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46 Cray T3E User’s Guide
!dir$ split
DOi=1,1000
a(i) = b(i) * c(i)
t = d(i) + a(i)
e(i)=f(i)+t*g(i)
h(i) = h(i) + e(i)
END DO
The directive is marked with the characters !dir$. If the source code
is written using the fixed source form, these characters must be at the
beginning of the line.
Due to the directive split, the compiler will split the above loop in two
as follows:
DOi=1,1000
a(i) = b(i) * c(i)
ta(i) = d(i) + a(i)
END DO
DOi=1,1000
e(i) = f(i) * ta(i) * g(i)
h(i) = h(i) + e(i)
END DO
This may make the code faster by reducing memory bandwidth. Instead
of loading and storing a lot of data in an iteration of the loop, the split
loop gives a better balance between computation and memory opera-
tions. This also improves the performance if we are using the streams
mechanism.
Here is an example of loop unrolling:
!dir$ unroll 2
DOi=1,10
DOj=1,100
a(j,i) = b(j,i) + 1
END DO
END DO
This results in the following unrolled loops (into two copies of the inner
loop):
DOi=1,10,2
DOj=1,100
a(j,i) = b(j,i) + 1
END DO
DOj=1,100
a(j,i+1) = b(j,i+1) + 1
END DO
END DO
The compiler may also fuse the two inner loops together to produce the
following final code:
DOi=1,10,2
DOj=1,100
- Cray T3E 1
- User’s Guide 1
- Cray T3E User’s Guide 3 3
- Contents 4
- Contents 5 5
- B Glossary 113 6
- Bibliography 118 6
- Index 120 6
- Introduction 7
- 1.2 Usage policy 8
- 1.3 Overview of the system 9
- 1.4 Programming environment 9
- 10 Cray T3E User’s Guide 10
- Chapter 1. Introduction 11 11
- 12 Cray T3E User’s Guide 12
- Using the Cray T3E at CSC 13
- 2.2 Files 14
- 2.3 Editing files 16
- 2.5 Executing in batch mode 17
- 2.6 More information 18
- The Cray T3E system 20
- 3.2 Distributed memory 21
- 3.3 Processing elements 21
- 3.4 Processor architecture 23
- 3.5 Local memory hierarchy 24
- 26 Cray T3E User’s Guide 26
- Processing Element 27
- I/O Controller 27
- Interconnection 27
- Network 27
- 3.9 File systems 28
- 3.10 Resource monitoring 29
- 30 Cray T3E User’s Guide 30
- 3.11 More information 31
- Program development 32
- 4.3.1 BLAS 34
- 4.3.2 LAPACK 34
- 4.3.3 BLACS 35
- 4.3.4 PBLAS and ScaLAPACK 35
- 4.3.5 Details 35
- 36 Cray T3E User’s Guide 36
- 4.6 More information 38
- Fortran programming 40
- 5.2 Basic usage 41
- 5.4 Compiler options 42
- 5.5 Optimization options 42
- 5.6 Optimizing for cache 44
- 5.7 Compiler directives 45
- 5.8 Fortran 90 modules 48
- 5.9 Source code preprocessing 49
- 5.10 More information 51
- C and C++ programming 52
- 6.2 The C compiler 53
- 6.3 Calling Fortran from C 54
- 6.4 C compiler options 55
- #pragma _CRI symmetric var 59
- #pragma _CRI unroll [n] 59
- 6.6 The C++ compiler 61
- 6.7 More information 61
- Interprocess communication 62
- 7.2.1 Format of the MPI calls 63
- 7.2.2 Some MPI routines 64
- 7.2.3 An example of using MPI 64
- 7.2.5 MPI data types 67
- 7.3.1 Using PVM on the T3E 68
- #include <mpp/shmem.h> 71
- INCLUDE ’mpp/shmem.fh’ 71
- 7.4.2 Data addresses 72
- 7.4.4 Reduction routines 74
- 7.4.6 Example of using SHMEM 76
- 78 Cray T3E User’s Guide 78
- 0:c= 10121416 78
- 1:c= 2468 78
- 80 Cray T3E User’s Guide 80
- Batch queuing system 81
- 82 Cray T3E User’s Guide 82
- # QSUB -l p_mpp_t=7000 82
- 8.3 Status of the NQE job 83
- 84 Cray T3E User’s Guide 84
- 8.4 Deleting an NQE batch job 86
- 8.5 Queues 86
- 8.6 More information 87
- Programming tools 88
- 9.2 Program Browser 89
- 9.3 Debugging programs 92
- 9.3.2 Examining core files 93
- 94 Cray T3E User’s Guide 94
- 9.4.1 MPP Apprentice 95
- 9.4.2 The appview command 98
- 9.4.3 PAT 98
- 9.5.1 The VAMPIRtrace library 100
- 102
- 9.5.3 More information 103
- Miscellaneous notes 104
- 10.1.3 The CPU clock timer 105
- 10.1.4 Example of timing 106
- 108 Cray T3E User’s Guide 108
- 10.4 More information 110
- About CSC 111
- 112 Cray T3E User’s Guide 112
- Glossary 113
- 114 Cray T3E User’s Guide 114
- Appendix B. Glossary 115 115
- Metacomputer Environment 116
- Bibliography 118
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