Cypress CSC-1200T Betriebsanweisung PDF herunterladen (Seite 57)

Chapter 6. C and C++ programming 57

In the previous format, var_list represents a list of variable names sepa-

rated by commas. In C, the cache_align directive can appear before or

after the declaration of the named objects. In C++, it must appear after

the declaration of all named objects.

noreduction

The noreduction compiler directive tells the compiler not to optimize

the loop that immediately follows the directive as a reduction loop. If

the loop is not a reduction loop, the directive is ignored.

You may choose to use this directive when the loop iteration count is

small or when the order of evaluation is numerically signiﬁcant. In the

latter case normal optimization may change the result of a reduction

loop, because it rearranges the operations.

The format of this directive is:

#pragma _CRI noreduction

The following example illustrates the use of the noreduction compiler

directive:

sum=0;

#pragma _CRI noreduction

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

sum += a[i];

}

Here we know that n will be a small number and therefore we do not

want to optimize this loop as a reduction loop.

split

The split directive instructs the compiler to attempt to split the follow-

ing loop into a set of smaller loops. Such a loop splitting improves single

processor performance by making the best use of the six stream buﬀers

of the Cray T3E system. The stream buﬀers reduce memory latency

and increase memory bandwidth by prefetching for long, small-strided

sequences of memory references.

The split directive may avoid performance problems with the stream

buﬀers by splitting an inner loop into a set of smaller loops, each of

which allocates no more than six stream buﬀers.

The split directive has the following form:

#pragma _CRI split

This compiler directive should be placed immediately before the loop

to be split. It should immediately precede a for, while, do or label

statement, but it should not appear in any other context.

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Cypress CSC-1200T Betriebsanweisung Seite 57