is it possible to enable FP128 in Flang binaries from https://apt.llvm.org/?

[jeffhammond]

The GAMESS team wants to use nightly binaries for CI testing to avoid building Flang from source every time. The Flang binaries from apt.llvm.org do not use the CMake option -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath and thus do not have all the features they need.

/usr/bin/ld: qeigen.o: in function `qjacdia_':
/gamess/object/qeigen.f:200:(.text+0xe1c): undefined reference to `_FortranASqrtF128'
/usr/bin/ld: /gamess/object/qeigen.f:201:(.text+0xe51): undefined reference to `_FortranASqrtF128'
/usr/bin/ld: /gamess/object/qeigen.f:202:(.text+0xe70): undefined reference to `_FortranASqrtF128'

I apologize if this is a dumb question. If there's a license issue with shipping libquadmath support then I guess it is unsolvable.

[sscalpone]

@vzakhari fyi

[vzakhari]

Are these binaries built using GCC or/and rely on GNU libc? If yes, then the dynamic linking with libquadmath is not different from dynamic linking with glibc - they all are under LGPL.

The LLVM/Flang compiler distribution does not have to ship libquadmath, but then the end users of LLVM/Flang compiler built with -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath will have to provide libquadmath on their own, otherwise, they will not be able to link any program with flang-new driver due to the missing libquadmath.

So, yes, the expectation here is that the vendor of such an LLVM/Flang compiler provides some version of libquadmath (which, under LGPL, may imply distributing the corresponding GCC source).

I can propose only one workaround to this: adding the compiler build (cmake) option that will change the behavior of flang-new driver when built with -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath, i.e. the driver will not try to link libquadmath by default and a special compiler option will be required to enable its linking. This way, we can always build LLVM/Flang with -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath -DFLANG_DRIVER_F128_MATH_NOLINK (assuming apt.llvm.org build environment has quadmath.h available), and the GAMESS team will have to provide some -flink-f128-math option for linking the programs that need F128 math, and the libquadmath itself (may require providing -Lpath).

FWIW, aarch64 libm provides float128 math support via long double type, so arm64 build from apt.llvm.org should have all the support by default.

[vzakhari]

Do you know who would be the best contact regarding apt.llvm.org build setup?

[JorgeG94]

I am building flang-new with the entire llvm, I've done:

cd build
cmake -DCMAKE_BUILD_TYPE=Release \
    -DBUILD_SHARED_LIBS=OFF \
    -DLLVM_BUILD_LLVM_DYLIB=ON \
    -DLLVM_TARGETS_TO_BUILD="X86" \
    -DLIBOMPTARGET_ENABLE_DEBUG=ON \
    -DLLVM_ENABLE_ASSERTIONS=ON \
    -DLLVM_ENABLE_PROJECTS="clang;mlir;flang;lldb" \
    -DLLVM_ENABLE_RUNTIMES="openmp;compiler-rt" \
    -DCMAKE_C_COMPILER=gcc \
    -DCMAKE_CXX_COMPILER=g++ \
    -DLLVM_ENABLE_BACKTRACES=ON \
    -DLLVM_ENABLE_WERROR=OFF \
    -DLLVM_ENABLE_RTTI=ON \
    -DOPENMP_ENABLE_LIBOMPTARGET=ON \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
    -DCMAKE_VERBOSE_MAKEFILE=OFF \
    -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath \
  ../llvm

and I didn't seem to build libquadmath :( does this need to be done by building flang standalone after building clang, etc?

[jeffhammond]

AFAIK libquadmath comes from GCC. This option just tells Flang to use it.

[vzakhari]

AFAIK libquadmath comes from GCC. This option just tells Flang to use it.

This is correct.

@JorgeG94 please double check that your GCC installation have access to quadmath.h and libquadmath (start with a simple test with #include <quadmath.h>).

[JorgeG94]

I forgot to load the module I created with the newly built flang-new...I am a talented software engineer. Thanks so much.