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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.
The first step in using these extensions is to provide the necessary data
types. This should be done using an appropriate typedef:
typedef int v4si __attribute__ ((vector_size (16)));
The int 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 v4si
type to be 16 bytes wide and divided into int sized units. For
a 32-bit int this means a vector of 4 units of 4 bytes, and the
corresponding mode of foo will be V4SI.
The vector_size attribute is only applicable to integral and
float scalars, although arrays, pointers, and function return values
are allowed in conjunction with this construct.
All the basic integer types can be used as base types, both as signed
and as unsigned: char, short, int, long,
long long. In addition, float and double can be
used to build floating-point vector types.
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 V4SI and your
architecture does not allow for this specific SIMD type, GCC will
produce code that uses 4 SIs.
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: +, -, *, /, unary minus, ^, |, &, ~, %.
The operations behave like C++ valarrays. 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 a will be
added to the corresponding 4 elements in b and the resulting
vector will be stored in c.
typedef int v4si __attribute__ ((vector_size (16)));
v4si a, b, c;
c = a + b;
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.
In C it is possible to use shifting operators <<, >> on
integer-type vectors. The operation is defined as following: {a0,
a1, ..., an} >> {b0, b1, ..., bn} == {a0 >> b0, a1 >> b1,
..., an >> bn}. Vector operands must have the same number of
elements.
For the convenience in C it is allowed to use a binary vector operation where one operand is a scalar. In that case the compiler will transform the scalar operand into a vector where each element is the scalar from the operation. The transformation will happen only if the scalar could be safely converted to the vector-element type. Consider the following code.
typedef int v4si __attribute__ ((vector_size (16)));
v4si a, b, c;
long l;
a = b + 1; /* a = b + {1,1,1,1}; */
a = 2 * b; /* a = {2,2,2,2} * b; */
a = l + a; /* Error, cannot convert long to int. */
In C vectors can be subscripted as if the vector were an array with the same number of elements and base type. Out of bound accesses invoke undefined behavior at runtime. Warnings for out of bound accesses for vector subscription can be enabled with -Warray-bounds.
In GNU C vector comparison is supported within standard comparison
operators: ==, !=, <, <=, >, >=. Comparison operands can be
vector expressions of integer-type or real-type. Comparison between
integer-type vectors and real-type vectors are not supported. The
result of the comparison is a vector of the same width and number of
elements as the comparison operands with a signed integral element
type.
Vectors are compared element-wise producing 0 when comparison is false and -1 (constant of the appropriate type where all bits are set) otherwise. Consider the following example.
typedef int v4si __attribute__ ((vector_size (16)));
v4si a = {1,2,3,4};
v4si b = {3,2,1,4};
v4si c;
c = a > b; /* The result would be {0, 0,-1, 0} */
c = a == b; /* The result would be {0,-1, 0,-1} */
Vector shuffling is available using functions
__builtin_shuffle (vec, mask) and
__builtin_shuffle (vec0, vec1, mask).
Both functions construct a permutation of elements from one or two
vectors and return a vector of the same type as the input vector(s).
The mask is an integral vector with the same width (W)
and element count (N) as the output vector.
The elements of the input vectors are numbered in memory ordering of vec0 beginning at 0 and vec1 beginning at N. The elements of mask are considered modulo N in the single-operand case and modulo 2*N in the two-operand case.
Consider the following example,
typedef int v4si __attribute__ ((vector_size (16)));
v4si a = {1,2,3,4};
v4si b = {5,6,7,8};
v4si mask1 = {0,1,1,3};
v4si mask2 = {0,4,2,5};
v4si res;
res = __builtin_shuffle (a, mask1); /* res is {1,2,2,4} */
res = __builtin_shuffle (a, b, mask2); /* res is {1,5,3,6} */
Note that __builtin_shuffle is intentionally semantically
compatible with the OpenCL shuffle and shuffle2 functions.
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).
You cannot operate between vectors of different lengths or different signedness without a cast.