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Code Generation Tools Tips & TricksFor ARM CGT v4.4.x

April 2008

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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ARM Processors Overview

Processor Arch Ver Switch -mt Neon VFP BE8

ARM7 ARMv4T -mv4 Thumb N N --be8

ARM9 ARMv5TE -mv5e Thumb N Y --be8

ARM11 ARMv6 -mv6 Thumb N Y Default

Cortex-A8 ARMv7A -mv7A8 Thumb2 Y Y Default

Cortex-R4 ARMv7R -mv7R4 Thumb2 N Y Default

Cortex-M3 ARMv7M -mv7M3 Nothing N N Default

• Details on next slide

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ARM Processors Overview

• Processor: Common name for the processor• Arch Ver: Version of the architecture

– Early non-TI ARM9 devices use ARMv4T• Switch: Compiler switch to use• -mt: What does –mt mean?

– Has no effect for Cortex-M3 always Thumb2• Neon: Is Neon SIMD supported?

– Enable with --neon• VFP: Is Vector Floating Point supported?

– Enable with --float_support=vfp_option– Must use EABI

• BE8: How to enable BE8 byte ordering– --be8: code is little endian, data is big endian– --be32: code and data is big endian– Linker build switch cl470 … -z --be8 …

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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EABI Preliminaries

• EABI includes– New object file format: ELF– Changes in runtime environment conventions

• Cannot link COFF and ELF together– Libraries from external vendors must be converted first– All other code must be recompiled

• Build option cl470 --abi=eabi– Default ABI remains TI_ARM9_ABI

• RTS lib must be EABI enabled– Chosen automatically when linking C/C++ code– Will include “eabi” in the name, e.g. rtsv5_T_be_eabi.lib

• Downstream utilities must understand ELF– cg_xml package provides roadmap

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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Conditional Linking Differences

• Conditional linking enabled for all input sections– COFF: Only sections marked .clink are so affected

• Section must be symbolically referenced through path starting at entry point, else it is ignored

• Disable: --unused_section_elimination=off• Use --retain on section that defines interrupt

vectors• To force a connection in assembly code

– .symdepend child section, parent section– .symdepend “mysect”, “.text” means .text uses mysect

even though there is no symbolic reference• Unusual cases that may require --retain

– Sections created entirely in LCF (link command file)– Sections where only symbol definitions are in LCF

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Reference Graphname

file Input section “name” from “file”.obj

“parent” section makes a reference to a “sym” definedin the “child” section

sym

.textboot

.textmain

.bssmain

.textuser

.datauser

main var1

var2

func_call etc …

• Initial symbol is entry point• For C code, this is _c_int00 defined in boot.obj

from the RTS library• A section must be in this graph to be included by

default

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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How Sections Change

Name COFF ELF

.text Code Same

.bss All RW Global Data Uninit’d RW Global Data

.data Not used by compiler Init’d RW Global Data

.const RO Global Data Same

.cinit Data init tables Similar, but compressed

.sysmem Heap Same

.stack Stack Same

.debug_* Dwarf Debug Info Same

• RW = Read or Write RO = Read only• Uninit’d = Uninitialized Init’d = Initialized• Ignores sections used only by C++ code• Heap or stack may be managed by an OS

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New ELF Sections

Name Contains.ARM.attributes Build attribute information.symtab Symbol table.strtab String table.shstrtab Section name string table(NULL) Empty

• None of these sections are loaded in memory• In COFF, similar functionality is provided in a

different, less flexible, manner• Most users can safely ignore these sections• C++ code adds more sections not shown• Do not need to add to link command file

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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C Callable Assembly

• No leading ‘_’ or ‘$’ on C variables– Always var. No longer _var or $var.

• SP must be maintained as 8-byte aligned– Type long long requires 8-byte alignment

• Register conventions see few changes– Sign extension details are different– long long args passed in specific register pairs

• Structs passed/returned differently• Bit fields are laid out differently

– Do you use bit fields to model HW registers?• Array alignment is different

– More relaxed under EABI

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Manage ABI Differences;*******************************************************************; Note old tools do not have built-in ABI symbols.; Must NOT use the default ARM9ABI of tools 4.1.X or higher.;*******************************************************************

.if $$isdefed("__TI_ARM9ABI_ASSEMBLER").if __TI_ARM9ABI_ASSEMBLER

.emsg "Must build with --abi=[tiabi|eabi]".endif

.endif

;*******************************************************************; Define your own variables for which ABI is being used.;*******************************************************************

