Compiling Stockfish with gcc on Termux

[Archimedes]

Here is a short description, how you can compile Stockfish for arm64-v8a based CPUs with gcc on Termux.

At first, forget the installation of gcc via the repository https://github.com/its-pointless/its-pointless.github.io. It works for Demolito, which is a C based chess engine, but not for Stockfish (and probably other C++ based chess engines).

If you have already installed gcc via the above repository, remove it with:

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apt-get remove gcc-11
apt-get autoremove

Use CCTools for that purpose. A detailed description, how you install the cctool packages under Termux can be found here:
https://cctools.info/index.php/Termux_repository_with_cctools_packages_(gcc_with_fortran_support_etc)

Follow the explanations step by step.

After that, download the Stockfish sources.

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git clone https://github.com/official-stockfish/Stockfish
cd Stockfish/src

Make changes to the makefile, as the following code pieces are shown:

Before:

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ifeq ($(COMP),gcc)
	comp=gcc
	CXX=g++
	CXXFLAGS += -pedantic -Wextra -Wshadow

	ifeq ($(arch),$(filter $(arch),armv7 armv8))
		ifeq ($(OS),Android)
			CXXFLAGS += -m$(bits)
			LDFLAGS += -m$(bits)

After:

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ifeq ($(COMP),gcc)
	comp=gcc
	CXX=g++
	CXXFLAGS += -pedantic -Wextra -Wshadow

	ifeq ($(arch),$(filter $(arch),armv7 armv8))
		ifeq ($(OS),Android)
			CXXFLAGS += 
			LDFLAGS += 

Before:

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### Sometimes gcc is really clang
ifeq ($(COMP),gcc)

After:

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### Sometimes gcc is really clang
ifeq ($(COMP),)

Before:

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### 3.3 Optimization
ifeq ($(optimize),yes)

	CXXFLAGS += -O3

	ifeq ($(comp),gcc)
		ifeq ($(OS), Android)
			CXXFLAGS += -fno-gcse -mthumb -march=armv7-a -mfloat-abi=softfp

After:

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### 3.3 Optimization
ifeq ($(optimize),yes)

	CXXFLAGS += -O3

	ifeq ($(comp),gcc)
		ifeq ($(OS), Android)
			CXXFLAGS += -march=native

Now you can compile Stockfish with gcc and PGO optimization with the following command:

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make -j profile-build ARCH=armv8 COMP=gcc