Add Base.is_serializing_code() for guarding precompilation code

[IanButterworth]

ccall(:jl_generating_output, Cint, ()) == 1 is used in the ecosystem widely to guard precompilation code. It seems like it should be formally in the API.

A slack discussion with @timholy arrived at this name. is_precompiling seemed like a good name at first, but is confused by precompile calls, which precompile code at a time where jl_generating_output != 1. And is_generating_output seemed a bit vague.

[timholy]

Thanks for doing this!

[timholy]

Would it be worth including an example of why you might want this kind of guard? For example, suppose you start julia with --compiled-modules=no. Then there's no reason to pay the price of the package author's precompile directives: those directives are meant to do the work once and make it reusable across multiple sessions, but that doesn't apply if you're not going to be using the cache. And then you might as well pay only for the things you actually need in that session.

[JeffBezanson]

How about is_saving_code? It's shorter, less jargon-y, and less tied to how the process is implemented.

[IanButterworth]

Yeah I share similar reservations about serializing

What about is_caching_code?

"Saving" made me think of saving human-readable code, like .jl files.

[ericphanson]

is_precompile_time()? That’s how I think of it whenever I need this but maybe it’s not accurate

[mkitti]

From triage:

Attending

• @mkitti
• @IanButterworth
• @Seelengrab
• @LilithHafner

Lasting thoughts:

• Wait until native code compiling work is settled. Support external linkage in "sysimages" #44527.
• Consider not putting this in Base. Put in Core.Compiler.
• Triage did not agree on a name

Discussed names

• Core.Compiler.is_saving_code - maybe it should not live in Base
• Core.Compiler.is_outputting - maybe it should not live in Base
• Core.Compiler.is_generating_output -matches the C function, but requires knowledge of compiler jargon
• is_caching_code - for precompile caches, specifically for .ji (output-ji)
• is_saving_compiled_code -
• is_saving_compiler_output
• is_caching_compiler_output
• is_generating_compiled_output

Bad names

• is_runtime -
• is_generating_compiled_code - we are always generating code

[JeffBezanson]

I will point out that this is really a question about what command-line options were passed, i.e. "is the julia runtime being run in a mode where what you do might be saved for later use?" so it is in fact system-global.

[IanButterworth]

Triage decided that with the creation of timholy/SnoopCompile.jl#282 which wraps this ccall, the need for this to be formalized in the API isn't necessary