.if $$isdefed("__TI_EABI_ASSEMBLER") ; Use predefined symEABI .set __TI_EABI_ASSEMBLER ; if it exists

.else ; otherwiseEABI .set 0 ; not using EABI

.endif

.if EABI ; TIABI is oppositeTIABI .set 0 ; of EABI

.elseTIABI .set 1

.endif

;*******************************************************************; Note old tools do not have built-in ABI symbols.; Must NOT use the default ARM9ABI of tools 4.1.X or higher.;*******************************************************************

.if $$isdefed("__TI_ARM9ABI_ASSEMBLER").if __TI_ARM9ABI_ASSEMBLER

.emsg "Must build with --abi=[tiabi|eabi]".endif

.endif

;*******************************************************************; Define your own variables for which ABI is being used.;*******************************************************************

.if $$isdefed("__TI_EABI_ASSEMBLER") ; Use predefined symEABI .set __TI_EABI_ASSEMBLER ; if it exists

.else ; otherwiseEABI .set 0 ; not using EABI

.endif

.if EABI ; TIABI is oppositeTIABI .set 0 ; of EABI

.elseTIABI .set 1

.endif

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Manage Symbols Between ABI’s.if EABI.asg global_data, _global_data.asg func_name, _func_name….endif

.if EABI

.asg global_data, _global_data

.asg func_name, _func_name….endif

• Good for changing over existing code• Every instance of _func_name is replaced by func_name

• Like C’s #define _func_name func_name

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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COFF –c ROM Init Model

burn ROM

.out file

.cinit .cinit ROM

.bss RAM

boot code

Memory

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COFF –cr RAM Init Model

Loader

.out file

.cinit

.bss RAM

Memory

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EABI -cr Initialization

loader

.out file

.data .data RAM

Memory

• .data handled like any other initialized section• Cannot be burned in ROM must be writable

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EABI –c Compressed ROM Init Model

.out file

compressed.cinit .cinit ROM

.data RAM

boot code

Memory

burn ROMELF linker.data

f1.obj

.data

f2.obj

.data

f3.obj• No .cinit in .obj files• Linker creates .cinit by compressing

.data sections• .data becomes uninit’d section like .bss

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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Recommended Build Options

• For fastest speed– When code size does not matter– -o3 –mf5 –oi200 -mvtarget

• Cortex Thumb2– Small code size– Fast speed, though not always the fastest– -mt -o3 –mf –oi200 –mvtarget

– This recommendation subject to change• For smallest code size

– -mt -o3 –mf0 –oi0 –mvtarget

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Recommended Build Options

• -o3– Highest of 5 levels of optimization– None, -o0, -o1, -o2, -o3

• -mfn– Control trade-off between speed and space

• -oin– Automatic function inlining threshold– Functions under size n are automatically inlined– See size of a function

• Build with –o3 –on2• Inspect .nfo file

– -oi0 disables automatic inlining

• -mt– Use Thumb instructions– Means Thumb2 for Cortex

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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GCC Extensions

• GCC defines several extensions to ANSI C (link)• v4.4 Compiler supports most of them

– Must use option --gcc– Details in Compiler User’s Guide

• Typical use case: existing code uses extensions• Example: Case Ranges

switch (val) {case 1 ... 5 : statements; // must have spaces around “...”case ‘A’ ... ‘Z’ : statements;

}

switch (val) {case 1 ... 5 : statements; // must have spaces around “...”case ‘A’ ... ‘Z’ : statements;

}

• Example: Deprecate Variablesextern int old_var __attribute__ ((deprecated)); // from header fileint old_var; // declare itint try() { return old_var; } // use it ==> get warning

extern int old_var __attribute__ ((deprecated)); // from header fileint old_var; // declare itint try() { return old_var; } // use it ==> get warning

• Many more …

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Agenda

• ARM Processors & Compiler Switches• Transition to EABI

– Preliminaries– Conditional Linking– How Sections Change– C Callable Assembly

• Compressed Auto-Initialization• Recommended Build Options• GCC Extensions• New RTS Library

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New RTS Library

• Previous lib remains the default• Comes from Dinkumware Ltd (link)• Big win for C++ users• Less so for C users• New C features include

– locale.h• Example: Format numbers based on local conventions

– time.h is more complete– printf

• Supports more %something formats• Uses locale.h settings• Thus it is bigger and slower vs older lib

• Library organization different in next release– Matters if you link with lib_name.a today