Loops can be formed with the usual DO-END DO and DO WHILE, or by using an IF/GOTO and a label. The loops must have a single entry and a single exit to be vectorized. The following examples illustrate loop constructs that can and cannot be vectorized.
Example: Vectorizable structure |
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subroutine vec(a, b, c) dimension a(100), b(100), c(100) integer i i = 1 do while (i .le. 100) a(i) = b(i) * c(i) if (a(i) .lt. 0.0) a(i) = 0.0 i = i + 1 enddo end subroutine vec |
The following example shows a loop that cannot be vectorized because of the inherent potential for an early exit from the loop.
Example: Non-vectorizable structure |
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subroutine no_vec(a, b, c) dimension a(100), b(100), c(100) integer i i = 1 do while (i .le. 100) a(i) = b(i) * c(i) ! The next statement allows early ! exit from the loop and prevents ! vectorization of the loop. if (a(i) .lt. 0.0) go to 10 i = i + 1 enddo 10 continue end subroutine no_vecN END |