[llvm-bugs] [Bug 52115] New: libomp.so: Missing linking against libm.so when compiled by Clang on Linux systems

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52115

            Bug ID: 52115
           Summary: libomp.so: Missing linking against libm.so when
                    compiled by Clang on Linux systems
           Product: OpenMP
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Runtime Library
          Assignee: unassignedbugs at nondot.org
          Reporter: arfrever.fta at gmail.com
                CC: llvm-bugs at lists.llvm.org

Gentoo report: https://bugs.gentoo.org/816831

As reported there, libomp.so compiled by Clang (instead of GCC) uses at least
fmaxl, which is defined in libm.so, but build system does not pass -lm to
consider linking against libm.so.

> $ nm -D /usr/lib64/libomp.so | grep fmaxl
>   U fmaxl
> $ readelf -d /usr/lib64/libomp.so | grep NEEDED
>  0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.0]
>  0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
>  0x0000000000000001 (NEEDED)             Shared library: [libdl.so.2]
>  0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
>  0x0000000000000001 (NEEDED)             Shared library: [ld-linux-x86-64.so.2]


https://github.com/llvm/llvm-project/blob/37109974af4b33189a599735afa67ed57c27b8ab/openmp/runtime/cmake/LibompHandleFlags.cmake#L138-L144
contains:
>   if(${CMAKE_SYSTEM_NAME} MATCHES "DragonFly|FreeBSD")
>     libomp_append(libflags_local "-Wl,--no-as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
>     libomp_append(libflags_local "-lm")
>     libomp_append(libflags_local "-Wl,--as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
>   elseif(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
>     libomp_append(libflags_local -lm)
>   endif()


I do not know why this code uses --no-as-needed to forcefully link against
libm.so on DragonFly and FreeBSD, but I am not asking to use --no-as-needed on
Linux.

Probable solutions:
Linux-specific solution:
     libomp_append(libflags_local "-Wl,--as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
-  elseif(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
+  elseif(${CMAKE_SYSTEM_NAME} MATCHES "Linux|NetBSD")
     libomp_append(libflags_local -lm)

More generic solution:
     libomp_append(libflags_local "-Wl,--as-needed" LIBOMP_HAVE_AS_NEEDED_FLAG)
-  elseif(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
+  else()
     libomp_append(libflags_local -lm)

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