Flang not respecting -module flag

[h-vetinari]

While trying to get flang working on a project that uses the meson build system, we ran into:

[133/1628] Compiling Fortran object scipy/special/libamos.a.p/amos_dgamln.f.obj
FAILED: scipy/special/libamos.a.p/amos_dgamln.f.obj 
"flang-new" "-Iscipy\special\libamos.a.p" "-Iscipy\special" "-I..\scipy\special" "-fcolor-diagnostics" "-DNDEBUG" "-D_FILE_OFFSET_BITS=64" "-O3" "-module" "scipy\special\libamos.a.p" -MD -MQ scipy/special/libamos.a.p/amos_dgamln.f.obj -MF "scipy\special\libamos.a.p\amos_dgamln.f.obj.d" -o scipy/special/libamos.a.p/amos_dgamln.f.obj "-c" ../scipy/special/amos/dgamln.f
flang-new: error: unknown argument: '-module'

flang-new: warning: scipy\special\libamos.a.p: 'linker' input unused

This is because -module is apparently so common in Fortran compilers, that it's the baseline assumption for how meson constructs its commandline invocations for most Fortran compilers (it's worth noting that this method gets overridden for several compilers, though notably, meson for classic flang defaults to -module).

It seems this assumption matches parts of flang's documentation (which do indicate -module support)...

llvm-project/flang/docs/ModFiles.md

Lines 106 to 121 in 2147e9e

	## Reading and writing module files
	### Options
	The compiler will have command-line options to specify where to search
	for module files and where to write them. By default it will be the current
	directory for both.
	For PGI, `-I` specifies directories to search for include files and module
	files. `-module` specifics a directory to write module files in as well as to
	search for them. gfortran is similar except it uses `-J` instead of `-module`.
	The search order for module files is:
	1. The `-module` directory (Note: for gfortran the `-J` directory is not searched).
	2. The current directory
	3. The `-I` directories in the order they appear on the command line

... but not others, which say:

-module-dir<dir>, -J<arg>

This option specifies where to put .mod files for compiled modules. It is also added to the list of directories to be searched by an USE statement. The default is the current directory.

I'm not sure if this was an oversight in the recent option-refactor. It's certainly conceivable to teach meson that (llvm-)flang needs -module-dir instead of -module. However, it's at least surprising (given the documentation divergence), so I thought I'd raise this.

CC @banach-space

[llvmbot]

@llvm/issue-subscribers-flang-driver

While [trying](https://github.com/conda-forge/scipy-feedstock/pull/252) to get flang working on a project that uses the meson build system, we ran into:

[133/1628] Compiling Fortran object scipy/special/libamos.a.p/amos_dgamln.f.obj
FAILED: scipy/special/libamos.a.p/amos_dgamln.f.obj 
"flang-new" "-Iscipy\special\libamos.a.p" "-Iscipy\special" "-I..\scipy\special" "-fcolor-diagnostics" "-DNDEBUG" "-D_FILE_OFFSET_BITS=64" "-O3" "-module" "scipy\special\libamos.a.p" -MD -MQ scipy/special/libamos.a.p/amos_dgamln.f.obj -MF "scipy\special\libamos.a.p\amos_dgamln.f.obj.d" -o scipy/special/libamos.a.p/amos_dgamln.f.obj "-c" ../scipy/special/amos/dgamln.f
flang-new: error: unknown argument: '-module'

flang-new: warning: scipy\special\libamos.a.p: 'linker' input unused

This is because -module is apparently so common in Fortran compilers, that it's the baseline assumption for how meson constructs its commandline invocations for most Fortran compilers (it's worth noting that this method gets overridden for several compilers, though notably, meson for classic flang defaults to -module).

It seems this assumption matches parts of flang's documentation (which do indicate -module support)...

llvm-project/flang/docs/ModFiles.md

Lines 106 to 121 in 2147e9e

	## Reading and writing module files
	### Options
	The compiler will have command-line options to specify where to search
	for module files and where to write them. By default it will be the current
	directory for both.
	For PGI, `-I` specifies directories to search for include files and module
	files. `-module` specifics a directory to write module files in as well as to
	search for them. gfortran is similar except it uses `-J` instead of `-module`.
	The search order for module files is:
	1. The `-module` directory (Note: for gfortran the `-J` directory is not searched).
	2. The current directory
	3. The `-I` directories in the order they appear on the command line

... but not others, which say:
> -module-dir&lt;dir>, -J&lt;arg>
>
> This option specifies where to put .mod files for compiled modules. It is also added to the list of directories to be searched by an USE statement. The default is the current directory.

I'm not sure if this was an oversight in the recent option-refactor. It's certainly conceivable to teach meson that (llvm-)flang needs -module-dir instead of -module. However, it's at least surprising (given the documentation divergence), so I thought I'd raise this.

CC @banach-space

[banach-space]

Hi @h-vetinari , sorry for the delay. You will find the relevant discussion here: https://lists.llvm.org/pipermail/flang-dev/2020-November/000597.html

flang-new supports -module-dir and -J. Perhaps you could use the latter? That would be compatible with GNU.

[h-vetinari]

Ok, thanks for the context and confirming that this is intentional. Closing