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C Language Extensions
By making a, b, and c restricted pointers, the programmer asserts to the compiler that
the pointers are in fact not aliased, which in this case means writes through c would
never overwrite elements of a or b. This changes the function prototype as follows:
void foo(const float* __restrict__ a,
const float* __restrict__ b,
float* __restrict__ c);
Note that all pointer arguments need to be made restricted for the compiler optimizer
to derive any benefit. With the __restrict keywords added, the compiler can now
reorder and do common sub-expression elimination at will, while retaining functionality
identical with the abstract execution model:
void foo(const float* __restrict__ a,
const float* __restrict__ b,
float* __restrict__ c)
{
float t0 = a[0];
float t1 = b[0];
float t2 = t0 * t2;
float t3 = a[1];
c[0] = t2;
c[1] = t2;
c[4] = t2;
c[2] = t2 * t3;
c[3] = t0 * t3;
c[5] = t1;
...
}
The effects here are a reduced number of memory accesses and reduced number of
computations. This is balanced by an increase in register pressure due to "cached" loads
and common sub-expressions.
Since register pressure is a critical issue in many CUDA codes, use of restricted pointers
can have negative performance impact on CUDA code, due to reduced occupancy.
B.3 Built-in Vector Types
B.3.1 char, short, int, long, longlong, float, double
These are vector types derived from the basic integer and floating-point types. They
are structures and the 1st, 2nd, 3rd, and 4th components are accessible through the
fields x, y, z, and w, respectively. They all come with a constructor function of the form
make_<type name>; for example,
int2 make_int2(int x, int y);
which creates a vector of type int2 with value(x, y).
In host code, the alignment requirement of a vector type is equal to the alignment
requirement of its base type. This is not always the case in device code as detailed in
Table 3 Alignment Requirements in Device Code.
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CUDA C Programming Guide
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