Guile FFI bindings for libflame/blis

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README.md

guile-ffi-blis

This is a set of Guile FFI bindings for the linear algebra subprograms library, BLIS1. It provides operations such as vector dot product, matrix-vector product, matrix-matrix product, and so on. These bindings are for BLIS' ‘typed’ API2.

To use the bindings, import (ffi blis). BLIS will be loaded from the default dynamic library path (see ‘Installation notes’ below). There are up to three bindings for each function, here using zgemm as an example:

  • bli_zgemm (raw binding): the raw C function by pointer->procedure. Don't use this if you aren't familiar with Guile's FFI. These functions take numeric constants for the trans_t parameters, etc. which have the same names as in C (like BLIS_TRANSPOSE, note the underscore).

  • blis-zgemm! (typed binding): takes array arguments of type 'c64 and operates by effect, without making copies. All the arguments must be properly sized. These functions take typed constants for the trans_t parameters, which have hyphened names (like BLIS-TRANSPOSE). This allows guile-ffi-blis to signal an error if you e.g. pass BLIS-NO-CONJUGATE for a trans_t parameter. For convenience, this function returns the output argument.

  • blis-zgemm (functional binding): takes array arguments of compatible types and returns a newly constructed array. The arguments will be converted as necessary, which may result in copies. The returned array will be of 'c64 type.

In principle, for the last two bindings, you don't need to care whether your array is row-major or column-major or what the strides are. The bindings will extract the required strides from the array arguments3.

If the function doesn't return an array (e.g. blis-cdot) then we only provide two bindings (e.g. bli_cdot and blis-cdot).

The bindings also provide type generic versions of the functions (e.g. blis-dotv for BLIS blis-sdotv blis-ddotv blis-cdotv blis-zdotv). These simply call one of the typed variants according to the type of the first array argument.

Enter ,help (ffi blis) at the Guile REPL to list all the bindings available4.

Installation notes

guile-ffi-blis uses dynamic-link to load the dynamic library for BLIS, so the names of the respective library files must be known. The default name libblis can be configured with the environment variable GUILE_FFI_BLIS_LIBNAME.

If your BLIS library isn't installed in the default dynamic library search path, you can configure specific paths for guile-ffi-blis with the environment variable GUILE_FFI_BLIS_LIBPATH. There are other variables that control where dynamic-link searches for libraries (LTDL_LIBRARY_PATH, LD_LIBRARY_PATH) and you may prefer to set those instead.

A previous version of this library also included CBLAS bindings, but now I have moved those to a separate library (guile-ffi-cblas). The following notes on using that previous version on Guix might still be useful for guile-ffi-blis: https://notabug.org/ZelphirKaltstahl/guile-ml#using-guile-ffi-cblas.

Running the tests

The tests use SRFI-64.

$GUILE -L mod -s test/test-ffi-blis.scm

Depending on your installation (see above) you might need

GUILE_FFI_BLIS_LIBNAME=libotherblas \
GUILE_FFI_BLIS_LIBPATH=/custom/path/lib \
$GUILE ... etc.

Coverage

BLIS level 1

  • setv copyv axpyv axpbyv swapv
  • dotv norm1v normfv normiv amaxv
  • setm copym axpym

BLIS level 2

  • gemv ger

BLIS level 3

  • gemm

¹ See https://github.com/flame/blis and also the related http://www.netlib.org/blas/, https://en.wikipedia.org/wiki/Basic_Linear_Algebra_Subprograms.

² See https://github.com/flame/blis/blob/master/docs/BLISTypedAPI.md.

³ By default (2020/07) BLIS will flag overlapping stride combinations in any of the array arguments as errors, even when the result is well defined (some discussion on the topic here). However, if you disable BLIS' internal error checking with (blis-error-checking-level-set! BLIS_NO_ERROR_CHECKING) BLIS will produce the correct result, as far as I've been able to verify. (ffi blis) performs independent shape checks on the typed and functional bindings, and the Guile array arguments have valid strides by construction, so the lack of error checking by BLIS itself isn't necessarily a problem. The test suite includes tests with a variety of overlapping stride combinations for gemm and gemv. Still, BLIS doesn't officially support these strides. Note that if the destination argument has overlapping strides, then the result depends on the order in which the operations are carried out and is pretty much undefined. (ffi blis) will not check the destination argument for this error.

This triggers a bug in the current version. You can use bli[TAB] as a substitute.