Cypress CSC-1200T Podręcznik Użytkownika Pobierz pdf (Strona 59)

Chapter 6. C and C++ programming 59

symmetric

The symmetric directive declares that an auto or register variable has

the same local address on all processing elements (PEs). This is useful

for global addressing using the SHMEM library functions. The format

for this compiler directive is:

#pragma _CRI symmetric var...

The symmetric directive must appear in local scope. Each variable listed

on the directive must:

• be declared in the same scope as the directive

• have auto or register storage class

• not be a function parameter.

Because all PEs must participate in the allocation of symmetric stack

variables, there is an implicit barrier before the ﬁrst executable state-

ment in a block containing symmetric variables.

unroll

The unroll directive allows the user to control unrolling for individual

loops. Loop unrolling can improve program performance by revealing

memory optimization opportunities such as read-after-write and read-

after-read. The eﬀects of loop unrolling also include:

• an improved loop scheduling by increasing the basic block size

• a reduced loop overhead

• improved chances for cache hits.

The format for this compiler directive is:

#pragma _CRI unroll [n]

Item n speciﬁes the total number of loop body copies to be generated.

The value of n must be in the range of 2 through 64. If you do not

specify a value for n, the compiler attempts to determine the number of

copies to be generated based on the number of statements in the loop

nest.

Warning: If placed prior to a non-innermost loop, the unroll directive

asserts that the following loop has no dependencies across iterations of

that loop. If dependencies exist, incorrect code could be generated.

The unroll compiler directive can be used only on loops whose iteration

counts can be calculated before entering the loop.

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Cypress CSC-1200T Podręcznik Użytkownika Strona 59