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Arm C Language Extensions

Document number: 101028

Date of Issue: 31/12/2019


This document specifies the Arm C Language Extensions to enable C/C++ programmers to exploit the Arm architecture with minimal restrictions on source code portability.


ACLE, ABI, C, C++, compiler, armcc, gcc, intrinsic, macro, attribute, Neon, SIMD, atomic

How to find the latest release of this specification or report a defect in it

Please check the Arm Information Center or Arm Developer Website for a later release if your copy is more than one year old.

Please report defects in this specification to arm dot acle at arm dot com.

Confidentiality status

This document is Non-Confidential.

About this document

Change control

Change history

Issue Date By Change
A 11/11/11 AG First release
B 13/11/13 AG Version 1.1. Editorial changes. Corrections and completions to intrinsics as detailed in 3.3. Updated for C11/C++11.
C 09/05/14 TB Version 2.0. Updated for Armv8 AArch32 and AArch64.
D 24/03/16 TB Version 2.1. Updated for Armv8.1 AArch32 and AArch64.
E 02/06/17 Arm Version ACLE Q2 2017. Updated for Armv8.2-A and Armv8.3-A.
F 30/04/18 Arm Version ACLE Q2 2018. Updated for Armv8.4-A.
G 30/03/19 Arm Version ACLE Q1 2019. Updated for Armv8.5-A and MVE. Various bugfixes.
H 30/06/19 Arm Version ACLE Q2 2019. Updated for TME and more Armv8.5-A intrinsics. Various bugfixes.
ACLE Q3 2019 30/09/19 Arm Version ACLE Q3 2019.
ACLE Q4 2019 31/12/19 Arm Version ACLE Q4 2019.

Changes between ACLE Q3 2019 and ACLE Q4 2019

  • BETA support for the Custom Datapath Extension.
  • MVE intrinsics updates and fixes.
  • Feature macros for Pointer Authentication and Branch Target Identification.

Changes between ACLE Q2 2019 and ACLE Q3 2019

  • Support added for Armv8.6-A features.
  • Support added for random number instruction intrinsics from Armv8.5-A [ARMARMv85].

Changes between ACLE Q1 2019 and ACLE Q2 2019

  • Support added for TME features.
  • Support added for rounding intrinsics from Armv8.5-A [ARMARMv85].

Changes between ACLE Q2 2018 and ACLE Q1 2019

  • Support added for features introduced in Armv8.5-A [ARMARMv85] (including the MTE extension).
  • Support added for MVE [MVE-spec] from the Armv8.1-M architecture.
  • Support added for Armv8.4-A half-precision extensions through Neon intrinsics.
  • Added feature detection macro for LSE atomic operations.
  • Added floating-point versions of intrinsics to access coprocessor registers.

Changes between ACLE Q2 2017 and ACLE Q2 2018

Most changes in ACLE Q2 2018 are updates to support features introduced in Armv8.3-A [ARMARMv83]. Support is added for the Complex addition and Complex MLA intrinsics. Armv8.4-A [ARMARMv84]. Support is added for the Dot Product intrinsics.


This document refers to the following documents.

[ARMARM]Arm, Arm Architecture Reference Manual (7-A / 7-R), Arm DDI 0406C
[ARMARMv8]Arm, Armv8-A Reference Manual (Issue A.b), Arm DDI0487A.B
[ARMARMv81]Arm, Armv8.1 Extension,
[ARMARMv82]Arm, Armv8.2 Extension,
[ARMARMv83]Arm, Armv8.3 Extension,
[ARMARMv84]Arm, Armv8.4 Extension,
[ARMARMv85](1, 2, 3) Arm, Armv8.5 Extension,
[ARMv7M]Arm, Arm Architecture Reference Manual (7-M), Arm DDI 0403C
[BA]Arm, EABI Addenda and Errata Build Attributes, Arm IHI 0045C
[CPP11]ISO, Standard C++ (based on draft N3337), ISO/IEC 14882:2011
[C11]ISO, Standard C (based on draft N1570), ISO/IEC 9899:2011
[C99]ISO, Standard C (C99), ISO 9899:1999
[cxxabi]Itanium C++ ABI,
[G.191]ITU-T, Software Tool Library 2005 User’s Manual, T-REC-G.191-200508-I
[GCC]GNU/FSF, GNU C Compiler Collection,
[IA-64]Intel, Intel Itanium Processor-Specific ABI, 245370-003
[IEEE-FP]IEEE, IEEE Floating Point, IEEE 754-2008
[CFP15]ISO/IEC, Floating point extensions for C, ISO/IEC TS 18661-3
[MVE-spec]Arm, Arm v8-M Architecture Reference Manual, Arm DDI0553B.F
[POSIX]IEEE / TOG, The Open Group Base Specifications, IEEE 1003.1
  1. Warren, Hacker’s Delight, pub. Addison-Wesley 2003
[SVE-ACLE]Arm, Arm C Language Extensions for SVE,
[Bfloat16]Arm, BFloat16 processing for Neural Networks on Armv8-A,

Terms and abbreviations

This document uses the following terms and abbreviations.

Term Meaning
AAPCS Arm Procedure Call Standard, part of the ABI, defined in [AAPCS].
ABI Arm Application Binary Interface.
ACLE Arm C Language Extensions, as defined in this document.
Advanced SIMD A 64-bit/128-bit SIMD instruction set defined as part of the Arm architecture.
build attributes Object build attributes indicating configuration, as defined in [BA].
ILP32 A 32-bit address mode where long is a 32-bit type.
LLP64 A 64-bit address mode where long is a 32-bit type.
LP64 A 64-bit address mode where long is a 64-bit type.
Neon An implementation of the Arm Advanced SIMD extensions.
SIMD Any instruction set that operates simultaneously on multiple elements of a vector data type.
Thumb The Thumb instruction set extension to Arm.
VFP The original Arm non-SIMD floating-point instruction set.
word A 32-bit quantity, in memory or a register.


The Arm C Language Extensions (ACLE) specification specifies source language extensions and implementation choices that C/C++ compilers can implement in order to allow programmers to better exploit the Arm architecture.

The extensions include:

  • Predefined macros that provide information about the functionality of the target architecture
  • Intrinsic functions
  • Attributes that can be applied to functions, data and other entities

This specification does not standardize command-line options, diagnostics or other external behavior of compilers.

The intended users of this specification are:

  • Application programmers wishing to adapt or hand-optimize applications and libraries for Arm targets
  • System programmers needing low-level access to Arm targets beyond what C/C++ provides for
  • Compiler implementors, who will implement this specification
  • Implementors of IDEs, static analysis and other similar tools who wish to deal with the C/C++ source language extensions when encountered in source code

ACLE is not a hardware abstraction layer (HAL), and does not specify a library component but it may make it easier to write a HAL or other low-level library in C rather than assembler.

Scalable Vector Extensions (SVE)

ACLE support for SVE is defined in the Arm C Language Extensions for SVE document [SVE-ACLE] available on the Arm Developer Website.