Introduction

This project contains crosstool-ng .config files to build OpenMP-enabled bare-metal GCC cross-compilers:

• aarch64 -- ARMv8
• arm-cortexm7 -- ARMv7 Cortex-M7
• arm-uboot -- ARMv7 Cortex-A9 with NEON (i.MX6Q) with aapcs-linux
• arm-cortexm0p -- ARMv7 Cortex-m0+
• arm-cortexm33 -- ARMv7 Cortex-m33

Normally bare metal compilers do not have OpenMP support, since it is assumed that considerable operating system support is required to make OpenMP work. However, it is actually not that difficult to get a subset of OpenMP working on a multi-core bare metal system.

To build a bare-metal OpenMP-enabled toolchain you first need a couple mods to crosstool-ng which are contained in the project at https://github.com/jrengdahl/crosstool-ng-openmp. After downloading my modified version of crosstool-ng, follow the instructions at https://crosstool-ng.github.io/docs to build and install ct-ng. You can then use the modified ct-ng and the .config files in this project to build the GCC toolchains.

Windows cross-compilers are built from a Cygwin64 bash shell. The Linux versions are built using Debian running under Windows Subsystem for Linux (WSL). These should work equally well under any real Linux system.

Cygwin issues

I built the Windows toolchains on a Windows PC using Cygwin. Following the ct-ng instructions, You need to enabled case-sensitive filenames in the C:/cross subdirectory. See https://learn.microsoft.com/en-us/windows/wsl/case-sensitivity. So I don't have to remember the command I created setcase.bat which contains the command: "fsutil file setCaseSensitiveInfo %1 enable". Remember that this only works if the directory is empty when the command is run.

I want the toolchains to be useable in both Cygwin and non-Cygwin environments. In order to do this, I need to copy two of the Cygwin DLLs into the toolchain's bin directory, and move or copy two directories. See the file SetupOpenMP.txt for detailed instructions.

More recently (early 2024) this stunt does not seem to work any more. I had better luck with creating a subdirectory arm-cortexm7-eabi/cygwin64/bin, copying ALL the Cygwin DLLs there, then puttingf that directory on the Eclipse PATH.

If you run the toolchains in a Cygwin environment it is necessary that the versions of the DLLs in the toolchain bin directory match the installed Cygwin version. I recommend that you update your Cygwin, then update the DLLs in the toolchain by copying them from C:/Cygwin64/bin. If you do not use Cygwin you should not need to do this.

In my world I run the toolchain in a Cygwin environment whenever I compile something by typing commands in a shell window. When I compile using an Eclipse project, the tools are run in a non-Cygwin environment automatically created by Eclipse. This is why I need the toolchain to work in both Cygwin and non-Cygwin environments.

What I really wanted to do was build the toolchains using Cygwin's MSYS compilers. Applications built with MSYS and statically linked are independent of any DLLs and are more portable. However, MSYS does not seem to be complete enough to build GCC.

Linux issues

For now, you need to copy omp.h from another toolchain into the new toolchain. TODO -- create a crosstool-ng patch to make this step unnecessary.

You may also need to copy in a gdb executable -- I have not yet tried integrating the toolchain into Eclipse under Linux.

.config mods

Here are some of the more significant changes made to the .config file:

• CT_PREFIX_DIR="${PWD}/../../${CT_TARGET}"
  (Cygwin only) This causes the toolchain to be installed under C:\cross, which is where I keep my cross toolchains. There is a possibility that moving the toolchain to another location may not work in some cases (such as in a non-Cygwin environment) due to how Cygwin translates path names. I suspect it has something to do with cygwin1.dll becoming offended at finding itself in <toolchain>/bin rather than in C:/cygwin64/bin. If moving the install location breaks for you, you may have to adjust this setting and re-build the toolchain for your environment, or just keep the toolchains in C:\cross.

• CT_TARGET_CFLAGS="-ffixed-x18" or CT_TARGET_CFLAGS="-ffixed-r9"
  This causes the compiler to build libraries that do not use the platform register as defined in the ARM or Aarch64 Procedure Call Standards (AAPCS):

  "The role of register r9 is platform specific. A virtual platform may assign any role to this register and must document this usage. For example, it may designate it as the static base (SB) in a position-independent data model, or it may designate it as the thread register (TR) in an environment with thread-local storage. The usage of this register may require that the value held is persistent across all calls. A virtual platform that has no need for such a special register may designate r9 as an additional callee-saved variable register."

  This is only needed if your bare metal thread switching code uses the platform register. Doing so enables significant optimization of bare metal thread switching.

• CT_TARGET_VENDOR="<something useful>"
  Since bare metal toolchains typically put "none" or "unknown" as the second field of the triplet, which is uninformative, I decided to put helpful identifiers such as "openmp" or "uboot" here.

• CT_THREADS_NONE=y
  This is set, since the bare metal environment does not support Pthreads or any other known operating system defined threading system. I am still learning OpenMP, but as far as I can tell at this point, OpenMP does not explicitly depend on Pthreads, therefore it doesn't make sense that the GCC build system enforces a dependency between OpenMP and Pthreads. Any Pthreads dependencies are hidden within libgomp. To use OpenMP under bare metal, the threading mechanism must be provided by an implementation of libgomp.

• CT_CC_GCC_LIBGOMP=y
  This flag enables OpenMP in the compiler. By default, the GCC config system will override this setting if CT_THREADS_NONE=y is set. A couple modifications to crosstool-ng were required to overcome this.

• CT_GDB_CROSS is not set
  I was not able to get crosstool-ng to successfully build GDB. It has something to do with the version of Python found by the configure script. Since I don't use GDB I simply turned this option off rather than spending time debugging the problem. Since the Eclipse toolchain manger requires the presence of gdb in the toolchain bin directory (even if it is not used), I copied in a gdb from another similar toolchain and renamed it to match the triplet. I don't know if this gdb would work.

Toolchain-specific comments

The ARMv8 toolchains are built for generic ARMv8 and multilib, but the ARMv7 versions are built for specific processors. I did this to have better control of library generation and hopefully better optimization for ARMv7. I am not expert at building GCC, so it remains to be seen if I know what I am doing here.

• arm-uboot-eabi
  This toolchain is built for

  • Cortex-A9
  • NEON
  • aapcs-linux

  This toolchain is optimized for the NXP i.MX6Q. I was looking for a multi-core ARM platform which is supported by Segger J-link and Ozone, and has an on-chip trace buffer. The i.MX6Q meets all these requirements, and I found a Sabre Lite board and several Karo Electronics TX6Q-1010 SoM modules on e-Bay for a good price. Most other readily available (and cheap) ARMv7-A implementations, such as Raspberry Pi or the Rockchip RK3188, have only external trace ports, which requires developers to purchase very expensive trace probes.

  The aapcs-linux setting causes the use of fixed-length enums rather than the default variable length, which allows this toolchain to be used to build u-boot and u-boot standalone apps. This is still experimental.

• arm-cortexm7
  Since the M7 is single-core it may not seem sensical to have an OpenMP-enabled compiler for it, but I found it useful to debug OpenMP in a single-core, multi-threaded environment. I will also use this toolchain for non-OpenMP Cortex-M7 projects.

##Releases:

• v0.0 First experimental release.
• v0.0.1 Fixed packaging issue for Windows binaries.
• v1.0 Oops, haven.t quite figured out tagging yet.
• v1.1 Add Cortex-M33, release GCC 12.2 for Cortex-m33.
• v1.2 Update Cortex-M33 and Cortex-M7 to GCC 13.2.