Cmake, optimization and README.md

[gareth-nx]

Description

README.md instructs users to control compiler flags in CMake builds like this (e.g. to get "-O3"):

export FFLAGS="-O3"
cmake -B build -G Ninja -DCMAKE_MAXIMUM_RANK:String=7 -DCMAKE_INSTALL_PREFIX=$HOME/.local

However this does not have the intended effect; by default there are other options that override this optimization.

To see that this is the case, we can insert the line set( CMAKE_VERBOSE_MAKEFILE on ) near the top of CMakeLists.txt, and build with the above commands. Then the compiler commands are printed out. Copying one (for illustration) we have:

/usr/bin/gfortran  -I/home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/build/src/mod_files  -O3 -O2 -g -DNDEBUG -Jmod_files/   -fimplicit-none -ffree-line-length-132 -Wall -Wextra -Wimplicit-procedure -pedantic-errors -std=f2018 -c /home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/build/src/stdlib_bitsets_64.f90 -o CMakeFiles/fortran_stdlib.dir/stdlib_bitsets_64.f90.o

Notice how right after the "-O3", we have "-O2 -g -DNDEBUG". This is overriding the "-O3" optimization which was intended. Our documentation doesn't make this clear, and this has confused some efforts to try optimising code.

Expected Behaviour

I expected the code to have been compiled with -O3, not -O2.

The reason I noticed this is that my code was slower when linking to Cmake builds, vs Makefile builds (and hence I was getting weird performance results).

Version of stdlib

15f3270e30f164bf2f63828a3f715e31e0077632

Platform and Architecture

All

Additional Information

It seems that by default, stdlib is having CMake add the flags for a CMAKE_BUILD_TYPE=RelWithDebInfo build.

One way to work-around this issue is to have the user make up some ad-hoc BUILD_TYPE (so that the defaults are not invoked). However presumably there are some options that we really do want by default -- so the questions are:

• What should we suggest in README.md?
• Should any of the CMake defaults be changed?

I think it's a good idea to use CMAKE_VERBOSE_MAKEFILE=On (or suggest that option to users in the documentation). This prints out the compiler commands (so users will likely notice if they differ from what was intended).

export FFLAGS="-O3 -fno-range-check"
cmake -B build -DCMAKE_MAXIMUM_RANK=7 -DCMAKE_INSTALL_PREFIX=/home/gareth/Code_Experiments/fortran/stdlib/my_branch/stdlib/local_install_gfortran/ -DCMAKE_BUILD_TYPE=MyCustomBuild -DCMAKE_VERBOSE_MAKEFILE=On

[awvwgk]

Optimization levels in CMake are mainly controlled by the -DCMAKE_BUILD_TYPE, we default to NoConfig, which takes whatever CMake thinks is the best default. For most projects I prefer RelWithDebInfo for development and Release build types for profiling. We should mention the possibility to adjust this via the standard CMake option.

[gareth-nx]

How should we recommend that users set -DCMAKE_BUILD_TYPE ?

If they use -DCMAKE_BUILD_TYPE=Release, then I believe that will append compiler flags that include -O3. This suffers the same problem that it can override other FFLAGS (e.g. say -Ofast was specified, it would be silently overridden). So IMO it isn't the right solution (unless further qualified).

We could tell users to set -DCMAKE_BUILD_TYPE=somethingWithoutADefault if they want to prevent their compiler flags being overridden. But it could be prone to other problems (e.g. gfortran-9 users better specify -fno-range-check to avoid problems with the hash routines).

[awvwgk]

We could provide an overwrite file like in LFortran to purge unwanted arguments from the profiles, or at least the default profile.

• definition: https://gitlab.com/lfortran/lfortran/-/blob/master/cmake/UserOverride.cmake
• usage: https://gitlab.com/lfortran/lfortran/-/blob/master/CMakeLists.txt#L3

[gareth-nx]

That sounds good.

So in the README, I imagine that, rather than explaining FFLAGS, we would say:

• Compiler options can be controlled by modifying the file cmake/UserOverride.cmake. Separate options can be provided for different compilers, and different build types. The latter can be specified via -DCMAKE_BUILD_TYPE when compiling stdlib (common values are -DCMAKE_BUILD_TYPE=Release for builds with high optimization, and -DCMAKE_BUILD_TYPE=Debug for debugging builds).

FYI I'm traveling from tomorrow until late January (without a computer), but will follow up in a few weeks if it hasn't been addressed already.

[awvwgk]

I don't really want to recommend users to overwrite checked in files in our repo. Instead we should provide a way to just use FFLAGS or have users set their arguments via -DCMAKE_Fortran_FLAGS="$FFLAGS" which will overwrite the CMake defaults. We could implement some logic to always use FFLAGS to overwrite the respective CMAKE_Fortran_FLAGS_* variables for all build-types as well.

I might look into this, if I find some motivation to write needlessly trivial but lengthy stuff in CMake, because it is not available by default.

And, of course, have a nice travel.