Since 6295b8e763503644961f9087382f80965b98d466 in llvm-project,
it's no longer possible to build openmp entirely standalone;
now one has to build it through the runtimes directory.
This allows simplification, this functionality was added long
ago to CMake.
Further motivation is to be able to re-direct the installation,
make/ninja install allow this by setting DESTDIR but this
generally doesnt work well with window paths (see [1]).
[1] - https://cmake.org/cmake/help/latest/envvar/DESTDIR.html#envvar:DESTDIR
Since a356782426f5bf54a00570e1f925345e5fda7b2e in llvm-project,
OpenMP can be built for mingw/aarch64, since
fb947c358661b3bd3d5e1fa776ec74cc0f308854 it should work correctly
in all currently tested cases, and since
97958c9bb83cec44b6ce13e732b53de0171a5d43 it can be built for
mingw/armv7.
When rebuilding in an already built directory, cmake does use
the cache file, containing what variables were set to previously.
When changing build configuration (building with a different prefix,
or changing build configurations) this can often result in failed
cmake runs.
Wipe the cache (and the rest of the cmake temp files) by default.
This makes sure that the full cmake configuration is run each time
when running the build scripts, just like it does the first time.
(This doesn't remove the build artifacts; if nothing of significance
has changed, ninja would still reuse old built files.)
By setting the env variable NO_RECONF, this behaviour can be skipped,
rerunning cmake without wiping the cache inbetween. (For cases where
the user knows that the configuration hasn't changed since the last
build, this can speed up the cmake execution time a bit.)
This command isn't a standard tool; different distributions have
different implementations, and e.g. Fedora doesn't ship it by
default.
Settle on using "command -v $tool >/dev/null" for tests about whether
a tool is available. (We could also use "hash $tool 2>/dev/null" as we
already had in build-all.sh, but settling on using "command -v"
everywhere, for consistency and simplicity.)
LLVM's OpenMP implementation only supports Windows on x86, and it
requires a MASM-compatible assembler to build it. Build UASM
for this purpose, for now. (UASM isn't available packaged in
debian/ubuntu repos.) In the future, llvm-ml should be able
to handle it instead, but it's not currently mature enough.
Not including it in the regular build-all.sh for now, as UASM
isn't generally available.
Martin Storsjö