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Subject: Re: [Boost-build] feature, properties, variants, and all the rest
From: Stefan Seefeld (stefan_at_[hidden])
Date: 2017-08-03 21:08:56

On 03.08.2017 16:47, Steven Watanabe via Boost-build wrote:
> On 08/03/2017 02:11 PM, Stefan Seefeld via Boost-build wrote:
>> Yes, figured. Can you draw out the dependency graph in more detail (or
>> just the bits involved in making 'C') ? If I request `b2 C`, what are
>> the steps involved, leading up to some file
>> "bin/gcc-7.1.1/release/.../C.exe" being generated ? I expect something
>> like this:
>> * C is declared in the Jamfile, as being depend on C-gen (a generator
>> rule that will actually instantiate some further dependency graph for C)
> I'm not quite sure what you mean by a generator rule.
> generators are based on target type. So with C as
> a typed-target{EXE}, we get:
> construct EXE from CPP:
> - composing generator OBJ -> EXE
> (required properties <toolset>gcc-7.1.1) is viable
> - Need OBJ, trying to construct OBJ from CPP
> gcc.compile.c++ is viable

I was referring to the fact that, as you confirmed in a previous reply,
"building C means to generate the dependency graph [for C]", so I called
that step that does that the "generator rule".

My question really was this: Once the Jamfile is read in, the target "C"
has to be registered, but the filename ".../C.exe" isn't, as it falls
out of the generation of the dependency graph, which will determine what
toolset to use, which in turn affects the qualified filename of C.exe.
Thus, I wondered how it is possible to submit to the engine a request to
build "C", and have .../C.exe being made in the process even though at
the time when the request is made nothing is known about it yet.
I thus imagined that one way to implement this was by making C dependent
on C-gen, then let C-gen as it is being made inject other dependencies
for C, notably all those that are concerned with the compilation from
C.cpp into C.exe.

>> * Thus, after reading the Jamfile, b2 does this:
>> - considering C (to be built from C.cpp)
>> - building C-gen (as C's "static" prerequisite)
>> * this will determine g++-7.1.1 as the tool to build C.exe from
>> C.cpp via C.o
>> * thus, C.exe is injected as a new prerequisite target for C, C.o
>> as dependency for C.exe
>> - building C.o from C.cpp
>> - building C.exe from C.o
>> - building C (nothing left to do, it's just an alias for C.exe)
>> Does this make sense, and more importantly, does it capture the logic in
>> b2 ? (By "Static prerequisite" above I'm referring to the fact that the
>> dependency from C to C-gen needs to be known before the engine is
>> started, as otherwise it wouldn't find any prerequisites for C at all,
>> and thus just make nothing in order for C to be considered up-to-date.)
> It's no quite correct to say that C is an alias for
> C.exe. There are actually three levels of targets.
> - A main target (such as C) is what is defined in the Jamfile
> - A virtual target is an intermediate layer representing
> a specific file. There can be multiple virtual targets
> for the same file as long as only one is actualized.
> (This mainly exists to allow backtracking in generators.
> Unlike a Jam target, creating a virtual-target doesn't
> affect global state, so it's safe to create and discard
> virtual-targets.)
> - A Jam target is the thing that gets passed to the
> build engine for processing.
> main-target.construct(properties) returns a list of virtual targets.
> virtual-target.actualize() returns a Jam target.

OK. But given that at least in my earlier case with config checks
affecting the properties passed down to main-target.construct(), the
call to "main-target.construct" needs itself to happen in a rule that is
invoked by the engine, after the config checks have been run, no ? So
Your last statement above ("main-target.construct(properties) returns a
list of virtual targets. virtual-target.actualize() returns a Jam
target.") thus needs to be performed in chunks executed by the engine,
rather than while the Jamfile is being processed. Right ?
And so it sounds like this transformation from main target over virtual
targets to jam targets is the equivalent of my above workflow from C
over C-gen to C.exe. Correct ?


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