Potential fix for --xcode build on M1.

[johnno1962]

Hi Apple,

I've been having difficulties building the Swift compiler on an M1 for some time now. This small CMake fix seems to resolve the problem and carry relatively little risk of regression. The problem was the generated SwiftInTheCompiler.xcodeproj was silently failing to build the static libraries containing Swift sources used in the bootstrapping process to link them into the compiler. This seems to be a feature of Xcode when the target doesn't contain a "Compile Sources" build phase and only links object files (I have no idea why this would be specific to M1.) By creating a dummy source file and including it in the project this forces Xcode to build the static library and subsequent bootstrapping stages can proceed.

I'm using Xcode 13.4 & 13.2.1, CMake 3.23.1 & 3.20.1

[philipturner]

I have had to manually pass --bootstrapping=off into the compiler for quite a while now, so I'm glad to see the need for that go away. However, I sometimes need to test an older commit in this repository to bisect where a bug originated. Could we add documentation to GettingStarted.md warning users about this flag?

[johnno1962]

I can do that even if perhaps it is better in a separate PR. Can you suggest some wording and where to add it?

[eeckstein]

For local development you should use --bootstrapping=hosttools. This avoids bootstrapping by compiling the swift sources with the installed Xcode. It speeds up the build

[johnno1962]

So we're happy leaving the default documented invocation for the build not working on M1?

[philipturner]

I can do that even if perhaps it is better in a separate PR. Can you suggest some wording and where to add it?

I don't think I have the time to do that myself, but I would definitely review a PR made by someone else.

[eeckstein]

lgtm

[johnno1962]

Thanks @eeckstein

[LucianoPAlmeida]

@swift-ci Please test

[johnno1962]

Thanks @LucianoPAlmeida

[LucianoPAlmeida]

Since it got the approval, let's

[johnno1962]

Terrific, thanks again! Does anybody have any ideas on what the problem with using utils/build-toolchain on an M1 might be? It doesn't seem to have been adapted for bootstrapping even if you turn it off. Pointers welcome as I may take a look at it myself though these days I just download one of the nightlies and symbolically link build products into it to work with Xcode.

[johnno1962]

I made some progress getting a toolchain out on an M1 by using "Rosetta2" in the end. #56426 (comment)