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<h3 class="section">6.47 Using vector instructions through built-in functions</h3>

<p>On some targets, the instruction set contains SIMD vector instructions that
operate on multiple values contained in one large register at the same time. 
For example, on the i386 the MMX, 3DNow! and SSE extensions can be used
this way.

 <p>The first step in using these extensions is to provide the necessary data
types.  This should be done using an appropriate <code>typedef</code>:

<pre class="smallexample">     typedef int v4si __attribute__ ((vector_size (16)));
</pre>
 <p>The <code>int</code> type specifies the base type, while the attribute specifies
the vector size for the variable, measured in bytes.  For example, the
declaration above causes the compiler to set the mode for the <code>v4si</code>
type to be 16 bytes wide and divided into <code>int</code> sized units.  For
a 32-bit <code>int</code> this means a vector of 4 units of 4 bytes, and the
corresponding mode of <code>foo</code> will be <acronym>V4SI</acronym>.

 <p>The <code>vector_size</code> attribute is only applicable to integral and
float scalars, although arrays, pointers, and function return values
are allowed in conjunction with this construct.

 <p>All the basic integer types can be used as base types, both as signed
and as unsigned: <code>char</code>, <code>short</code>, <code>int</code>, <code>long</code>,
<code>long long</code>.  In addition, <code>float</code> and <code>double</code> can be
used to build floating-point vector types.

 <p>Specifying a combination that is not valid for the current architecture
will cause GCC to synthesize the instructions using a narrower mode. 
For example, if you specify a variable of type <code>V4SI</code> and your
architecture does not allow for this specific SIMD type, GCC will
produce code that uses 4 <code>SIs</code>.

 <p>The types defined in this manner can be used with a subset of normal C
operations.  Currently, GCC will allow using the following operators
on these types: <code>+, -, *, /, unary minus, ^, |, &amp;, ~, %</code>.

 <p>The operations behave like C++ <code>valarrays</code>.  Addition is defined as
the addition of the corresponding elements of the operands.  For
example, in the code below, each of the 4 elements in <var>a</var> will be
added to the corresponding 4 elements in <var>b</var> and the resulting
vector will be stored in <var>c</var>.

<pre class="smallexample">     typedef int v4si __attribute__ ((vector_size (16)));
     
     v4si a, b, c;
     
     c = a + b;
</pre>
 <p>Subtraction, multiplication, division, and the logical operations
operate in a similar manner.  Likewise, the result of using the unary
minus or complement operators on a vector type is a vector whose
elements are the negative or complemented values of the corresponding
elements in the operand.

 <p>You can declare variables and use them in function calls and returns, as
well as in assignments and some casts.  You can specify a vector type as
a return type for a function.  Vector types can also be used as function
arguments.  It is possible to cast from one vector type to another,
provided they are of the same size (in fact, you can also cast vectors
to and from other datatypes of the same size).

 <p>You cannot operate between vectors of different lengths or different
signedness without a cast.

 <p>A port that supports hardware vector operations, usually provides a set
of built-in functions that can be used to operate on vectors.  For
example, a function to add two vectors and multiply the result by a
third could look like this:

<pre class="smallexample">     v4si f (v4si a, v4si b, v4si c)
     {
       v4si tmp = __builtin_addv4si (a, b);
       return __builtin_mulv4si (tmp, c);
     }
     
</pre>
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