Get BLAS implementation information from CMake

Question

In a project I am using

find_package(BLAS REQUIRED)

to detect BLAS.

Is there a way to tell which implementation of BLAS was found after this?

According to the documentation BLA_VENDOR can be used to require a certain implementation, but it doesn't report which one was found.

Unfortunatelly I need to know which BLAS was found because different implementations have subtle differences in their interfaces, for example MKL uses zdotu with 6 arguments rather than 5 (first is the pointer to the result value).

It seems that script [FindBLAS.cmake](https://github.com/Kitware/CMake/blob/master/Modules/FindBLAS.cmake) doesn't store information about BLAS provider. If you want to choose API usage between several cases, the most direct approach is to try all these cases with [try_compile](https://cmake.org/cmake/help/latest/command/try_compile.html) or similar mechanisms. E.g. you may try compile a source file, which calls `zdotu` with 6 arguments. If it succeed, then you would know how to use this function. — Tsyvarev, Nov 25 '20 at 08:46
@Tsyvarev thanks. It is a pity that FindBLAS gets that information and then throws it away. — alfC, Nov 25 '20 at 17:59
@Tsyvarev the worst part of the interface difference is that it is not a matter of compiling or not it is a difference in the ABI. So it would be hard to even detect at runtime (it produces a segfault if you are lucky). — alfC, Nov 25 '20 at 23:16
Actually, you may call `find_package(BLAS)` **several times**, with different `BLA_VENDOR`. So, once a call will succeed, you could be confident about the provider. This approach could be simpler would `BLA_VENDOR` accept several vendors specification at once, but even single vendor per `find_package` call is usable. — Tsyvarev, Nov 26 '20 at 11:13
@Tsyvarev, that is a good trick. An extra complication is that one might need to try with every "mkl" version. — alfC, Nov 27 '20 at 23:11

alfC · Accepted Answer · 2022-06-24T22:12:17.220

Per @Tsyvarev suggestion I ended up doing this

set(BLA_VENDOR Intel10_64lp)
find_package(BLAS)
if(BLAS_FOUND)
    message("MKL environment detected")
    add_definitions(-DRETURN_BY_STACK)
else()
    unset(BLA_VENDOR)
    find_package(BLAS REQUIRED)
endif()

It seems that -DRETURN_BY_STACK (or -DFORTRAN_COMPLEX_FUNCTIONS_RETURN_VOID) is a way recognized by some BLAS headers (e.g. cblas.h) to have MKL-compatible declarations.

Technically one might need to try with every variant of intel mkl BLAS, and choose one somehow, I am putting it here for completeness.

Intel10_32 (intel mkl v10 32 bit)
Intel10_64lp (intel mkl v10+ 64 bit, threaded code, lp64 model)
Intel10_64lp_seq (intel mkl v10+ 64 bit, sequential code, lp64 model)
Intel10_64ilp (intel mkl v10+ 64 bit, threaded code, ilp64 model)
Intel10_64ilp_seq (intel mkl v10+ 64 bit, sequential code, ilp64 model)
Intel

Get BLAS implementation information from CMake

1 Answers1