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Compiler Reference Guide

Preface Arm Compiler Tools Overview armclang Reference armclang Command-line Options Summary of armclang command-line options -C (armclang) -c (armclang) -D (armclang) -E -e -fbare-metal-pie -fbracket-depth=N -fcommon, -fno-common -fdata-sections, -fno-data-sections -ffast-math, -fno-fast-math -ffixed-rN -ffp-mode -ffunction-sections, -fno-function-sections -fident, -fno-ident @file -fldm-stm, -fno-ldm-stm -fno-builtin -fno-inline-functions -flto, -fno-lto -fexceptions, -fno-exceptions -fomit-frame-pointer, -fno-omit-frame-pointer -fpic, -fno-pic -fropi, -fno-ropi -fropi-lowering, -fno-ropi-lowering -frwpi, -fno-rwpi -frwpi-lowering, -fno-rwpi-lowering -fsanitize -fshort-enums, -fno-short-enums -fshort-wchar, -fno-short-wchar -fstack-protector, -fstack-protector-all, -fstack- -fstrict-aliasing, -fno-strict-aliasing -fsysv, -fno-sysv -ftrapv -fvectorize, -fno-vectorize -fvisibility -fwrapv -g, -gdwarf-2, -gdwarf-3, -gdwarf-4 (armclang) -I -include -L -l -M, -MM -MD, -MMD -MF -MG -MP -MT -march -marm -masm -mbig-endian -mbranch-protection -mcmodel -mcmse -mcpu -mexecute-only -mfloat-abi -mfpu -mimplicit-it -mlittle-endian -mno-neg-immediates -moutline, -mno-outline -mpixolib -munaligned-access, -mno-unaligned-access -mthumb -nostdlib -nostdlibinc -O (armclang) -o (armclang) -pedantic -pedantic-errors -Rpass -S (armclang) -save-temps -shared (armclang) -std --target -U -u (armclang) -v (armclang) --version (armclang) --version_number (armclang) --vsn (armclang) -W (armclang) -Wl -Xlinker -x (armclang) -### Compiler-specific Keywords and Operators Compiler-specific keywords and operators __alignof__ __asm __declspec attributes __declspec(noinline) __declspec(noreturn) __declspec(nothrow) __inline __promise __unaligned Global named register variables Compiler-specific Function, Variable, and Type Att Function attributes __attribute__((always_inline)) function attribute __attribute__((cmse_nonsecure_call)) function attr __attribute__((cmse_nonsecure_entry)) function att __attribute__((const)) function attribute __attribute__((constructor(priority))) function at __attribute__((format_arg(string-index))) function __attribute__((interrupt("type"))) function attrib __attribute__((malloc)) function attribute __attribute__((naked)) function attribute __attribute__((noinline)) function attribute __attribute__((nomerge)) function attribute __attribute__((nonnull)) function attribute __attribute__((noreturn)) function attribute __attribute__((not_tail_called)) function attribut __attribute__((nothrow)) function attribute __attribute__((pcs("calling_convention"))) functio __attribute__((pure)) function attribute __attribute__((section("name"))) function attribut __attribute__((target("options"))) function attrib __attribute__((unused)) function attribute __attribute__((used)) function attribute __attribute__((value_in_regs)) function attribute __attribute__((visibility("visibility_type"))) fun __attribute__((weak)) function attribute __attribute__((weakref("target"))) function attrib Type attributes __attribute__((aligned)) type attribute __attribute__((packed)) type attribute __attribute__((transparent_union)) type attribute Variable attributes __attribute__((alias)) variable attribute __attribute__((aligned)) variable attribute __attribute__((deprecated)) variable attribute __attribute__((packed)) variable attribute __attribute__((section("name"))) variable attribut __attribute__((unused)) variable attribute __attribute__((used)) variable attribute __attribute__((visibility("visibility_type"))) var __attribute__((weak)) variable attribute __attribute__((weakref("target"))) variable attrib Compiler-specific Intrinsics __breakpoint intrinsic __current_pc intrinsic __current_sp intrinsic __disable_fiq intrinsic __disable_irq intrinsic __enable_fiq intrinsic __enable_irq intrinsic __force_stores intrinsic __memory_changed intrinsic __schedule_barrier intrinsic __semihost intrinsic __vfp_status intrinsic Compiler-specific Pragmas #pragma clang system_header #pragma clang diagnostic #pragma clang section #pragma once #pragma pack(...) #pragma unroll[(n)], #pragma unroll_completely #pragma weak symbol, #pragma weak symbol1 = symbol Other Compiler-specific Features ACLE support Predefined macros Inline functions Volatile variables Half-precision floating-point data types Half-precision floating-point number format Half-precision floating-point intrinsics Library support for _Float16 data type BFloat16 floating-point number format TT instruction intrinsics Non-secure function pointer intrinsics Supported architecture feature combinations for sp armclang Integrated Assembler Syntax of assembly files for integrated assembler Assembly expressions Alignment directives Data definition directives String definition directives Floating-point data definition directives Section directives Conditional assembly directives Macro directives Symbol binding directives Org directive AArch32 target selection directives AArch64 target selection directives Space-filling directives Type directive Integrated assembler support for the CSDB instruct armclang Inline Assembler Inline Assembly File-scope inline assembly Inline assembly statements within a function Assembly string Output and input operands Clobber list volatile Inline assembly constraint strings Constraint modifiers Constraint codes Constraint codes common to AArch32 state and AArch Constraint codes for AArch32 state Constraint codes for AArch64 state Using multiple alternative operand constraints Inline assembly template modifiers Template modifiers common to AArch32 state and AAr Template modifiers for AArch32 state Template modifiers for AArch64 state Forcing inline assembly operands into specific reg Symbol references and branches into and out of inl Duplication of labels in inline assembly statement armlink Reference fromelf Reference armar Reference armasm Legacy Assembler Reference Appendixes

