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Build System

docs/build_system.md

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!!! info "Supported build systems"

This page describes the Make-based build, which is the
default/authoritative build method. Note that the OpenBLAS repository also
supports building with CMake (not described here) - that generally works
and is tested, however there may be small differences between the Make and
CMake builds.

Makefile dependency graph

<!--- An easy way to update this diagram is to copy it into https://mermaid.live and edit it interactively. -->
mermaid
flowchart LR
    A[Makefile] -->|included by many of the Makefiles in the subdirectories!| B(Makefile.system)
        B -->|triggered, not included, once by Makefile.system, and runs before any of the actual library code is built. builds and runs the 'getarch' tool for cpu identification, runs the compiler detection scripts c_check/f_check| C{Makefile.prebuild}
            C -->|either this or Makefile_kernel.conf is generated| D[Makefile.conf]
            C -->|temporary Makefile.conf during DYNAMIC_ARCH builds| E[Makefile_kernel.conf]
        B -->|defaults for build options that can be given on the make command line| F[Makefile.rule]
        B -->|architecture-specific compiler options and OpenBLAS buffer size values| G[Makefile.$ARCH]
    A --> exports
    A -->|directories: test, ctest, utest, cpp_thread_test| H(test directories)
    A --> I($BLASDIRS)
        I --> interface
        I --> driver/level2
        I --> driver/level3
        I --> driver/others
    A -->|for each target in DYNAMIC_CORE if DYNAMIC_ARCH=1| kernel
    A -->|subdirs: timing, testing, testing/EIG, testing/LIN| J($NETLIB_LAPACK_DIR)
    A --> relapack

Important Variables

Most of the tunable variables are found in Makefile.rule, along with their detailed descriptions.

Most of the variables are detected automatically in Makefile.prebuild, if they are not set in the environment.

The most commonly used variables are documented below. There are more options though - please read the linked Makefiles if you want to see all variables.

  • ARCH: target architecture (e.g., x86-64).
  • DYNAMIC_ARCH: For building library for multiple TARGETs (does not lose any optimizations, but increases library size).
  • DYNAMIC_LIST: optional user-provided subset of the DYNAMIC_CORE list in Makefile.system.
  • TARGET: target CPU architecture. In case of DYNAMIC_ARCH=1, it means that the library will not be usable on less capable CPUs.
  • TARGET_CORE: override TARGET internally during each CPU-specific cycle of the build for DYNAMIC_ARCH.
  • CC: TARGET C compiler used for compilation (can be cross-toolchains).
  • FC: TARGET Fortran compiler used for compilation (can be cross-toolchains, set NOFORTRAN=1 if the used cross-toolchain has no Fortran compiler).
  • COMMON_OPT: flags to add to all invocations of the target C and Fortran compilers (overrides CFLAGS/FFLAGS - prefer using COMMON_OPT)
  • CCOMMON_OPT: flags to add to all invocations of the target C compiler (overrides CFLAGS)
  • FCOMMON_OPT: flags to add to all invocations of the target Fortran compiler (overrides FFLAGS)
  • LDFLAGS: flags to add to all target linker invocations
  • AR, AS, LD, RANLIB: TARGET toolchain helpers used for compilation (can be cross-toolchains).
  • HOSTCC: compiler of build machine, needed to create proper config files for the target architecture.
  • HOST_CFLAGS: flags for the build machine compiler.

Library kind and bitness options

  • BINARY: whether to build a 32-bit or 64-bit library (default is 64, set to 32 on a 32-bit platform).
  • INTERFACE64: build with 64-bit (ILP64) integer representations to support large array index values (incompatible with the standard 32-bit integer (LP64) API).
  • NO_STATIC: if set to 1, don't build a static library (default is 0)
  • NO_SHARED: if set to 1, don't build a shared library (default is 0)

Data type options

  • BUILD_SINGLE: build the single-precision real functions of BLAS and (if it's built) LAPACK
  • BUILD_DOUBLE: build the double-precision real functions
  • BUILD_COMPLEX: build the single-precision complex functions
  • BUILD_COMPLEX16: build the double-precision complex functions
  • BUILD_BFLOAT16: build the "half precision brainfloat" real functions
  • EXPRECISION: (do not use, this is a work in progress) option to use long double functions

By default, the single- and double-precision real and complex floating-point functions are included in the build, while the half- and extended-precision functions are not.

Threading options

  • USE_THREAD: Use a multithreading backend (defaults to pthreads).
  • USE_LOCKING: implement locking for thread safety even when USE_THREAD is not set (so that the single-threaded library can safely be called from multithreaded programs).
  • USE_OPENMP: Use OpenMP as multithreading backend
  • NUM_THREADS: define this to the maximum number of parallel threads you expect to need (defaults to the number of cores in the build CPU).
  • NUM_PARALLEL: define this to the number of OpenMP instances that your code may use for parallel calls into OpenBLAS (the default is 1, see below).

OpenBLAS uses a fixed set of memory buffers internally, used for communicating and compiling partial results from individual threads. For efficiency, the management array structure for these buffers is sized at build time - this makes it necessary to know in advance how many threads need to be supported on the target system(s).

With OpenMP, there is an additional level of complexity as there may be calls originating from a parallel region in the calling program. If OpenBLAS gets called from a single parallel region, it runs single-threaded automatically to avoid overloading the system by fanning out its own set of threads. In the case that an OpenMP program makes multiple calls from independent regions or instances in parallel, this default serialization is not sufficient as the additional caller(s) would compete for the original set of buffers already in use by the first call. So if multiple OpenMP runtimes call into OpenBLAS at the same time, then only one of them will be able to make progress while all the rest of them spin-wait for the one available buffer. Setting NUM_PARALLEL to the upper bound on the number of OpenMP runtimes that you can have in a process ensures that there are a sufficient number of buffer sets available.

Library and symbol name options

  • FIXED_LIBNAME: if set to 1, uses a non-versioned name for the library and no symbolic linking to variant names (default is 0)
  • LIBNAMEPREFIX: prefix that, if given, will be inserted in the library name before openblas (e.g., xxx will result in libxxxopenblas.so)
  • LIBNAMESUFFIX: suffix that, if given, will be inserted in the library name after openblas, separated by an underscore (e.g., yyy will result in libopenblas_yyy.so)
  • SYMBOLPREFIX: prefix that, if given, will be added to all symbol names and to the library name
  • SYMBOLSUFFIX: suffix that, if given, will be added to all symbol names and to the library name

BLAS and LAPACK options

By default, the Fortran and C interfaces to BLAS and LAPACK are built, including deprecated functions, while ReLAPACK is not.

  • NO_CBLAS: if set to 1, don't build the CBLAS interface (default is 0)
  • ONLY_CBLAS: if set to 1, only build the CBLAS interface (default is 0)
  • NO_LAPACK: if set to 1, don't build LAPACK (default is 0)
  • NO_LAPACKE: if set to 1, don't build the LAPACKE interface (default is 0)
  • BUILD_LAPACK_DEPRECATED: if set to 0, don't build deprecated LAPACK functions (default is 1)
  • BUILD_RELAPACK: if set to 1, build Recursive LAPACK on top of LAPACK (default is 0)