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android-gfortran

## Introduction

This repository is intended as a tutorial for building the GNU toolchain on Android, with support for Fortran. Prebuilt versions can be found in the Releases section.

It is based on my experience building OpenBLAS with LAPACK for Android and on several other sources, including this stackoverflow post and this Google group.

Procedure

The whole process has become fairly easy, thanks to the new Python building scripts. A few modifications are still required, and the building process takes time.

Goals & environment

This tutorial aims at building the GNU toolchain 4.9 with Android NDK r13b. It has been tested on Linux x86_64, but I expect that it should work with small changes on other systems supported by the NDK.

Past versions of this README covered building with Android NDK r11c & r12b. Please refer to those for special instructions.

Windows

Although the NDK supports Windows (32-bit & 64-bit variants), the toochain can only be built from Linux. The process is the roughly the same, with a few extra steps detailed along the way.

Steps

Requirements

A few tools are required for building the toolchain, namely:

  • git
  • repo
  • make
  • gcc
  • g++
  • m4
  • texinfo
  • bison
  • flex

On Debian-based Linux, you can run:

sudo apt-get install git repo make gcc g++ m4 texinfo bison flex

Note: When building the toolchain for Windows, mingw-w64 is required. For 32-bit, gcc-multilib and g++-multilib are also needed.

Android NDK

The easiest way to setup all required sources is to follow the official steps (found here).

In this repository, call:

repo init -u https://android.googlesource.com/platform/manifest -b gcc

You can then use repo sync to clone all parts of the toolchain, and repo forall -c git checkout ndk-r13-release to checkout the latest r13 version.

Adding support for Fortran

In toolchain/gcc/build-gcc.sh, find the line that contains:

ENABLE_LANGUAGES="c,c++"

and replace it with:

ENABLE_LANGUAGES="c,c++,fortran"

Building

If you are planning to build the ARM or AArch64 toolchains for Linux 64-bit or Windows 32-bit, that shoule be sufficient. Simply call build.py under toolchain/gcc which will take care of everything. You can specify which toolchain to build. For instance:

./build.py --toolchain arm-linux-androideabi

See ./build.py -h for possible values. If nothing is specified, it will build all of them.

When building the toolchain for Windows, add --host windows (or --host windows64 for 64-bit).

Other targets / hosts

When building the x86 or x86_64 toolchains, additional changes are required. There is an issue in libgfortran for the x86 and x86_64 targets (see #2 and this issue) causing an error when building it. See the x86.diff.

When building the toolchain for Windows 64-bit, you need to change toolchain/binutils/binutils-2.25/gold/aarch64.cc (see #1 and this issue). Find line 2028 that says:

Insntype adr_insn = adrp_insn & ((1 << 31) - 1);

and replace it by:

Insntype adr_insn = adrp_insn & ((1u << 31) - 1);

Deploying

The generated toolchain are not standalone as few includes are packaged. This is because NDK lets you choose which platform version and which STL you want to use.

To allow ndk-build to use your new toolchain, extract the archive under ndk/toolchains/$(TOOLCHAIN)/prebuilt/$(HOST_ARCH). Don't forget to back up the toolchain that was already packaged with the NDK. Warning: This refers to the full NDK downloadable from Google, not the directory created by repo.

For instance, on Linux x86_64 for the AArch64 toolchain, unpack the archive as ndk/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64.

If you want to create a standalone toolchains (i.e. you're not using ndk-build), do the previous step, and then follow this guide. You will want to specify which API level you want to use. It is safe to always use the latest available, even if you are targetting older devices.