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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 -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 -save-temps -shared (armclang) -std --target -U -u (armclang) -v (armclang) --version (armclang) --version_number (armclang) --vsn (armclang) -W -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__((nonnull)) function attribute __attribute__((noreturn)) function attribute __attribute__((nothrow)) function attribute __attribute__((pcs("calling_convention"))) functio __attribute__((pure)) function attribute __attribute__((section("name"))) function attribut __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 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 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

-ffixed-rN

B1.12 -ffixed-rN

Prevents the compiler from using the specified core register, unless the use is required for Arm ABI compliance. You must use this option if you want to reserve registers for use as a global named register variable.

Default

By default, the compiler is free to use core registers for any purpose, such as for temporary storage of local variables, within the requirements of the Arm ABI.

Syntax

-ffixed-rN

Parameters

N specifies the register number, which can be any number from 5 to 11. This enables you to reserve core registers R5 to R11.

Restrictions

This feature is only available for AArch32 state.

If you use -mpixolib, then you must not use the following registers as global named register variables:

  • R8
  • R9

If you use -fwrpi or -fwrpi-lowering, then you must not use register R9 as a global named register variable.

Arm recommends that you do not use the following registers as global named register variables because the Arm ABI reserves them for use as a frame pointer if needed. You must carefully analyze your code, to avoid side effects, if you want to use these registers as global named register variables:

  • R7 in T32 state.
  • R11 in A32 state.
Code size

Declaring a core register as a global named register variable means that the register is not available to the compiler for other operations. If you declare too many global named register variables, code size increases significantly. In some cases, your program might not compile, for example if there are insufficient registers available to compute a particular expression.

Operation

-ffixed-rN reserves the specified core register so that the compiler does not use the specified register unless required for Arm ABI compliance. You must reserve the register if you want to use the register as a global named register variable. You can also use -ffixed-rN for generating compatible objects, for example to generate objects that you want to link with other objects that have been built with -frwpi.

For example -ffixed-r5 reserves register R5 so that the compiler cannot use R5 for storing temporary variables.

Note:

The specified registers might still be used in other object files, for example library code, that have not been compiled using the -ffixed-rN option.

Examples

The following example demonstrates the effect of the -ffixed-rN option.

Source file foo.c contains the code below:

int foo(int a1, int a2, int a3, int a4, int a5, int a6)
{
   return  a1/a2 + a3/a4 + a5/a6;
}

Compile the above code without any -ffixed-rN option:

armclang --target=arm-arm-none-eabi -march=armv8-a -O0 -S foo.c -o foo.s

The generated assembly file, foo.s, saves the registers it needs to use, which are {r4, r5, r6, r7, r11, lr}:

foo:
        .fnstart
@ %bb.0:
        .save   {r4, r5, r6, r7, r11, lr}
        push    {r4, r5, r6, r7, r11, lr}
        .pad    #40
        sub     sp, sp, #40
       
        /* Code in between is hidden */

        add     sp, sp, #40
        pop     {r4, r5, r6, r7, r11, pc}
.Lfunc_end0:

To ensure that the compiler does not use registers R5 and R6, compile the same code in foo.c with the -ffixed-r5 and -ffixed-r6 options:

armclang --target=arm-arm-none-eabi -march=armv8-a -O0 -ffixed-r5 -ffixed-r6 -S foo.c -o foo.s

The generated assembly file, foo.s, saves the registers it needs to use, which are {r4, r7, r8, r9, r11, lr}. In this foo.s, the compiler uses registers R8 and R9 instead of R5 and R6:

foo:
        .fnstart
@ %bb.0:
        .save   {r4, r7, r8, r9, r11, lr}
        push    {r4, r7, r8, r9, r11, lr}
        .pad    #40
        sub     sp, sp, #40
       
        /* Code in between is hidden */

        add     sp, sp, #40
        pop     {r4, r7, r8, r9, r11, pc}
.Lfunc_end0:
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