Keil Logo

Technical Support

On-Line Manuals

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

Symbol references and branches into and out of inline assembly

B8.7 Symbol references and branches into and out of inline assembly

Symbols that are defined in an inline assembly statement can only be referred to from the same inline assembly statement.

The compiler can optimize functions containing inline assembly, which can result in the removal or duplication of the inline assembly statements. To define symbols in assembly and use them elsewhere, use file-scope inline assembly, or a separate assembly language source file.

With the exception of function calls, it is not permitted to branch out of an inline assembly block, including branching to other inline assembly blocks. The optimization passes of the compiler assume that inline assembly statements only exit by reaching the end of the assembly block, and optimize the surrounding function accordingly.

It is valid to call a function from inside inline assembly, as that function will return control-flow back to the inline assembly code.

Arm does not recommend directly referencing global data or functions from inside an assembly block by using their names in the assembly string. Often such references appear to work, but the compiler does not know about the reference.

If the global data or functions are only referenced inside inline assembly statements, then the compiler might remove these global data or functions.

To prevent the compiler from removing global data or functions which are referenced from inline assembly statements, you can:

  • use __attribute__((used)) with the global data or functions.
  • pass the reference to global data or functions as operands to inline assembly statements.

Arm recommends passing the reference to global data or functions as operands to inline assembly statements so that if the final image does not require the inline assembly statements and the referenced global data or function, then they can be removed.

Example

static void foo(void) { /* ... */ }
				
// This function attempts to call the function foo from inside inline assembly.
// In some situations this may appear to work, but if foo is not referenced
// anywhere else (including if all calls to it from C got inlined), the
// compiler could remove the definition of foo, so this would fail to link.
void bar() {
	__asm volatile(
			"bl foo"
		: /* no outputs */
		: /* no inputs */
		: "r0", "r1", "r2", "r3", "r12", "lr");
}

// This function is the same as above, except it passes a reference to foo into
// the inline assembly as an operand. This lets the compiler know about the
// reference, so the definition of foo will not be removed (unless, the
// definition of bar_fixed can also be removed). In C++, this has the
// additional advantage that the operand uses the source name of the function,
// not the mangled name (_ZL3foov) which would have to be used if writing the
// symbol name directly in the assembly string.
void bar_fixed() {
	__asm volatile(
			"bl %[foo]"
		: /* no outputs */
		: [foo] "i" (foo)
		: "r0", "r1", "r2", "r3", "r12", "lr");
}
Non-ConfidentialPDF file icon PDF version101754_0613_00_en
Copyright © 2019 Arm Limited or its affiliates. All rights reserved. 
  Arm logo
Important information

This site uses cookies to store information on your computer. By continuing to use our site, you consent to our cookies.

Change Settings

Privacy Policy Update

Arm’s Privacy Policy has been updated. By continuing to use our site, you consent to Arm’s Privacy Policy. Please review our Privacy Policy to learn more about our collection, use and transfers
of your data.