Intel® High Level Synthesis Compiler Standard Edition: Reference Manual

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ID 683310
Date 12/18/2019
Public
Document Table of Contents
Document Table of Contents x
1. Intel® HLS Compiler Standard Edition Reference Manual 2. Compiler 3. C Language and Library Support 4. Component Interfaces 5. Component Memories (Memory Attributes) 6. Loops in Components 7. Component Concurrency 8. Arbitrary Precision Math Support 9. Component Target Frequency 10. Intel® High Level Synthesis Compiler Standard Edition Compiler Reference Summary A. Supported Math Functions B. Intel® HLS Compiler Standard Edition Reference Manual Archives C. Document Revision History of the Intel® HLS Compiler Standard Edition Reference Manual
2. Compiler x
2.1. Intel® HLS Compiler Standard Edition Command Options 2.2. Using Libraries in Your Component 2.3. Compiler Interoperability 2.4. Intel® HLS Compiler Pipeline Approach
3. C Language and Library Support x
3.1. Supported C and C++ Subset for Component Synthesis 3.2. C and C++ Libraries 3.3. Intel® HLS Compiler Standard Edition Compiler-Defined Preprocessor Macros
4. Component Interfaces x
4.1. Component Invocation Interface 4.2. Avalon® Streaming Interfaces 4.3. Avalon® Memory-Mapped Master Interfaces 4.4. Slave Interfaces 4.5. Component Invocation Interface Arguments 4.6. Unstable and Stable Component Arguments 4.7. Global Variables 4.8. Structs in Component Interfaces 4.9. Reset Behavior
4.1. Component Invocation Interface x
4.1.1. Scalar Parameters 4.1.2. Pointer and Reference Parameters 4.1.3. Interface Definition Example: Component with Both Scalar and Pointer Arguments
4.3. Avalon® Memory-Mapped Master Interfaces x
4.3.1. Memory-Mapped Master Testbench Constructor 4.3.2. Implicit and Explicit Examples of Describing a Memory Interface
4.4. Slave Interfaces x
4.4.1. Control and Status Register (CSR) Slave 4.4.2. Slave Memories
5. Component Memories (Memory Attributes) x
5.1. Static Variables
6. Loops in Components x
6.1. Loop Initiation Interval (ii Pragma) 6.2. Loop-Carried Dependencies (ivdep Pragma) 6.3. Loop Coalescing (loop_coalesce Pragma) 6.4. Loop Concurrency (max_concurrency Pragma) 6.5. Loop Unrolling (unroll Pragma)
7. Component Concurrency x
7.1. Serial Equivalence within a Memory Space or I/O 7.2. Concurrency Control (hls_max_concurrency Attribute)
8. Arbitrary Precision Math Support x
8.1. Declaring ac_int Data Types 8.2. Integer Promotion and ac_int Data Types 8.3. Debugging Your Use of the ac_int Data Type 8.4. Declaring ac_fixed Data Types 8.5. AC Data Types and Native Compilers
8.1. Declaring ac_int Data Types x
8.1.1. Important Usage Information on the ac_int Data Type
10. Intel® High Level Synthesis Compiler Standard Edition Compiler Reference Summary x
10.1. Intel® HLS Compiler Standard Edition i++ Command-Line Arguments 10.2. Intel® HLS Compiler Standard Edition Header Files 10.3. Intel® HLS Compiler Standard Edition Compiler-Defined Preprocessor Macros 10.4. Intel® HLS Compiler Standard Edition Keywords 10.5. Intel® HLS Compiler Standard Edition Simulation API (Testbench Only) 10.6. Intel® HLS Compiler Standard Edition Component Memory Attributes 10.7. Intel® HLS Compiler Standard Edition Loop Pragmas 10.8. Intel® HLS Compiler Standard Edition Component Attributes 10.9. Intel® HLS Compiler Standard Edition Component Default Interfaces 10.10. Intel® HLS Compiler Standard Edition Component Invocation Interface Arguments 10.11. Intel® HLS Compiler Standard Edition Component Macros 10.12. Intel® HLS Compiler Standard Edition Streaming Input Interfaces 10.13. Intel® HLS Compiler Standard Edition Streaming Output Interfaces 10.14. Intel® HLS Compiler Standard Edition Memory-Mapped Interfaces 10.15. Intel® HLS Compiler Standard Edition Arbitrary Precision Data Types
A. Supported Math Functions x
A.1. Math Functions Provided by the math.h Header File A.2. Math Functions Provided by the extendedmath.h Header File A.3. Math Functions Provided by the ac_fixed_math.h Header File
1. Intel® HLS Compiler Standard Edition Reference Manual
2. Compiler
3. C Language and Library Support
4. Component Interfaces
5. Component Memories (Memory Attributes)
6. Loops in Components
7. Component Concurrency
8. Arbitrary Precision Math Support
9. Component Target Frequency
10. Intel® High Level Synthesis Compiler Standard Edition Compiler Reference Summary
A. Supported Math Functions
B. Intel® HLS Compiler Standard Edition Reference Manual Archives
C. Document Revision History of the Intel® HLS Compiler Standard Edition Reference Manual

