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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

-fshort-enums, -fno-short-enums

B1.29 -fshort-enums, -fno-short-enums

Allows the compiler to set the size of an enumeration type to the smallest data type that can hold all enumerator values.

The -fshort-enums option can improve memory usage, but might reduce performance because narrow memory accesses can be less efficient than full register-width accesses.

Note:

All linked objects, including libraries, must make the same choice. It is not possible to link an object file compiled with -fshort-enums, with another object file that is compiled without -fshort-enums.

Note:

The -fshort-enums option is not supported for AArch64. The Procedure Call Standard for the Arm® 64-bit Architecture states that the size of enumeration types must be at least 32 bits.

Default

The default is -fno-short-enums. That is, the size of an enumeration type is at least 32 bits regardless of the size of the enumerator values.

Example

This example shows the size of four different enumeration types: 8-bit, 16-bit, 32-bit, and 64-bit integers.

#include <stdio.h>

// Largest value is 8-bit integer
enum int8Enum  {int8Val1 =0x01, int8Val2 =0x02, int8Val3 =0xF1 }; 

// Largest value is 16-bit integer              
enum int16Enum {int16Val1=0x01, int16Val2=0x02, int16Val3=0xFFF1 };             

// Largest value is 32-bit integer
enum int32Enum {int32Val1=0x01, int32Val2=0x02, int32Val3=0xFFFFFFF1 }; 

// Largest value is 64-bit integer        
enum int64Enum {int64Val1=0x01, int64Val2=0x02, int64Val3=0xFFFFFFFFFFFFFFF1 }; 

int main(void)
{
  printf("size of int8Enum  is %zd\n", sizeof (enum int8Enum));
  printf("size of int16Enum is %zd\n", sizeof (enum int16Enum));
  printf("size of int32Enum is %zd\n", sizeof (enum int32Enum));
  printf("size of int64Enum is %zd\n", sizeof (enum int64Enum));
}

When compiled without the -fshort-enums option, all enumeration types are 32 bits (4 bytes) except for int64Enum which requires 64 bits (8 bytes):

armclang --target=arm-arm-none-eabi -march=armv8-a enum_test.cpp
				
size of int8Enum  is 4
size of int16Enum is 4
size of int32Enum is 4
size of int64Enum is 8

When compiled with the -fshort-enums option, each enumeration type has the smallest size possible to hold the largest enumerator value:

armclang -fshort-enums --target=arm-arm-none-eabi -march=armv8-a enum_test.cpp
                
size of int8Enum  is 1
size of int16Enum is 2
size of int32Enum is 4
size of int64Enum is 8

Note:

ISO C restricts enumerator values to the range of int. By default armclang does not issue warnings about enumerator values that are too large, but with -Wpedantic a warning is displayed.
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