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Compiler Reference Guide

Preface Arm Compiler Tools Overview armclang Reference armlink Reference armlink Command-line Options --any_contingency --any_placement=algorithm --any_sort_order=order --api, --no_api --autoat, --no_autoat --bare_metal_pie --base_platform --be8 --be32 --bestdebug, --no_bestdebug --blx_arm_thumb, --no_blx_arm_thumb --blx_thumb_arm, --no_blx_thumb_arm --bpabi --branchnop, --no_branchnop --callgraph, --no_callgraph --callgraph_file=filename --callgraph_output=fmt --callgraph_subset=symbol[,symbol,...] --cgfile=type --cgsymbol=type --cgundefined=type --comment_section, --no_comment_section --cppinit, --no_cppinit --cpu=list (armlink) --cpu=name (armlink) --crosser_veneershare, --no_crosser_veneershare --datacompressor=opt --debug, --no_debug --diag_error=tag[,tag,…] (armlink) --diag_remark=tag[,tag,…] (armlink) --diag_style={arm|ide|gnu} (armlink) --diag_suppress=tag[,tag,…] (armlink) --diag_warning=tag[,tag,…] (armlink) --dll --dynamic_linker=name --eager_load_debug, --no_eager_load_debug --eh_frame_hdr --edit=file_list --emit_debug_overlay_relocs --emit_debug_overlay_section --emit_non_debug_relocs --emit_relocs --entry=location --errors=filename --exceptions, --no_exceptions --export_all, --no_export_all --export_dynamic, --no_export_dynamic --filtercomment, --no_filtercomment --fini=symbol --first=section_id --force_explicit_attr --force_so_throw, --no_force_so_throw --fpic --fpu=list (armlink) --fpu=name (armlink) --got=type --gnu_linker_defined_syms --help (armlink) --import_cmse_lib_in=filename --import_cmse_lib_out=filename --import_unresolved, --no_import_unresolved --info=topic[,topic,…] (armlink) --info_lib_prefix=opt --init=symbol --inline, --no_inline --inline_type=type --inlineveneer, --no_inlineveneer input-file-list (armlink) --keep=section_id (armlink) --keep_intermediate --largeregions, --no_largeregions --last=section_id --legacyalign, --no_legacyalign --libpath=pathlist --library=name --library_security=protection --library_type=lib --list=filename --list_mapping_symbols, --no_list_mapping_symbols --load_addr_map_info, --no_load_addr_map_info --locals, --no_locals --lto, --no_lto --lto_keep_all_symbols, --no_lto_keep_all_symbols --lto_intermediate_filename --lto_level --lto_relocation_model --mangled, --unmangled --map, --no_map --max_er_extension=size --max_veneer_passes=value --max_visibility=type --merge, --no_merge --merge_litpools, --no_merge_litpools --muldefweak, --no_muldefweak -o filename, --output=filename (armlink) --output_float_abi=option --overlay_veneers --override_visibility -Omax (armlink) --pad=num --paged --pagesize=pagesize --partial --pie --piveneer, --no_piveneer --pixolib --pltgot=type --pltgot_opts=mode --predefine="string" --preinit, --no_preinit --privacy (armlink) --ref_cpp_init, --no_ref_cpp_init --ref_pre_init, --no_ref_pre_init --reloc --remarks --remove, --no_remove --ro_base=address --ropi --rosplit --rw_base=address --rwpi --scanlib, --no_scanlib --scatter=filename --section_index_display=type --shared --show_cmdline (armlink) --show_full_path --show_parent_lib --show_sec_idx --soname=name --sort=algorithm --split --startup=symbol, --no_startup --stdlib --strict --strict_flags, --no_strict_flags --strict_ph, --no_strict_ph --strict_preserve8_require8 --strict_relocations, --no_strict_relocations --strict_symbols, --no_strict_symbols --strict_visibility, --no_strict_visibility --symbols, --no_symbols --symdefs=filename --symver_script=filename --symver_soname --sysv --tailreorder, --no_tailreorder --tiebreaker=option --unaligned_access, --no_unaligned_access --undefined=symbol --undefined_and_export=symbol --unresolved=symbol --use_definition_visibility --userlibpath=pathlist --veneerinject, --no_veneerinject --veneer_inject_type=type --veneer_pool_size=size --veneershare, --no_veneershare --verbose --version_number (armlink) --via=filename (armlink) --vsn (armlink) --xo_base=address --xref, --no_xref --xrefdbg, --no_xrefdbg --xref{from|to}=object(section) --zi_base=address Linking Models Supported by armlink Overview of linking models Bare-metal linking model overview Partial linking model overview Base Platform Application Binary Interface (BPABI) Base Platform linking model overview SysV linking model overview Concepts common to both BPABI and SysV linking mod Image Structure and Generation The structure of an Arm ELF image Views of the image at each link stage Input sections, output sections, regions, and prog Load view and execution view of an image Methods of specifying an image memory map with the Image entry points Restrictions on image structure Simple images Types of simple image Type 1 image structure, one load region and contig Type 2 image structure, one load region and non-co Type 3 image structure, multiple load regions and Section placement with the linker Default section placement Section placement with the FIRST and LAST attribut Section alignment with the linker Linker support for creating demand-paged files Linker reordering of execution regions containing Linker-generated veneers What is a veneer? Veneer sharing Veneer types Generation of position independent to absolute ven Reuse of veneers when scatter-loading Generation of secure gateway veneers Command-line options used to control the generatio Weak references and definitions How the linker performs library searching, selecti How the linker searches for the Arm standard libra Specifying user libraries when linking How the linker resolves references The strict family of linker options Linker Optimization Features Elimination of common section groups Elimination of unused sections Optimization with RW data compression How the linker chooses a compressor Options available to override the compression algo How compression is applied Considerations when working with RW data compressi Function inlining with the linker Factors that influence function inlining About branches that optimize to a NOP Linker reordering of tail calling sections Restrictions on reordering of tail calling section Linker merging of comment sections Merging identical constants Accessing