Use swift-frontend from the host for SupportedFeatures.json

[edymtt]

Addresses rdar://71282911

[edymtt]

@swift-ci please build toolchain

[edymtt]

@swift-ci please smoke test

[edymtt]

Follow up of #34655

[compnerd]

Hmm, Im not sure how this is supposed to work. There are two scenarios here:

1. you are building the standard library only and you specify the SWIFT_NATIVE_SWIFT_TOOLS_PATH
2. you are building the Swift toolchain for testing which will build the standard library. In such a case, SWIFT_NATIVE_SWIFT_TOOLS_PATH is not set, and swift-frontend should be used from SWIFT_RUNTIME_OUTPUT_INTDIR.

In fact, I would argue that the first case doesn't apply to this at all - the SupportedFeatures.json should be generated when generating the toolchain, so it should always be executed from the output directory. Even in the cross-compilation case, it should be the version of the toolchain that is being built that determines the features of the toolchain, not the host toolchain that may be used in the generation of the toolchain.

Perhaps the right thing to do here is to only generate it if the toolchain is not being cross-compiled, that is:
if(NOT CMAKE_CROSSCOMPILING).

[edymtt]

SWIFT_NATIVE_SWIFT_TOOLS_PATH is used during the compilation of the macOS toolchain -- in particular when we are cross compiling we are setting that to the host toolchain we just built (see https://github.com/apple/swift/blob/main/utils/build-script-impl#L1791), which should be what we want.

This target seems only run if we are building bin/swift-frontend, so the change should not affect building only the standard library

[edymtt]

@swift-ci please test Windows

[edymtt]

@swift-ci please test stdlib with toolchain

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 3055b68

[compnerd]

This target seems only run if we are building bin/swift-frontend, so the change should not affect building only the standard library

Thats what I had assumed and hence why I had said

I would argue that the first case doesn't apply to this at all - the SupportedFeatures.json should be generated when generating the toolchain

This code path is specific to building the toolchain. But, I wanted to verify that the assumption was correct. It seems that it is.

SWIFT_NATIVE_SWIFT_TOOLS_PATH is used during the compilation of the macOS toolchain -- in particular when we are cross compiling we are setting that to the host toolchain we just built (see https://github.com/apple/swift/blob/main/utils/build-script-impl#L1791), which should be what we want.

This is where I think that the problem actually is.

Perhaps the right thing to do here is to only generate it if the toolchain is not being cross-compiled, that is:
if(NOT CMAKE_CROSSCOMPILING).

AIUI, the features are meant to be the features of the toolchain being built, not the features with which the toolchain was built. This means that we need to run the binary that is being generated. This requires that we are not cross-compiling. In the case that we are cross-compiling, we have no good way to ensure that the toolchain was built at the same revision and same configuration (configuration can impact the feature set in theory).

[edymtt]

Sorry for not understanding your point in the first place -- I blindly based my fix on a similar issue (#33402) without realizing that in this case that the role of the executable invoked is more relevant.

[finagolfin]

Just a note about this point,

you are building the Swift toolchain for testing which will build the standard library.
In such a case, SWIFT_NATIVE_SWIFT_TOOLS_PATH is not set

Even if SWIFT_NATIVE_SWIFT_TOOLS_PATH is not externally passed in by CMake, it is internally defined and used, so it has always been set to something and can be used for invocations like this. I don't know about features.json, so not saying anything about the overall pull.

[swift-ci]

Linux Toolchain (Ubuntu 16.04)
Download Toolchain
Git Sha - 3055b68

Install command
tar zxf swift-PR-34686-494-ubuntu16.04.tar.gz
More info

[nkcsgexi]

This won't work because we intend to use the just-built compiler to emit this file.

[edymtt]

Thanks everyone for the feedback!

Closing this for the sake of clarity.

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - 3055b68

Install command
tar -zxf swift-PR-34686-767-osx.tar.gz --directory ~/