CMake build issues with GCC

[3e33]

I'm building from source on Ubuntu with GCC (from apt install build-essential) and CMake, the compilation runs fine, these are my build flags:

cmake -S . -B build -G "Ninja" \
 -DDYNAMIC_ARCH=OFF \
 -DBUILD_SHARED_LIBS=OFF \
 -DCMAKE_BUILD_TYPE=Release \
 -DBUILD_TESTING=OFF \
 -DBUILD_STATIC_LIBS=ON \
 -DNOFORTRAN=ON \
 -DTARGET=NEHALEM

Everything compiles fine, until I try to use it with:

find_package(BLAS REQUIRED)
find_package(LAPACK REQUIRED)

In version OpenBLAS-0.3.21 find BLAS works, but find LAPACK fails its /usr/bin/cc -DCHECK_FUNCTION_EXISTS=cheev_ -g check with error:

OpenBLAS/lib/libopenblas.a(cheev.c.o): in function `cheev_':
cheev.c:(.text+0x50b): undefined reference to `sqrtf'

In version OpenBLAS-0.3.34, find BLAS also fails, with similar errors.

I get no errors during compilation, only when CMake's find_package does its function check. I have compiled and used OpenBLAS successfully in the past but I used Clang as the compiler, this time I'm using GCC 11.4 and I can't seem to get it to work.

What's strange to me is that find_package(BLAS) works on 0.3.21 and doesn't on 0.3.34.

[martin-frbg]

That's an implicit dependency on the standard math library (libm), and apparently CMake's find script doesn't set CMAKE_REQUIRED_LIBRARY=-lm when searching for OpenBLAS. You could try if adding EXTRALIB=-lm to the build options for OpenBLAS works around this

[3e33]

@martin-frbg I actually already tried that, and the find script became:
/usr/bin/cc -DCHECK_FUNCTION_EXISTS=cheev_ -g -lm

But it made no difference.

[martin-frbg]

Hmm, the toplevel CMakeLists.txt already sets target_link_libraries to add libm, not sure what's going wrong here. (That line - with an older comment about Android needing libm - was added in 0.3.22 as PR #3895 but I get a github internal error when I try to open that PR to recall the context)

[3e33]

@martin-frbg I found a solution:
The problem is doing set(CMAKE_REQUIRED_LINK_OPTIONS "-lm") doesn't work because it puts the -lm too early in the commandline, the flag isn't used. But you can do set(CMAKE_REQUIRED_LINK_OPTIONS "-Wl,--no-as-needed;-lm"), then find_package does the linking properly.

[martin-frbg]

It isn't clear to me yet if this is something OpenBLAS is doing wrong, or could be doing better. I cannot even seem to find anybody else who encountered this problem in the past

[3e33]

I've used both LLVM and MSVC to compile OpenBLAS without such issues, so I think this is only a problem with GCC. Let this be a warning to others. : )

The only solution I can think of is for OpenBLAS to provide its own find_package script that doesn't rely on check_function_exists.