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

15.6 Lazy Binding

For large programs, load-time symbol resolution can significantly degrade program startup time. Lazy binding is a mechanism that delays resolution of function symbols until they are actually called by the program, thus reducing startup time and improving overall performance since only functions that are actually called need to be resolved.

The general approach is that the first call through a PLT vectors control through a resolver function in the dynamic linker, which performs the resolution and re-routes future calls directly to the function itself.

The resolver requires two arguments. The first is a module id that identifies the current module (the one containing the reference). The representation of the module id is unspecified by the ABI, to be determined by the loader. The second argument specifies the relocation entry corresponding to the target function. The relocation entry in turn provides both the name of the target symbol, and the location of the reference in the GOT. The relocation entry is specified by its byte offset in the object file from the address in the DT_RELPLT tag in the file's .dynamic section.

Since all this happens behind the caller's back, the mechanism must preserve any state that affects the standard function-call interface. In particular, it must not disturb any registers used for argument passing or the return address register, and it must preserve any callee-saved registers it modifies. To avoid disturbing the normal argument registers, the resolver's two arguments are passed in B0 and B1.

Two slots in the Global Offset Table are reserved for use by the dynamic loader to implement lazy binding. GOT[0] is used by the loader to hold the address of the resolver function. GOT[1] is used to hold the module id.

The following sequence describes the mechanism:

  1. The static linker identifies candidates for lazy binding. A candidate is a GOT entry that is only referred to by a PLT entry; that is, only used for calling an imported function.
  2. The static linker generates, or includes from a library, a special resolver stub. In this description the stub is called PLT0, although the ABI does not specify its name or location.
  3. The static linker initializes candidate GOT entries with the address of PLT0, and marks them with R_C6000_JUMP_SLOT relocations. The linker locates any such relocation in the section of the dynamic relocation table marked with the DT_JMPREL tag.
  4. The PLT entry is generated with an additional instruction for use in lazy resolution. The instruction loads register B0 with the first of the resolver's two arguments: the byte offset of the R_C6000_JUMP_SLOT relocation entry relative to the dynamic relocation table, indicated by the DT_REL[A] tag. It then loads the target address from the GOT in the usual way and jumps to it. As a result of the initialization in step 3, the first time this jump happens, control transfers to PLT0.
  5. PLT0 loads the second of the resolver's two arguments from GOT[1] into B1: a loader-defined value that identifies the current module. It then loads the address of the loader's resolver function from GOT[0] and tail-calls it.
  6. The resolver function uses its two arguments to find the specified dynamic relocation in the object file specified by the module id. It looks up the symbol in the dynamic symbol table to get the actual address of the function, and replaces the GOT entry with that address. Finally, it jumps to that address, effectively tail-calling the target function.
  7. When the PLT entry is entered for subsequent calls, the GOT has been updated with the actual address, so control passes directly to the function.

Lazy Binding PLT Entry

   $sym$plt:
         MVK   reloc_offset(sym),B0   ;byte offset of GOT reloc entry from DT_RELPLT
         MVKH  reloc_offset(sym),B0
         LDW   *+DP($GOT(sym)),tmp    ;&PLT0 first time, &sym after that
         B     tmp

Resolver Stub—PLT0

   PLT0:
         LDW   *+DP($GOT(0)),tmp      ; address of resolver
         LDW   *+DP($GOT(4)),B1       ; module id
         B tmp                        ; tail-call resolver

Global Offset Table

               ; $GOT(0) reserved, initialized to module id
               ; $GOT(4) reserved, initialized to &resolver function
               ; ...
               ; $GOT(sym) R_C6000_JUMP_SLOT initialized to &PLT0
               ; updated to &sym by resolver