Using Vector Instructions through Built-in Functions#
On some targets, the instruction set contains SIMD vector instructions which operate on multiple values contained in one large register at the same time. For example, on the x86 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 is V4SI.
The vector_size attribute is only applicable to integral and
floating scalars, although arrays, pointers, and function return values
are allowed in conjunction with this construct. Only sizes that are
positive power-of-two multiples of the base type size are currently allowed.
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
causes 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
produces code that uses 4 SIs.
The types defined in this manner can be used with a subset of normal C
operations. Currently, GCC allows 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 is
added to the corresponding 4 elements in b and the resulting
vector is 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.
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 convenience, it is allowed to use a binary vector operation where one operand is a scalar. In that case the compiler transforms the scalar operand into a vector where each element is the scalar from the operation. The transformation happens 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. */
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 run time. Warnings for out of bound
accesses for vector subscription can be enabled with
-Warray-bounds.
Vector comparison is supported with 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} */
In C++, the ternary operator ?: is available. a?b:c, where
b and c are vectors of the same type and a is an
integer vector with the same number of elements of the same size as b
and c, computes all three arguments and creates a vector
{a[0]?b[0]:c[0], a[1]?b[1]:c[1], ...}. Note that unlike in
OpenCL, a is thus interpreted as a != 0 and not a < 0.
As in the case of binary operations, this syntax is also accepted when
one of b or c is a scalar that is then transformed into a
vector. If both b and c are scalars and the type of
true?b:c has the same size as the element type of a, then
b and c are converted to a vector type whose elements have
this type and with the same number of elements as a.
In C++, the logic operators !, &&, || are available for vectors.
!v is equivalent to v == 0, a && b is equivalent to
a!=0 & b!=0 and a || b is equivalent to a!=0 | b!=0.
For mixed operations between a scalar s and a vector v,
s && v is equivalent to s?v!=0:0 (the evaluation is
short-circuit) and v && s is equivalent to v!=0 & (s?-1:0).
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.
Vector shuffling is available using the
__builtin_shufflevector (vec1, vec2, index...)
function. vec1 and vec2 must be expressions with
vector type with a compatible element type. The result of
__builtin_shufflevector is a vector with the same element type
as vec1 and vec2 but that has an element count equal to
the number of indices specified.
The index arguments are a list of integers that specify the
elements indices of the first two vectors that should be extracted and
returned in a new vector. These element indices are numbered sequentially
starting with the first vector, continuing into the second vector.
An index of -1 can be used to indicate that the corresponding element in
the returned vector is a don’t care and can be freely chosen to optimized
the generated code sequence performing the shuffle operation.
Consider the following example,
typedef int v4si __attribute__ ((vector_size (16)));
typedef int v8si __attribute__ ((vector_size (32)));
v8si a = {1,-2,3,-4,5,-6,7,-8};
v4si b = __builtin_shufflevector (a, a, 0, 2, 4, 6); /* b is {1,3,5,7} */
v4si c = {-2,-4,-6,-8};
v8si d = __builtin_shufflevector (c, b, 4, 0, 5, 1, 6, 2, 7, 3); /* d is a */
Vector conversion is available using the
__builtin_convertvector (vec, vectype)
function. vec must be an expression with integral or floating
vector type and vectype an integral or floating vector type with the
same number of elements. The result has vectype type and value of
a C cast of every element of vec to the element type of vectype.
Consider the following example,
typedef int v4si __attribute__ ((vector_size (16)));
typedef float v4sf __attribute__ ((vector_size (16)));
typedef double v4df __attribute__ ((vector_size (32)));
typedef unsigned long long v4di __attribute__ ((vector_size (32)));
v4si a = {1,-2,3,-4};
v4sf b = {1.5f,-2.5f,3.f,7.f};
v4di c = {1ULL,5ULL,0ULL,10ULL};
v4sf d = __builtin_convertvector (a, v4sf); /* d is {1.f,-2.f,3.f,-4.f} */
/* Equivalent of:
v4sf d = { (float)a[0], (float)a[1], (float)a[2], (float)a[3] }; */
v4df e = __builtin_convertvector (a, v4df); /* e is {1.,-2.,3.,-4.} */
v4df f = __builtin_convertvector (b, v4df); /* f is {1.5,-2.5,3.,7.} */
v4si g = __builtin_convertvector (f, v4si); /* g is {1,-2,3,7} */
v4si h = __builtin_convertvector (c, v4si); /* h is {1,5,0,10} */
Sometimes it is desirable to write code using a mix of generic vector
operations (for clarity) and machine-specific vector intrinsics (to
access vector instructions that are not exposed via generic built-ins).
On x86, intrinsic functions for integer vectors typically use the same
vector type __m128i irrespective of how they interpret the vector,
making it necessary to cast their arguments and return values from/to
other vector types. In C, you can make use of a union type:
#include <immintrin.h>
typedef unsigned char u8x16 __attribute__ ((vector_size (16)));
typedef unsigned int u32x4 __attribute__ ((vector_size (16)));
typedef union {
__m128i mm;
u8x16 u8;
u32x4 u32;
} v128;
for variables that can be used with both built-in operators and x86 intrinsics:
v128 x, y = { 0 };
memcpy (&x, ptr, sizeof x);
y.u8 += 0x80;
x.mm = _mm_adds_epu8 (x.mm, y.mm);
x.u32 &= 0xffffff;
/* Instead of a variable, a compound literal may be used to pass the
return value of an intrinsic call to a function expecting the union: */
v128 foo (v128);
x = foo ((v128) {_mm_adds_epu8 (x.mm, y.mm)});