="declare-variables-in-the-scope-where-they-re-used">
<h2>Declare Variables In The Scope Where They're Used</h2>
<p>Performance is slightly improved by declaring variables at the same block
scope where they are first used. For example, in code like the
following, if the lifetime of <tt class="docutils literal">foo</tt> is only within the scope of the
<tt class="docutils literal">if</tt> clause, write the code like this:</p>
<pre class="literal-block">
float func() {
....
if (x < y) {
float foo;
... use foo ...
}
}
</pre>
<p>Try not to write code as:</p>
<pre class="literal-block">
float func() {
float foo;
....
if (x < y) {
... use foo ...
}
}
</pre>
<p>Doing so can reduce the amount of masked store instructions that the
compiler needs to generate.</p>
</div>
<div class="section" id="instrumenting-ispc-programs-to-understand-runtime-behavior">
<h2>Instrumenting ISPC Programs To Understand Runtime Behavior</h2>
<p><tt class="docutils literal">ispc</tt> has an optional instrumentation feature that can help you
understand performance issues. If a program is compiled using the
<tt class="docutils literal"><span class="pre">--instrument</span></tt> flag, the compiler emits calls to a function with the
following signature at various points in the program (for
example, at interesting points in the control flow, when scatters or
gathers happen.)</p>
<pre class="literal-block">
extern "C" {
void ISPCInstrument(const char *fn, const char *note,
int line, uint64_t mask);
}
</pre>
<p>This function is passed the file name of the <tt class="docutils literal">ispc</tt> file running, a short
note indicating what is happening, the line number in the source file, and
the current mask of active program instances in the gang. You must provide an
implementation of this function and link it in with your application.</p>
<p>For example, when the <tt class="docutils literal">ispc</tt> program runs, this function might be called
as follows:</p>
<pre class="literal-block">
ISPCInstrument("foo.ispc", "function entry", 55, 0xfull);
</pre>
<p>This call indicates that at the currently executing program has just
entered the function defined at line 55 of the file <tt class="docutils literal">foo.ispc</tt>, with a
mask of all lanes currently executing (assuming a four-wide gang size
target machine).</p>
<p>For a fuller example of the utility of this functionality, see
<tt class="docutils literal">examples/aobench_instrumented</tt> in the <tt class="docutils literal">ispc</tt> distribution. This
example includes an implementation of the <tt class="docutils literal">ISPCInstrument()</tt> function
that collects aggregate data about the program's execution behavior.</p>
<p>When running this example, you will want to direct to the <tt class="docutils literal">ao</tt> executable
to generate a low resolution image, because the instrumentation adds
substantial execution overhead. For example:</p>
<pre class="literal-block">
% ./ao 1 32 32
</pre>
<p>After the <tt class="docutils literal">ao</tt> program exits, a summary report along the following lines
will be printed. In the first few lines, you can see how many times a few
functions were called, and the average percentage of SIMD lanes that were
active upon function entry.</p>
<pre class="literal-block">
ao.ispc(0067) - function entry: 342424 calls (0 / 0.00% all off!), 95.86% active lanes
ao.ispc(0067) - return: uniform control flow: 342424 calls (0 / 0.00% all off!), 95.86% active lanes
ao.ispc(0071) - function entry: 1122 calls (0 / 0.00% all off!), 97.33% active lanes
ao.ispc(0075) - return: uniform control flow: 1122 calls (0 / 0.00% all off!), 97.33% active lanes
ao.ispc(0079) - function entry: 10072 calls (0 / 0.00% all off!), 45.09% active lanes
ao.ispc(0088) - function entry: 36928 calls (0 / 0.00% all off!), 97.40% active lanes
...
</pre>
</div>
<div class="section" id="choosing-a-target-vector-width">
<h2>Choosing A Target Vector Width</h2>
<p>By default, <tt class="docutils literal">ispc</tt> compiles to the natural vector width of the target
instruction set. For example, for SSE2 and SSE4, it compiles four-wide,
and for AVX, it complies 8-wide. For some programs, higher performance may
be seen if the program is compiled to a doubled vector width--8-wide for
SSE and 16-wide for AVX.</p>
<p>For workloads that don't require many of registers, this method can lead to
significantly more efficient execution thanks to greater instruction level
parallelism and amortization of various overhead over more program
instances. For other workloads, it may lead to a slowdown due to higher
register pressure; trying both approaches for key kernels may be
worthwhile.</p>
<p>This option is only available for each of the SSE2, SSE4 and AVX targets.
It is selected with the <tt class="docutils literal"><span class="pre">--target=sse2-x2</span></tt>, <tt class="docutils literal"><span class="pre">--target=sse4-x2</span></tt> and
<tt class="docutils literal"><span class="pre">--target=avx-x2</span></tt> options, respectively.</p>
</div>
</div>
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<h1>Optimization Notice</h1>
<p>Intel compilers, associated libraries and associated development tools may
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