SPRAB89A September   2011  – March 2014

 

  1. 1 Introduction
    1. 1.1  ABIs for the C6000
    2. 1.2  Scope
    3. 1.3  ABI Variants
    4. 1.4  Toolchains and Interoperability
    5. 1.5  Libraries
    6. 1.6  Types of Object Files
    7. 1.7  Segments
    8. 1.8  C6000 Architecture Overview
    9. 1.9  Reference Documents
    10. 1.10 Code Fragment Notation
  2. 2 Data Representation
    1. 2.1 Basic Types
    2. 2.2 Data in Registers
    3. 2.3 Data in Memory
    4. 2.4 Complex Types
    5. 2.5 Structures and Unions
    6. 2.6 Arrays
    7. 2.7 Bit Fields
      1. 2.7.1 Volatile Bit Fields
    8. 2.8 Enumeration Types
  3. 3 Calling Conventions
    1. 3.1 Call and Return
      1. 3.1.1 Return Address Computation
      2. 3.1.2 Call Instructions
      3. 3.1.3 Return Instruction
      4. 3.1.4 Pipeline Conventions
      5. 3.1.5 Weak Functions
    2. 3.2 Register Conventions
    3. 3.3 Argument Passing
    4. 3.4 Return Values
    5. 3.5 Structures and Unions Passed and Returned by Reference
    6. 3.6 Conventions for Compiler Helper Functions
    7. 3.7 Scratch Registers for Inter-Section Calls
    8. 3.8 Setting Up DP
  4. 4 Data Allocation and Addressing
    1. 4.1 Data Sections and Segments
    2. 4.2 Allocation and Addressing of Static Data
      1. 4.2.1 Addressing Methods for Static Data
        1. 4.2.1.1 Near DP-Relative Addressing
        2. 4.2.1.2 Far DP-Relative Addressing
        3. 4.2.1.3 Absolute Addressing
        4. 4.2.1.4 GOT-Indirect Addressing
        5. 4.2.1.5 PC-Relative Addressing
      2. 4.2.2 Placement Conventions for Static Data
        1. 4.2.2.1 Abstract Conventions for Placement
        2. 4.2.2.2 Abstract Conventions for Addressing
        3. 4.2.2.3 Linker Requirements
      3. 4.2.3 Initialization of Static Data
    3. 4.3 Automatic Variables
    4. 4.4 Frame Layout
      1. 4.4.1 Stack Alignment
      2. 4.4.2 Register Save Order
        1. 4.4.2.1 Big-Endian Pair Swapping
        2. 4.4.2.2 Examples
      3. 4.4.3 DATA_MEM_BANK
      4. 4.4.4 C64x+ Specific Stack Layouts
        1. 4.4.4.1 _ _C6000_push_rts Layout
        2. 4.4.4.2 Compact Frame Layout
    5. 4.5 Heap-Allocated Objects
  5. 5 Code Allocation and Addressing
    1. 5.1 Computing the Address of a Code Label
      1. 5.1.1 Absolute Addressing for Code
      2. 5.1.2 PC-Relative Addressing
      3. 5.1.3 PC-Relative Addressing Within the Same Section
      4. 5.1.4 Short-Offset PC-Relative Addressing (C64x)
      5. 5.1.5 GOT-Based Addressing for Code
    2. 5.2 Branching
    3. 5.3 Calls
      1. 5.3.1 Direct PC-Relative Call
      2. 5.3.2 Far Call Trampoline
      3. 5.3.3 Indirect Calls
    4. 5.4 Addressing Compact Instructions
  6. 6 Addressing Model for Dynamic Linking
    1. 6.1 Terms and Concepts
    2. 6.2 Overview of Dynamic Linking Mechanisms
    3. 6.3 DSOs and DLLs
    4. 6.4 Preemption
    5. 6.5 PLT Entries
      1. 6.5.1 Direct Calls to Imported Functions
      2. 6.5.2 PLT Entry Via Absolute Address
      3. 6.5.3 PLT Entry Via GOT
    6. 6.6 The Global Offset Table
      1. 6.6.1 GOT-Based Reference Using Near DP-Relative Addressing
      2. 6.6.2 GOT-Based Reference Using Far DP-Relative Addressing
    7. 6.7 The DSBT Model
      1. 6.7.1 Entry/Exit Sequence for Exported Functions
      2. 6.7.2 Avoiding DP Loads for Internal Functions
      3. 6.7.3 Function Pointers
      4. 6.7.4 Interrupts
      5. 6.7.5 Compatibility With Non-DSBT Code
    8. 6.8 Performance Implications of Dynamic Linking
  7. 7 Thread-Local Storage Allocation and Addressing
    1. 7.1 About Multi-Threading and Thread-Local Storage