and Managing Symbols with armlink About mapping symbols Linker-defined symbols Region-related symbols Types of region-related symbols Image$$ execution region symbols Load$$ execution region symbols Load$$LR$$ load region symbols Region name values when not scatter-loading Linker defined symbols and scatter files Methods of importing linker-defined symbols in C a Methods of importing linker-defined symbols in Arm Section-related symbols Types of section-related symbols Image symbols Input section symbols Access symbols in another image Creating a symdefs file Outputting a subset of the global symbols Reading a symdefs file Symdefs file format Edit the symbol tables with a steering file Specifying steering files on the linker command-li Steering file command summary Steering file format Hide and rename global symbols with a steering fil Use of $Super$$ and $Sub$$ to patch symbol definit Scatter-loading Features The scatter-loading mechanism Overview of scatter-loading When to use scatter-loading Linker-defined symbols that are not defined when s Placing the stack and heap with a scatter file Scatter-loading command-line options Scatter-loading images with a simple memory map Scatter-loading images with a complex memory map Root region and the initial entry point Effect of the ABSOLUTE attribute on a root region Effect of the FIXED attribute on a root region Methods of placing functions and data at specific Placing functions and data in a named section Placing __at sections at a specific address Restrictions on placing __at sections Automatically placing __at sections Manually placing __at sections Placing a key in flash memory with an __at section Example of how to explicitly place a named section Placement of unassigned sections Default rules for placing unassigned sections Command-line options for controlling the placement Prioritizing the placement of unassigned sections Specify the maximum region size permitted for plac Examples of using placement algorithms for .ANY se Example of next_fit algorithm showing behavior of Examples of using sorting algorithms for .ANY sect Behavior when .ANY sections overflow because of li Placing veneers with a scatter file Placement of CMSE veneer sections for a Secure ima Reserving an empty block of memory Characteristics of a reserved empty block of memor Example of reserving an empty block of memory Placement of Arm C and C++ library code Placing code in a root region Placing Arm C library code Placing Arm C++ library code Aligning regions to page boundaries Aligning execution regions and input sections Preprocessing a scatter file Default behavior for armclang -E in a scatter file Using other preprocessors in a scatter file Example of using expression evaluation in a scatte Equivalent scatter-loading descriptions for simple Command-line options for creating simple images Type 1 image, one load region and contiguous execu Type 2 image, one load region and non-contiguous e Type 3 image, multiple load regions and non-contig How the linker resolves multiple matches when proc How the linker resolves path names when processing Scatter file to ELF mapping Scatter File Syntax BNF notation used in scatter-loading description s Syntax of a scatter file Load region descriptions Components of a load region description Syntax of a load region description Load region attributes Inheritance rules for load region address attribut Inheritance rules for the RELOC address attribute Considerations when using a relative address +offs Execution region descriptions Components of an execution region description Syntax of an execution region description Execution region attributes Inheritance rules for execution region address att Considerations when using a relative address +offs Input section descriptions Components of an input section description Syntax of an input section description Examples of module and input section specification Expression evaluation in scatter files Expression usage in scatter files Expression rules in scatter files Execution address built-in functions for use in sc ScatterAssert function and load address related fu Symbol related function in a scatter file AlignExpr(expr, align) function GetPageSize() function SizeOfHeaders() function Example of aligning a base address in execution sp Scatter files containing relative base address loa BPABI and SysV Shared Libraries and Executables About the Base Platform Application Binary Interfa Platforms supported by the BPABI Features common to all BPABI models About importing and exporting symbols for BPABI mo Symbol visibility for BPABI models Automatic import and export for BPABI models Manual import and export for BPABI models Symbol versioning for BPABI models RW compression for BPABI models SysV linking model SysV standard memory model Using the C and C++ libraries Using a dynamic Linker Automatic dynamic symbol table rules in the SysV l Symbol definitions defined for SysV compatibility Addressing modes in the SysV linking model Thread local storage in the SysV linking model Linker command-line options for the SysV linking m Bare metal and DLL-like memory models BPABI standard memory model Customization of the BPABI standard memory model Linker command-line options for bare metal and DLL Mandatory symbol versioning in the BPABI DLL-like Automatic dynamic symbol table rules in the BPABI Addressing modes in the BPABI DLL-like model C++ initialization in the BPABI DLL-like model Symbol versioning Overview of symbol versioning Embedded symbols The symbol versioning script file Example of creating versioned symbols Linker options for enabling implicit symbol versio Features of the Base Platform Linking Model Restrictions on the use of scatter files with the Scatter files for the Base Platform linking model Placement of PLT sequences with the Base Platform Linker Steering File Command Reference EXPORT steering file command HIDE steering file command IMPORT steering file command RENAME steering file command REQUIRE steering file command RESOLVE steering file command SHOW steering file command fromelf Reference armar Reference armasm Legacy Assembler Reference Appendixes

