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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, ^, |, &, ~, %</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); |
| } |
| |
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