-fno-builtin

B1.18 -fno-builtin

Tells the compiler not to use generic (non-Arm specific) handling and optimization of standard C and C++ library functions and operators, for example for the printf(), strlen(), and malloc() functions from the C standard library, or for the new and delete operators from the C++ standard library.

When compiling without -fno-builtin, the compiler can replace calls to certain standard library functions with inline code or with calls to other library functions. The Run-time ABI for the Arm® Architecture lists library functions that the compiler can use. This means that your re-implementations of the standard library functions might not be used, and might be removed by the linker.

Note:

The -fno-builtin option does not prevent Arm specific transformations from within the Arm implementation of the C Standard Library. Use the -nostdlib option to tell the compiler not to use the Arm standard C and C++ libraries.

Default

-fno-builtin is disabled by default.

Example without -fno-builtin

The following test program uses the printf() function:

#include "stdio.h"

void foo( void )
{
    printf("Hello\n");
}
  1. Compile the program without -fno-builtin:

    armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none foo.c -o foo.o
  2. Run the following fromelf command to show the disassembled output:

    fromelf --disassemble foo.o -o foo.s
    ...
    ||foo|| PROC
            ADR      r0,|L3.8|
            B        puts
    |L3.8|
            DCD      0x6c6c6548
            DCD      0x0000006f
            ENDP
    ...

    Note:

    The compiler has replaced the printf() function with the puts() function.

Example with -fno-builtin

  1. Compile the same test program with -fno-builtin:

    armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -fno-builtin foo.c -o foo.o

  2. Run the following fromelf command to show the disassembled output:

    fromelf --disassemble foo.o -o foo.s
    ...
    ||foo|| PROC
            ADR      r0,|L3.8|
            B        __2printf
    |L3.8|
            DCD      0x6c6c6548
            DCD      0x00000a6f
            ENDP
    ...

    Note:

    The compiler has not replaced the printf() function with the puts() function when using the -fno-builtin option. Instead, it has replaced it with __2printf from the Arm standard C library. In order to prevent that, you need to use the -nostdlib option as well.

Example with -fno-builtin and -nostdlib

  1. Compile the test program with -fno-builtin and -nostdlib:

    armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -fno-builtin -nostdlib foo.c -o foo.o
  2. Run the following fromelf command to show the disassembled output:

    fromelf --disassemble foo.o -o foo.s
    						...
    						||foo|| PROC
    						ADR      r0,|L3.8|
    						B        printf
    						|L3.8|
    						DCD      0x6c6c6548
    						DCD      0x00000a6f
    						ENDP
    						...

    Note:

    The compiler has now not replaced the printf() function at all.
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