10.7. Intel® HLS Compiler Standard Edition Loop Pragmas

Use the Intel® HLS Compiler loop pragmas to control how the compiler pipelines the loops in your component.

Table 24.   Intel® HLS Compiler Standard Edition Loop Pragmas Summary
Pragma Description
ii Forces a loop to have a loop initiation interval (II) of a specified value.
ivdep Ignores memory dependencies between iterations of this loop.
loop_coalesce Tries to fuse all loops nested within this loop into a single loop.
max_concurrency Limits the number of iterations of a loop that can simultaneously execute at any time.
unroll Unrolls the loop completely or by a number of times.

ii Loop Pragma

Syntax
#pragma ii N
Description
Forces the loop to which you apply this pragma to have a loop initiation interval (II) of <N>, where <N> is a positive integer value.

Forcing a loop II value can have an adverse effect on the fMAX of your component because using this pragma to get a lower loop II combines pipeline stages together and creates logic with a long propagation delay.

Using this pragma with a larger loop II inserts more pipeline stages and can give you a better component fMAX value.

Example: 
#pragma ii 2
for (int i = 0; i < 8; i++) {
 // Loop body
}

ivdep Loop Pragma

Syntax
#pragma ivdep safelen(N) array(array_name)
Description
Tells the compiler to ignore memory dependencies between iterations of this loop.

It can accept an optional argument that specifies the name of the array. If array is not specified, all component memory dependencies are ignored. If there are loop-carried dependencies, your generated RTL produces incorrect results.

The safelen parameter specifies the dependency distance. The dependency distance is the number of iterations between successive load/stores that depend on each other. It is safe to not include safelen is only when the dependence distance is infinite (that is, there are no real dependencies).

Example: 
#pragma ivdep safelen(2)
for (int i = 0; i < 8; i++) {
 // Loop body
}

To learn more, review the tutorial: <quartus_installdir>/hls/examples/tutorials/best_practices/loop_memory_dependency.

loop_coalesce Loop Pragma

Syntax
#pragma loop_coalesce N
Description
Tells the compiler to try to fuse all loops nested within this loop into a single loop. This pragma accepts an optional value N which indicates the number of levels of loops to coalesce together. 
#pragma loop_coalesce 2
for (int i = 0; i < 8; i++) {
 for (int j = 0; j < 8; j++) {
 // Loop body 
 } 
}

max_concurrency Loop Pragma

Syntax
#pragma max_concurrency N
Description
This pragma limits the number of iterations of a loop that can simultaneously execute at any time.

This pragma is useful mainly when private copies of are created to improve the throughput of the loop. This is mentioned in the details pane for the loop in the Loop Analysis pane and the Bank view of the Component Memory Viewer of the high level design report (report.html).

This can occur only when the scope of a component memory (through its declaration or access pattern) is limited to this loop. Adding this pragma can be used to reduce the area that the loop consumes at the cost of some throughput.

Example: 
// Without this pragma,
// multiple private copies 
// of the array "arr"
#pragma max_concurrency 1
for (int i = 0; i < 8; i++) {
 int arr[1024];
 // Loop body
}

unroll Loop Pragma

Syntax
#pragma unroll N
Description
This pragma unrolls the loop completely or by <N> times, where <N> is optional and is a positive integer value.
Example: 
#pragma unroll 8
for (int i = 0; i < 8; i++) {
 // Loop body
}

To learn more, review the tutorial: <quartus_installdir>/hls/examples/best_practices/resource_sharing_filter.

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