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Home > Using the NEON Vectorizing Compiler > Carry-around scalar variables and vectorization |

Scalar variables that are used and then set in a loop can cause problems for vectorization.

A scalar variable that is used but not set in a loop is replicated in each position in a vector register and the replication is used in the vector calculation.

A scalar that is set and then used in a loop is promoted to
a vector. These variables generally hold temporary scalar values
in a loop that now has to hold temporary vector values. In the following example, `x`

is
a used scalar and `y`

is a promoted scalar.

Vectorizable loop:

float a[99], b[99], x, y; int i, n; ... for (i = 0; i < n; i++) { y = x + b[i]; a[i] = y + 1/y; };

A scalar that is used and then set in a loop is called a
carry-around scalar. These variables are a problem for
vectorization because the value computed in one pass of the loop
is carried forward into the next pass. In the following example, `x`

is a carry-around
scalar.

Nonvectorizable loop

float a[99], b[99], x; int i, n; ... for (i = 0; i < n; i++) { a[i] = x + b[i]; x = a[i] + 1/x; };