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

5.1 Software Pipelining Processing Stages

The C7000 compiler goes through three basic stages when software pipelining a loop. The three stages are:

  1. Qualify the loop for software pipelining
  2. Collect loop resource and dependency graph information
  3. Attempt to software pipeline the loop

By the time the compiler tries to software pipeline an inner loop, the compiler may have applied certain transformations to the code in the loop, and also may have combined adjacent or nested loops.

Stage 1: Qualification

Several conditions must be met before software pipelining is allowed, or found to be legal, from the compiler’s point of view. Two of the most common conditions that cause software pipelining to fail are:

  • The loop cannot have too many instructions. Loops that are too big typically require more registers than are available and require a longer compilation time.
  • Another function cannot be called from within the loop unless the called function is inlined. Any break in control flow makes it impossible to software pipeline, since multiple iterations are executing in parallel.

If any conditions for software pipelining are not met, qualification of the pipeline halts and a disqualification messages appears. See Section 5.4.1. for troubleshooting and Section 5.2.1 for information provided during this stage.

If all conditions for software pipelining are met, the compiler continues to Stage 2.

Stage 2: Collecting Loop and Dependency Information

The second stage of software pipelining involves collecting loop resource and dependency graph information. See Section 5.2.2 for information about output from this stage.

Stage 3: Software Pipelining Attempts

Once the compiler has qualified the loop for software pipelining, partitioned it, and analyzed the necessary loop carry and resource requirements, it can attempt software pipelining.

The compiler attempts to software pipeline a loop starting at a certain initiation interval (ii). Each time a compiler software pipelining attempt at a particular initiation interval fails, the ii is increased, and another software pipelining attempt is made. This can be seen in the Software Pipeline Information comment block. This process continues until a software pipelining attempt succeeds or ii is equal to the length of a scheduled loop with no software pipelining. If ii reaches the length of a scheduled loop with no software pipelining, attempts to software pipeline stop and the compiler generates a non-software pipelined loop. See Section 5.2.3 for more about the information provided during this stage.

If a software pipelining attempt is not successful, the compiler provides additional feedback to help explain why. See Section 5.4.2 for a list of the most common software pipeline failures and strategies for mitigation.

After a successful software pipeline schedule and register allocation is found at a particular initiation interval, more information about the loop is displayed. See Section 5.2.4, Section 5.2.5, Section 5.2.6, Section 5.2.7, and Section 5.2.8.