Cypress CSC-1200T Betriebsanweisung PDF herunterladen (Seite 58)

58 Cray T3E User’s Guide

The split directive merely asserts that the loop can proﬁt by splitting.

It will not cause incorrect code.

The compiler splits the loop only if it is safe. Generally, a loop is safe to

split under the same conditions that a loop is vectorizable. The compiler

only splits inner loops, but it may not split loops with conditional code.

The split directive also causes the original loop to be stripmined, and

therefore the data is processed in blocks small enough to ﬁt in the cache.

Loop splitting can reduce the execution time of a loop by as much as

40%. Even loops with as few as 40 iterations may be split. The loops

must contain more than six diﬀerent memory references with strides

less than 16.

Note that there is a slight risk on increasing the execution time of certain

loops. Loop splitting also increases compilation time, especially when

loop unrolling is also enabled.

Here is an example of loop splitting:

#pragma _CRI split

for (i=0; i<1000; i++) {

a[i] = b[i] * c[i];

t = d[i] + a[i];

e[i]=f[i]+t*g[i];

h[i] = h[i] + e[i];

}

First, the compiler generates the following loop:

for (i=0; i<1000; i++) {

a[i] = b[i] * c[i];

ta[i] = d[i] + a[i];

}

for (i=0; i<1000; i++) {

e[i] = f[i] * ta[i] * g[i];

h[i] = h[i] + e[i];

}

Finally, the compiler stripmines the loops to increase the potential for

cache hits and reduces the size of arrays created for scalar expansion:

for (i1=0; i1<1000; i1+=256) {

i2 = (i1+256 < 1000) ? i1+256 : 1000;

for (i=i1; i<i2; i++) {

a[i] = b[i] * c[i]

ta[i-i1] = d[i] + a[i]

}

for (i=i1; i<i2; i++) {

e[i] = f[i] * ta[i-i1] * g[i]

h[i] = h[i] + e[i]

}

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