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

3.2 Selecting Compiler Options for Performance

After your application has been fully debugged and is working properly, it is time to begin the optimization process. First, you need to select appropriate compiler options. The following compiler options affect performance. See the C7000 C/C++ Compiler User’s Guide (SPRUIG8) for more details on command-line options.

  • --opt_level=3 (-o3). The compiler performs function-level optimization at -opt_level=2 and file-level optimization and function inlining at --opt_level=3. At both --opt_level=2 and --opt_level=3, the compiler performs various loop optimizations, such as software pipelining, vectorization, and loop coalescing. By default, the --opt_level switches optimize for performance. Such optimization can increase code size. If code size is an issue, do not reduce the level of optimization. Instead use the --opt_for_speed (-mf) switch to change the optimization goal (performance versus code size) and the –oi option to control the amount of automatic inlining.
  • --opt_level=4 (-o4). Consider using this option to perform optimizations across all files at link-time. Using this option can increase compile time significantly. If used for any step, this option must be used at all compilation and linking steps. Source files can be compiled separately, as long as they are all compiled with --opt_level=4. This optimization level cannot be used with --program_level_compile (-pm).
  • --gen_func_subsections (-mo). Consider using this option if the source code uses many functions that are never called. This option places each function in its own input subsection, so the linker can exclude that function from the executable if it is never referenced. However, this optimization can increase code size, because there are minimum section alignment requirements the compiler must apply.
  • --opt_for_speed=0 (-mf0) or --opt_for_speed=1 (-mf1). If code size is a concern, use these options when compiling files with functions that are not executed often or are not critical to performance. This tells the compiler to optimize for code size instead of performance. Do not lower the optimization level (--opt_level) in an attempt to lower code size.

Do not use the --disable_software_pipelining (-mu) option if you are concerned about performance. This option turns off software pipelining. Software pipelining is critical to achieving high performance on most loops. This option can be a debugging tool, as it makes the assembly code easier to understand.

The following options provide additional information for debugging and performance evaluation purposes:

  • --src_interlist (-s). This option causes the compiler to emit into the compiler-generated assembly files a copy of what the source code looks like after high-level optimization. This output is placed in the assembly files as comments among the assembly code. The comments output from the optimizer look like C code and show the high-level transformations that have been applied such as inlining, loop coalescing, and vectorization. This option can be useful in helping you understand the assembly code and some of what the compiler is doing to optimize the performance of the code. This option turns on the --keep_asm (-k) option, so the compiler-generated assembly (.asm) files will not be deleted.

    --debug_software_pipeline (-mw). This option emits extra information about software-pipelined loops, including the single-scheduled iteration of the loop. This information is used in loop tuning examples presented later in this document. This option turns on the --keep_asm (-k) option, so the compiler-generated assembly (.asm) files will not be deleted.

    --gen_opt_info=2 (-on2). This option creates a .nfo file with the same base name as the .obj file. This file contains summary information regarding the high-level optimizations that have been applied, as well as providing advice.