guile-ffi-cblas

guile-ffi-cblas

This is a set of Guile FFI bindings for two libraries of linear algebra subprograms, CBLAS and BLIS. They provide operations such as vector dot product, matrix-vector product, matrix-matrix product, and so on.

The bindings for either library are independent of each other; you do not need to have BLIS installed to use the CBLAS bindings or viceversa. I am packaging them together because they share a fair amount of code.

CBLAS (or BLAS) is by far the more popular library¹, and there are many different implementations. However BLIS is much more regular and nicer to program for. These bindings are for the BLIS' ‘typed’ API.²

Both sets of bindings work in the same way. Upon importing (ffi cblas) (or (ffi blis)), CBLAS (or 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:

• cblas_zgemm (raw binding): the raw C function by pointer->procedure. Don't use this if you aren't familiar with Guile's FFI.

• zgemm! (typed binding): takes array arguments of type 'c64 and operates by effect, without making copies. All the arguments must be properly sized. The return value is unspecified.

• 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 arguments. However, since CBLAS doesn't support arbitrary strides (e.g. it only supports a column stride for matrix arguments, assuming column-major order), some array arguments will cause the typed binding to fail, or result in extra copies with the functional binding³.

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

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

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

Note that this package is a work in progress and that there are bugs. For example, negative strides require specific handling for CBLAS and are not supported yet. BLIS doesn't have this problem.

Installation notes

guile-ffi-cblas uses dynamic-link to load the dynamic libraries for CBLAS/BLIS. To do this, the names of the respective library files must be known. The default names libcblas/libblis can be configured with the environment variables GUILE_FFI_CBLAS_LIBNAME/GUILE_FFI_BLIS_LIBNAME. The name for BLIS is a safe bet but the name for CBLAS is sadly variable; possible alternative names are libblas or libatlas.

If your CBLAS/BLIS libraries are not installed in the default dynamic library search path, you can configure specific paths for guile-ffi-cblas with the environment variables GUILE_FFI_CBLAS_LIBPATH/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. See also https://notabug.org/ZelphirKaltstahl/guile-ml#using-guile-ffi-cblas for notes on using guile-ffi-cblas on Guix.

Running the tests

The tests use SRFI-64.

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

Depending on your installation (see above) you might need

GUILE_FFI_CBLAS_LIBNAME=libotherblas \
GUILE_FFI_CBLAS_LIBPATH=/custom/path/lib \
$GUILE ... etc.

and similarly for BLIS.

Coverage

CBLAS level 1

• srotg drotg crotg zrotg ⁴
• sscal dscal cscal zscal csscal zdscal
• sswap dswap cswap zswap
• scopy dcopy ccopy zcopy
• saxpy daxpy caxpy zaxpy
• sdot ddot cdotu zdotu cdotc zdotc
• snrm2 dnrm2 scnrm2 dznrm2
• sasum dasum scasum dzasum
• isamax idamax icamax izamax

CBLAS level 2

• sgemv dgemv cgemv zgemv
• sger dger cgeru zgeru cgerc zgerc

CBLAS level 3

• sgemm dgemm cgemm zgemm

BLIS level 1

• sdaxpy ddaxpy cdaxpy zdaxpy
• sdaxpby ddaxpby cdaxpby zdaxpby
• sdotv ddotv cdotv zdotv

BLIS level 2

• sgemv dgemv cgemv zgemv
• sger dger cger zger

BLIS level 3

• sgemm dgemm cgemm zgemm

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¹ See http://www.netlib.org/blas/ or https://en.wikipedia.org/wiki/Basic_Linear_Algebra_Subprograms. ↩

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

³ The situation with BLIS is a bit complicated. 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 (bli-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 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. ↩

⁴ Some distributions of libcblas do not provide these. guile-ffi-cblas will still work, just without these bindings. ↩