SPRUIV4C May   2020  – December 2023

 

  1.   1
  2.   Read This First
    1.     About This Manual
    2.     Related Documentation
    3.     Trademarks
  3. 2Introduction
    1. 2.1 C7000 Digital Signal Processor CPU Architecture Overview
    2. 2.2 C7000 Split Datapath and Functional Units
  4. 3C7000 C/C++ Compiler Options
    1. 3.1 Overview
    2. 3.2 Selecting Compiler Options for Performance
    3. 3.3 Understanding Compiler Optimization
      1. 3.3.1 Software Pipelining
      2. 3.3.2 Vectorization and Vector Predication
      3. 3.3.3 Automatic Use of Streaming Engine and Streaming Address Generator
      4. 3.3.4 Loop Collapsing and Loop Coalescing
      5. 3.3.5 Automatic Inlining
      6. 3.3.6 If Conversion
  5. 4Basic Code Optimization
    1. 4.1  Signed Types for Iteration Counters and Limits
    2. 4.2  Floating-Point Division
    3. 4.3  Loop-Carried Dependencies and the Restrict Keyword
      1. 4.3.1 Loop-Carried Dependencies
      2. 4.3.2 The Restrict Keyword
      3. 4.3.3 Run-Time Alias Disambiguation
    4. 4.4  Function Calls and Inlining
    5. 4.5  MUST_ITERATE and PROB_ITERATE Pragmas and Attributes
    6. 4.6  If Statements and Nested If Statements
    7. 4.7  Intrinsics
    8. 4.8  Vector Types
    9. 4.9  C++ Features to Use and Avoid
    10. 4.10 Streaming Engine
    11. 4.11 Streaming Address Generator
    12. 4.12 Optimized Libraries
    13. 4.13 Memory Optimizations
  6. 5Understanding the Assembly Comment Blocks
    1. 5.1 Software Pipelining Processing Stages
    2. 5.2 Software Pipeline Information Comment Block
      1. 5.2.1 Loop and Iteration Count Information
      2. 5.2.2 Dependency and Resource Bounds
      3. 5.2.3 Initiation Interval (ii) and Iterations
      4. 5.2.4 Constant Extensions
      5. 5.2.5 Resources Used and Register Tables
      6. 5.2.6 Stage Collapsing
      7. 5.2.7 Memory Bank Conflicts
      8. 5.2.8 Loop Duration Formula
    3. 5.3 Single Scheduled Iteration Comment Block
    4. 5.4 Identifying Pipeline Failures and Performance Issues
      1. 5.4.1 Issues that Prevent a Loop from Being Software Pipelined
      2. 5.4.2 Software Pipeline Failure Messages
      3. 5.4.3 Performance Issues
  7. 6Revision History

5.2.1 Loop and Iteration Count Information

If the compiler qualifies the loop for software pipelining, the first few lines look like the following example:

;*----------------------------------------------------------------------------* 
;*   SOFTWARE PIPELINE INFORMATION
;*
;*      Loop found in file               : s.cpp
;*      Loop source line                 : 5
;*      Loop opening brace source line   : 6
;*      Loop closing brace source line   : 8
;*      Known Minimum Iteration Count    : 768                    
;*      Known Maximum Iteration Count    : 1024                    
;*      Known Max Iteration Count Factor : 256

The loop counter is called the "iteration counter" because it is the number of iterations through a loop. The statistics provided in this section of the block are:

  • Loop found in file, Loop source line, Loop opening brace source line, Loop closing brace source line: Information about where the loop is located in the original C/C++ source code.
  • Known Minimum Iteration Count: The minimum number of times the loop might execute given the amount of information available to the compiler.
  • Known Maximum Iteration Count: The maximum number of times the loop might execute given the amount of information available to the compiler.
  • Known Max Iteration Count Factor: The maximum number that will divide evenly into the iteration count. Even though the exact value of the iteration count is not deterministic, it may be known that the value is a multiple of 2, 4, etc., which may allow more aggressive packed data/SIMD optimization.

The compiler tries to identify information about the loop counter such as minimum value (known minimum iteration count), and whether it is a multiple of something (has a known maximum iteration count factor).

If a Max Iteration Count Factor greater than 1 is known, the compiler might be more aggressive in packed data processing and loop unrolling optimizations. For example, if the exact value of a loop counter is not known but it is known that the value is a multiple of some number, the compiler may be better able to unroll the loop to improve performance.