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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
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Version 6.16

-mthumb

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Arm® Compiler Reference GuideVersion 6.16
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B1.66 -mthumb

Requests that the compiler targets the T32 instruction set.

Most Arm®v7‑A (and earlier) processors support two instruction sets. These are the A32 instruction set (formerly ARM), and the T32 instruction set (formerly Thumb). Armv8‑A processors in AArch32 state continue to support these two instruction sets, but with additional instructions. The Armv8‑A architecture additionally introduces the A64 instruction set, used in AArch64 state.

Different architectures support different instruction sets:

  • Armv8‑A processors in AArch64 state execute A64 instructions.
  • Armv8‑A processors in AArch32 state, in addition to Armv7 and earlier A- and R- profile processors execute A32 and T32 instructions.
  • M-profile processors execute T32 instructions.

Note:

  • The -mthumb option is not valid for targets in AArch64 state, for example --target=aarch64-arm-none-eabi. The compiler ignores the -mthumb option and generates a warning when compiling for a target in AArch64 state.
  • The -mthumb option is recognized when using armclang as a compiler, but not when using it as an assembler. To request armclang to assemble using the T32 instruction set for your assembly source files, you must use the .thumb or .code 16 directive in the assembly files.

Default

The default for all targets that support A32 instructions is -marm.

Example

armclang -c --target=arm-arm-none-eabi -march=armv8-a -mthumb test.c
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