Worse performance than pyerfa on Windows

[dcherrie-dstl]

On windows I have found ~2-3X worse performance compared to pyerfa. Using cmakelists (liberfa/erfa#75) and a clang compiler I was able to substitute the dll resulting in comparative (slightly better) performance.

[helgee]

Which compiler flags did you use?

[giordano]

Looking to the logs of ERFA_jll (they are in all tarballs), the library was compiled with -g -O2. CMake configured with -DCMAKE_BUILD_TYPE=Release instead would typically use -O3, that can make quite some difference

[dcherrie-dstl]

I used Visual Studio 2019 and set the target to 'x64-Clang-Release'. PSB a snippet of the ninja build output (build.ninja). Not sure if its helpful; I dont have much experience with build systems and compilation.

build src\CMakeFiles\erfa.dir\apcg13.c.obj: C_COMPILER__erfa_RelWithDebInfo ......\src\apcg13.c || cmake_object_order_depends_target_erfa
DEFINES = -DHAVE_CONFIG_H -Derfa_EXPORTS
FLAGS = -m64 -fdiagnostics-absolute-paths /DWIN32 /D_WINDOWS /W3 /MD /Zi /O2 /Ob1 /DNDEBUG
INCLUDES = -I. -I......\src
OBJECT_DIR = src\CMakeFiles\erfa.dir
OBJECT_FILE_DIR = src\CMakeFiles\erfa.dir
TARGET_COMPILE_PDB = src\CMakeFiles\erfa.dir
TARGET_PDB = src\erfa.pdb

[helgee]

Thanks for posting the build log @dcherrie-dstl. I can see that your binaries were compiled with -O2 as well. It seems that Clang produces more performant binaries than our GCC cross-compiler. Could this be the case @giordano? I guess there is not much we can do here?

Out of curiosity, if you use the DLL from Clang and run the tests (import Pkg; Pkg.test("ERFA")), do they pass?

[giordano]

Yes, it's possible that Clang is doing a better job at building more performant binaries, but the factor 2-3x looks a bit surprising. Maybe GCC is being very conservative about what to optimise? I think that trying to build with -O3, for example by switching to the CMake build system, is worth a try

[helgee]

Unfortunately, the CMake build system has not been merged, yet. I will give it a try locally on macOS.

[giordano]

Ugh, right, I missed that 😅

[helgee]

Running the tests on macOS is virtually identical with -O2 and -O3.