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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

-moutline, -mno-outline

B1.63 -moutline, -mno-outline

The outliner searches for identical sequences of code and puts them in a separate function. The outliner then replaces the original sequences of code with calls to this function.

Outlining reduces code size, but it can increase the execution time. The operation of -moutline depends on the optimization level and the complexity of the code.

Note:

-moutline is only supported in AArch64 state.

Default

If the optimization level is -Oz, the default is -moutline. Otherwise the default is -mno-outline.

Syntax

-moutline
Enables outlining.
-mno-outline
Disables outlining.

Parameters

None.

Restrictions

Inline assembly might prevent the outlining of functions.

Examples

This example uses the following C file to show the effects of -moutline:

// foo.c

int func3(int x)
{
    return x*x;
}

int func1(int x)
{
    int i = x;
    i = i * i;
    i+=1;
    //i=func3(i);
    i+=2;
    
    return i;
}

char func2(char x)
{
    int i = x;
    i = i * i;
    i+=1;
    //i=func3(i);
    i+=2;
    
    return i;
}

Compile using -S to output the disassembly to a file as follows:

armclang.exe --target=aarch64-arm-none-eabi -march=armv8.5-a -moutline foo.c -S -o foo.s
armclang.exe --target=aarch64-arm-none-eabi -march=armv8.5-a -moutline foo.c -S -O1 -o foo_O1.s
armclang.exe --target=aarch64-arm-none-eabi -march=armv8.5-a -moutline foo.c -S -O2 -o foo_O2.s

Enable the calls to func3 in foo.c, recompile with -O1 and -O2 and _func3 appended to the output assembly filenames. The following tables show comparisons of the output:

  • With no optimization and with -O1 and the calls to func3 disabled.
  • With no optimization and with -O2 and the calls to func3 enabled.

Compiler-generated comments have been removed for brevity.

Functions func1 and func2 are outlined at -O1, as shown in foo_01.s.

If you enable the calls to func3 and recompile with -O1, then no outlining occurs as shown in foo_01_func3.s. This case is not shown in the tables.

If you enable the calls to func3 and recompile with -O2, then outlining occurs as shown in foo_02_func3.s.

Table B1-9 Comparison of disassembly for -moutline with and without -O1 optimization

No optimization (foo.s) -O1 (foo_01.s)
func3:
// %bb.0:
  sub	sp, sp, #16
  str	w0, [sp, #12]
  ldr	w0, [sp, #12]
  ldr	w8, [sp, #12]
  mul	w0, w0, w8
  add	sp, sp, #16
  ret
.Lfunc_end0:

...

func1:
// %bb.0:
  sub	sp, sp, #16
  str	w0, [sp, #12]
  ldr	w0, [sp, #12]

...

  ldr	w0, [sp, #8]
  add	sp, sp, #16
  ret
.Lfunc_end1:

...

func2:
// %bb.0:
	sub	sp, sp, #16
	strb	w0, [sp, #15]
	ldrb	w0, [sp, #15]

...

	mov	w0, w8
	add	sp, sp, #16
	ret
.Lfunc_end2:
func3:
// %bb.0:
	mul	w0, w0, w0
	ret
.Lfunc_end0:

...

func1:
// %bb.0:
  b	OUTLINED_FUNCTION_0
.Lfunc_end1:

...

func2:
// %bb.0:
  b	OUTLINED_FUNCTION_0

...

  .section	.text.OUTLINED_FUNCTION_0,"ax",@progbits
  .p2align	2
  .type	OUTLINED_FUNCTION_0,@function
OUTLINED_FUNCTION_0:
  .cfi_sections .debug_frame
  .cfi_startproc
// %bb.0:
  orr	w8, wzr, #0x3
  madd	w0, w0, w0, w8
  ret
.Lfunc_end3:
  .size	OUTLINED_FUNCTION_0, .Lfunc_end3-OUTLINED_FUNCTION_0
  .cfi_endproc

Table B1-10 Comparison of disassembly for -moutline with and without -O2 optimization and with func3 enabled

No optimization (foo_func3.s) -O2 and func3 enabled (foo_02_func3.s)
func3:
// %bb.0:
  sub	sp, sp, #16
  str	w0, [sp, #12]
  ldr	w0, [sp, #12]
  ldr	w8, [sp, #12]
  mul	w0, w0, w8
  add	sp, sp, #16
  ret
.Lfunc_end0:

...

func1:
// %bb.0:
  sub	sp, sp, #32
  str	x30, [sp, #16]
  str	w0, [sp, #12]

...

  ldr	x30, [sp, #16]
  add	sp, sp, #32
  ret
.Lfunc_end1:

...

func2:
// %bb.0:
	sub	sp, sp, #32
	str	x30, [sp, #16]
	strb	w0, [sp, #15]

...

	ldr	x30, [sp, #16]
	add	sp, sp, #32
	ret
.Lfunc_end2:
func3:
// %bb.0:
  mul	w0, w0, w0
  ret
.Lfunc_end0:

...

func1:
// %bb.0:
  orr	w8, wzr, #0x1
  madd	w8, w0, w0, w8
  b	OUTLINED_FUNCTION_0
.Lfunc_end1:

...

func2:
// %bb.0:
  and	w8, w0, #0xff
  orr	w9, wzr, #0x1
  madd	w8, w8, w8, w9
  b	OUTLINED_FUNCTION_0
.Lfunc_end2:
  .size	func2, .Lfunc_end2-func2

  .section	.text.OUTLINED_FUNCTION_0,"ax",@progbits
  .p2align	2
  .type	OUTLINED_FUNCTION_0,@function
OUTLINED_FUNCTION_0:
  .cfi_sections .debug_frame
  .cfi_startproc
// %bb.0:
  orr	w9, wzr, #0x2
  madd	w0, w8, w8, w9
  ret
.Lfunc_end3:
  .size	OUTLINED_FUNCTION_0, .Lfunc_end3-OUTLINED_FUNCTION_0
  .cfi_endproc
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