[PATCH] D137059: [Driver] [Modules] Introduce -fsave-std-c++-module-file= to specify the path of the module file (2/2)

Thu Nov 3 15:48:28 PDT 2022

dblaikie added a subscriber: rsmith.
dblaikie added a comment.

In D137059#3899339 <https://reviews.llvm.org/D137059#3899339>, @ben.boeckel wrote:

> There is another motivating factor for 1-phase: the build graph is far simpler. With 2-phase, CMake will have to write out rules to perform:
>
> - source -> .bmi
> - .bmi -> .withbmi.o
> - source -> .o
>
> because we do not know if a BMI is needed or not. If it isn't we use the latter. If it is, we use the former. Note that this also means we need 2 different `.o` filenames (as neither `make` nor `ninja` doesn't support multiple rules making the same output). This also means that the collator needs to generate a response file for the linker to direct which `.o` file to use for each TU based on the contents.

I /think/ from @rsmith's comments in the discourse thread, we're more likely to skip/remove the ability to go from ".bmi" -> ".o" and possibly have 2 path options (this is all from @rsmith's comments on discourse) either ".cppm -> {.pcm, .o}" or ".cppm -> .o" + ".cppm -> .pcm" - this'd avoid the need to maintain full V slim pcm, there would never be a pcm that could produce a .o, .pcm would only be sufficient for users, not implementation.

But yeah, maybe we end up with all 3 options in the interim. Though I'd really like to keep the surface area as small as possible, while still allowing room for experimentation. Perhaps experimentation via -Xclang flags until data shows the options are worthwhile beyond those experiments.

Pulling in your (Ben) comment from D137058 <https://reviews.llvm.org/D137058>:

> Plus the other compilers offer controls over it; why does Clang have to be different?

Which compilers/flags are you referring to? Arguments from compatibility with GCC are relatively easy to make (though I still have more hesitance for these flags since there's not wide-scale adoption, and I think there's still room to shape the world we want to see and limit the width of the interface/variations we end up having to support long term) & might side-step some of the discussions here.

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https://reviews.llvm.org/D137059