    2. 7.2 Terms and Concepts
    3. 7.3 User Interface
    4. 7.4 ELF Object File Representation
    5. 7.5 TLS Access Models
      1. 7.5.1 C6x Linux TLS Models
        1. 7.5.1.1 General Dynamic TLS Access Model
        2. 7.5.1.2 Local Dynamic TLS Access Model
        3. 7.5.1.3 Initial Exec TLS Access Model
          1. 7.5.1.3.1 Thread Pointer
          2. 7.5.1.3.2 Initial Exec TLS Addressing
        4. 7.5.1.4 Local Exec TLS Access Model
      2. 7.5.2 Static Executable TLS Model
        1. 7.5.2.1 Static Executable Addressing
        2. 7.5.2.2 Static Executable TLS Runtime Architecture
        3. 7.5.2.3 Static Executable TLS Allocation
          1. 7.5.2.3.1 TLS Initialization Image Allocation
          2. 7.5.2.3.2 Main Thread’s TLS Allocation
          3. 7.5.2.3.3 Thread Library’s TLS Region Allocation
        4. 7.5.2.4 Static Executable TLS Initialization
          1. 7.5.2.4.1 Main Thread’s TLS Initialization
          2. 7.5.2.4.2 TLS Initialization by Thread Library
        5. 7.5.2.5 Thread Pointer
      3. 7.5.3 Bare-Metal Dynamic Linking TLS Model
        1. 7.5.3.1 Default TLS Addressing for Bare-Metal Dynamic Linking
        2. 7.5.3.2 TLS Block Creation
    6. 7.6 Thread-Local Symbol Resolution and Weak References
      1. 7.6.1 General and Local Dynamic TLS Weak Reference Addressing
      2. 7.6.2 Initial and Local Executable TLS Weak Reference Addressing
      3. 7.6.3 Static Exec and Bare Metal Dynamic TLS Model Weak References
  8. 8 Helper Function API
    1. 8.1 Floating-Point Behavior
    2. 8.2 C Helper Function API
    3. 8.3 Special Register Conventions for Helper Functions
    4. 8.4 Helper Functions for Complex Types
    5. 8.5 Floating-Point Helper Functions for C99
  9. 9 Standard C Library API
    1. 9.1  Reserved Symbols
    2. 9.2  <assert.h> Implementation
    3. 9.3  <complex.h> Implementation
    4. 9.4  <ctype.h> Implementation
    5. 9.5  <errno.h> Implementation
    6. 9.6  <float.h> Implementation
    7. 9.7  <inttypes.h> Implementation
    8. 9.8  <iso646.h> Implementation
    9. 9.9  <limits.h> Implementation
    10. 9.10 <locale.h> Implementation
    11. 9.11 <math.h> Implementation
    12. 9.12 <setjmp.h> Implementation
    13. 9.13 <signal.h> Implementation
    14. 9.14 <stdarg.h> Implementation
    15. 9.15 <stdbool.h> Implementation
    16. 9.16 <stddef.h> Implementation
    17. 9.17 <stdint.h> Implementation
    18. 9.18 <stdio.h> Implementation
    19. 9.19 <stdlib.h> Implementation
    20. 9.20 <string.h> Implementation
    21. 9.21 <tgmath.h> Implementation
    22. 9.22 <time.h> Implementation
    23. 9.23 <wchar.h> Implementation
    24. 9.24 <wctype.h> Implementation
  10. 10C++ ABI
    1. 10.1  Limits (GC++ABI 1.2)
    2. 10.2  Export Template (GC++ABI 1.4.2)
    3. 10.3  Data Layout (GC++ABI Chapter 2)
    4. 10.4  Initialization Guard Variables (GC++ABI 2.8)
    5. 10.5  Constructor Return Value (GC++ABI 3.1.5)
    6. 10.6  One-Time Construction API (GC++ABI 3.3.2)
    7. 10.7  Controlling Object Construction Order (GC++ ABI 3.3.4)
    8. 10.8  Demangler API (GC++ABI 3.4)
    9. 10.9  Static Data (GC++ ABI 5.2.2)
    10. 10.10 Virtual Tables and the Key function (GC++ABI 5.2.3)
    11. 10.11 Unwind Table Location (GC++ABI 5.3)
  11. 11Exception Handling
    1. 11.1  Overview
    2. 11.2  PREL31 Encoding
    3. 11.3  The Exception Index Table (EXIDX)
      1. 11.3.1 Pointer to Out-of-Line EXTAB Entry
      2. 11.3.2 EXIDX_CANTUNWIND
      3. 11.3.3 Inlined EXTAB Entry
    4. 11.4  The Exception Handling Instruction Table (EXTAB)
      1. 11.4.1 EXTAB Generic Model
