Regent: Detect how many dimensions Legion was compiled with

[manopapad]

To properly interface with Legion's newly implemented support for a variable number of index space dimensions, Regent needs to know the value of the LEGION_MAX_DIM constant that Legion was compiled against when it calls terralib.includec on legion_c.h.

One option would be to autodetect this by inspecting libregent.so, the way it's done for CUDA and OpenMP, and add an -f flag so the user can set it manually when auto-detection isn't available (and do a dynamic consistency check, when the code starts running).

A cleaner option might be to have the Makefile generate a legion_defines.h header, like the one generated by CMake. This has also been suggested as a way to communicate the link flags that Legion's runtime.mk infers to the compiler (see #341).

[elliottslaughter]

As a workaround I've added a flag -flegion-dim that you can use to tell Regent about the value of LEGION_MAX_DIM.

[lightsighter]

Would it help if you had a runtime call to query this value from the runtime?

[elliottslaughter]

Unfortunately no, any autodetect mechanism has to work without actually loading the runtime because:

(a) on platforms like Summit we just can't do that right now

(b) even then we need to set LEGION_MAX_DIM at the point where we include the Legion header file. So if you need to load Legion in order to include that header, you've got a circular dependency.

In my opinion the most promising approach is to have legion_defines.h be generated as part of the build in both Make and CMake. That way any build-influencing variables can always be depended on being set, which is the simplest way for users (most won't even have to think about this at all) and makes this future proof as well.

[lightsighter]

What about a static method on the runtime so you can call it without having actually started the runtime?

[elliottslaughter]

No, we can't even load libregent.so because the ffi implementation in Lua is incomplete (and LuaJIT doesn't support PPC64le). Anything that requires calling a C or Terra function during compilation, or even just loading a shared library, is not going to work.

Futhermore, the static method still doesn't resolve the cyclic dependency, because presumably the way I call the static method is by running terralib.includec("legion.h") and the set of defines is already fixed at that point.

[streichler]

@elliottslaughter why can't terra just include legion_c.h with a large LEGION_MAX_DIM and then check afterwards if the symbols aren't actually in the .so file?

[elliottslaughter]

This doesn't work because LEGION_MAX_DIM affects the size of domains/domain points so it's not just a matter of ignoring symbols that are not defined.

@lightsighter had a partial patch for this but it's currently disabled because it is very inefficient in the current implementation.

[elliottslaughter]

To be clear, the partial patch is for #496 .

[elliottslaughter]

As of ff00fb8 we generate legion_defines.h which includes the value for LEGION_MAX_DIM so that it no longer needs to be set on the command line. -flegion-dim is no longer required or supported.