Using the C and C++ libraries

C8.4.2 Using the C and C++ libraries

You can use either the Arm® C and C++ libraries or platform libraries with the SysV linking model.

Use of the Arm® C and C++ libraries

You can use the Arm C and C++ libraries with the SysV linking model by statically linking the main executable with them. You must appropriately retarget the library for the platform.

Note:

When performing the standard library selection as described in C3.10 How the linker searches for the Arm® standard libraries, the linker selects the best-suited variants of the C and C++ libraries with the SysV linking model by statically linking the main executable with them. You must appropriately retarget the library for the platform.Arm C and C++ libraries based only on the attributes of input objects that are used to build the main executable. Shared libraries used in the link and their input objects do not affect the library selection

Integration with a dynamic loader

  • The Arm C and C++ libraries with the SysV linking model by statically linking the main C library executes pre-initialization (.prenit_array) and initialization functions (.init_array) that are present only in the main executable. The library is not aware of initialization functions in loaded shared objects.

    To enable running initialization routines in the whole program, you can link the main executable with armlink --no_preinit --no_cppinit and provide custom implementation of __arm_preinit_() and __cpp_initialize__aeabi_(). The overridden functions must integrate with a platform dynamic loader to execute all initialization functions.

    The dynamic loader can use dynamic entries DT_PREINIT_ARRAY, DT_INIT_ARRAY, DT_INIT to obtain initialization functions in the executable and each shared object.

  • The Arm C++ library by default supports exceptions only in the main executable. To allow exceptions in loaded shared objects, you can provide implementation of __arm_find_exidx_section() (in AArch32 state) and __arm_find_eh_frame_hdr_section() (in AArch64 state):
    /* AArch32 hook */
    int __arm_find_exidx_section(uintptr_t target_addr, uintptr_t *base, size_t *length);
    
    /* AArch64 hook */
    int __arm_find_eh_frame_hdr_section(uintptr_t target_addr, uintptr_t *base, size_t *length);

    The functions receive an address of code that needs to be unwound and must find an exception-index section associated with this location. Parameter target_addr specifies an address of code that needs to be unwound. Parameters base and length point to values that must be set by the function to the address and size of the found exception-index section. Return value 0 indicates success, non-zero value indicates a failure. The dynamic loader can use segments PT_ARM_EXIDX (in AArch32 state) and PT_GNU_EH_FRAME (in AArch64 state) to locate the exception-index sections.

Use of the platform C and C++ libraries

It is possible to use system libraries that come with the target platform.

The code of the program must be compiled with the -nostdlib and -nostdlibinc armclang command-line options to indicate to the compiler to not use the Arm C and C++ libraries.

Executable and shared objects should be linked with the --no_scanlib armlink command-line option.

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