      2. 11.4.2 EXTAB Compact Model
      3. 11.4.3 Personality Routines
    5. 11.5  Unwinding Instructions
      1. 11.5.1 Common Sequence
      2. 11.5.2 Byte-Encoded Unwinding Instructions
      3. 11.5.3 24-Bit Unwinding Encoding
    6. 11.6  Descriptors
      1. 11.6.1 Encoding of Type Identifiers
      2. 11.6.2 Scope
      3. 11.6.3 Cleanup Descriptor
      4. 11.6.4 Catch Descriptor
      5. 11.6.5 Function Exception Specification (FESPEC) Descriptor
    7. 11.7  Special Sections
    8. 11.8  Interaction With Non-C++ Code
      1. 11.8.1 Automatic EXIDX Entry Generation
      2. 11.8.2 Hand-Coded Assembly Functions
    9. 11.9  Interaction With System Features
      1. 11.9.1 Shared Libraries
      2. 11.9.2 Overlays
      3. 11.9.3 Interrupts
    10. 11.10 Assembly Language Operators in the TI Toolchain
  12. 12DWARF
    1. 12.1 DWARF Register Names
    2. 12.2 Call Frame Information
    3. 12.3 Vendor Names
    4. 12.4 Vendor Extensions
  13. 13ELF Object Files (Processor Supplement)
    1. 13.1 Registered Vendor Names
    2. 13.2 ELF Header
    3. 13.3 Sections
      1. 13.3.1 Section Indexes
      2. 13.3.2 Section Types
      3. 13.3.3 Extended Section Header Attributes
      4. 13.3.4 Subsections
      5. 13.3.5 Special Sections
      6. 13.3.6 Section Alignment
    4. 13.4 Symbol Table
      1. 13.4.1 Symbol Types
      2. 13.4.2 Common Block Symbols
      3. 13.4.3 Symbol Names
      4. 13.4.4 Reserved Symbol Names
      5. 13.4.5 Mapping Symbols
    5. 13.5 Relocation
      1. 13.5.1 Relocation Types
      2. 13.5.2 Relocation Operations
      3. 13.5.3 Relocation of Unresolved Weak References
  14. 14ELF Program Loading and Dynamic Linking (Processor Supplement)
    1. 14.1 Program Header
      1. 14.1.1 Base Address
      2. 14.1.2 Segment Contents
      3. 14.1.3 Bound and Read-Only Segments
      4. 14.1.4 Thread-Local Storage
    2. 14.2 Program Loading
    3. 14.3 Dynamic Linking
      1. 14.3.1 Program Interpreter
      2. 14.3.2 Dynamic Section
      3. 14.3.3 Shared Object Dependencies
      4. 14.3.4 Global Offset Table
      5. 14.3.5 Procedure Linkage Table
      6. 14.3.6 Preemption
      7. 14.3.7 Initialization and Termination
    4. 14.4 Bare-Metal Dynamic Linking Model
      1. 14.4.1 File Types
      2. 14.4.2 ELF Identification
      3. 14.4.3 Visibility and Binding
      4. 14.4.4 Data Addressing
      5. 14.4.5 Code Addressing
      6. 14.4.6 Dynamic Information
  15. 15Linux ABI
    1. 15.1  File Types
    2. 15.2  ELF Identification
    3. 15.3  Program Headers and Segments
    4. 15.4  Data Addressing
      1. 15.4.1 Data Segment Base Table (DSBT)
      2. 15.4.2 Global Offset Table (GOT)
    5. 15.5  Code Addressing
    6. 15.6  Lazy Binding
    7. 15.7  Visibility
    8. 15.8  Preemption
    9. 15.9  Import-as-Own Preemption
    10. 15.10 Program Loading
    11. 15.11 Dynamic Information
    12. 15.12 Initialization and Termination Functions
    13. 15.13 Summary of the Linux Model
  16. 16Symbol Versioning
    1. 16.1 ELF Symbol Versioning Overview
    2. 16.2 Version Section Identification
  17. 17Build Attributes
    1. 17.1 C6000 ABI Build Attribute Subsection
    2. 17.2 C6000 Build Attribute Tags
  18. 18Copy Tables and Variable Initialization
    1. 18.1 Copy Table Format
    2. 18.2 Compressed Data Formats
      1. 18.2.1 RLE
      2. 18.2.2 LZSS Format
    3. 18.3 Variable Initialization
  19. 19Extended Program Header Attributes
    1. 19.1 Encoding
    2. 19.2 Attribute Tag Definitions
    3. 19.3 Extended Program Header Attributes Section Format
  20. 20Revision History

7.1 About Multi-Threading and Thread-Local Storage

Complex multi-threaded programs can be better structured and easier to develop if the threads can use variables with static storage duration and that are specific to the thread. That is, other threads cannot see or access such thread-specific variables with static storage duration. Consider the following C code:

    int global_x;
    foo()  {
        int local_x;
        static int static_x = 0;
        ... 
    }

The global_x and static_x variables are allocated once per process, and all threads share the same instance. In contrast, local_x is allocated from the stack. Since each thread gets its own stack, the variable local_x is thread specific, while static_x is not. However, there is no easy way to define a global/static variable on a per thread basis. The POSIX thread interface allows creating thread-specific static storage variables using pthread getspecific and pthread setspecific . But this interface is cumbersome to use.

To solve this issue, Thread-Local Storage (TLS) is a class of storage that allows a program to define thread-specific variables with static storage durations. A TLS variable or "thread-local" is a global/static variable that is instanced once per thread.

Memory used for TLS is allocated statically for the full time the program runs. Each thread has its own instance of all the thread-local variables (even the ones it doesn't declare or use) that are defined by all of the dynamic modules that are loaded at the time a thread is created. When a thread is created, its TLS block is allocated and initialized by the underlying OS thread support library. A thread’s TLS block is reinitialized if a thread completes and then runs again within the same program run. TLS variables are not re-initialized if the thread is suspended or blocked by other threads and then resumes execution after it becomes un-blocked.

The way a TLS variable is accessed depends on how the OS or RTOS creates and manages thread-local storage for each thread. Linux systems need to support TLS allocation for multiple dynamic libraries and libraries loaded during runtime using dlopen(). Also, Linux systems may require allocating TLS storage lazily only when the thread-local is accessed. This requires sophisticated TLS storage management and affects how the thread-local is accessed. On the other hand, a static executable that includes an RTOS needs only to manage a single TLS block and the access can be simple.

After an overview of thread-local concepts, this document describes how thread-locals are specified in source code and how they are represented in the ELF object file (Section 7.5). Then it describes how thread-locals are accessed for C6x Linux, static executable, and bare-metal dynamic linking TLS models (Section 7.6) and how weak references to thread-local variables are resolved. (Section 7.7).

The C6000 TLS mechanism is based on industry-standard conventions, for example the mechanism described in the ELF Handling for Thread-Local Storage paper by Ulrich Drepper.