[build] Don't infer Clang path when crosscompiling

[edymtt]

Addresses rdar://71240426

[edymtt]

@swift-ci build toolchain macOS

[edymtt]

@swift-ci please smoke test

[edymtt]

Follow up to #34437

[edymtt]

@swift-ci please test stdlib with toolchain

[edymtt]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f5afc7e

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f5afc7e

[edymtt]

@swift-ci Please Test Source Compatibility

[edymtt]

@swift-ci please smoke test Linux

[shahmishal]

16:12:18 Failed Tests (2):
16:12:18   Swift(macosx-x86_64) :: stdlib/KeyPathAppending.swift
16:12:18   Swift(macosx-x86_64) :: stdlib/ArrayDiagnostics.swift

Not sure why it's failing on this change

[edymtt]

Those failures seem to be happening on the "main" job as well and makes me think they are unrelated to this change

https://ci.swift.org/job/oss-swift-test-stdlib-with-toolchain/62/

I would need to double check locally just in case

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - f5afc7e

Install command
tar -zxf swift-PR-34664-762-osx.tar.gz --directory ~/

[compnerd]

Hey @edymtt could you share a bit more context on this? I don't understand what this is supposed to do/what it does.

From ~CMakeLists.txt:465

if("${SWIFT_NATIVE_LLVM_TOOLS_PATH}" STREQUAL "")
  set(SWIFT_CROSS_COMPILING FALSE)
else()
  set(SWIFT_CROSS_COMPILING TRUE)
endif()

The NOT SWIFT_CROSS_COMPILING implies that SWIFT_NATIVE_TOOLS_LLVM_PATH would be "", which means that this was just setting the variable to empty (so it should be a no-op)?

[edymtt]

To the best of my understanding, when we are building Swift for the host architecture, we expect SWIFT_NATIVE_TOOLS_LLVM_PATH/SWIFT_NATIVE_CLANG_TOOLS_PATH to not be set when CMake is invoked (as per toolchain job log) -- the code in cmake/modules/SwiftSharedCMakeConfig.cmake will then leverage the values obtained by finding the LLVM package to set the path for the compiler.

When we are cross compiling, those values are set when invoking CMake and we want to consider them -- as such my patch is basically mimicking what we are already doing at https://github.com/apple/swift/pull/34664/files#diff-af70c8d589bfadd011360af736827b9f3e3bd1655f7ceb9f08db8fc3045acaeeR62 for SWIFT_NATIVE_TOOLS_LLVM_PATH, when we sense SWIFT_CROSS_COMPILING to understand if that what's the case and avoid overwriting the value

Let me know if anything is missing in my explanation.

[compnerd]

Ah, if the checks are for the value being set rather than empty, that would explain that. Thanks!

[edymtt]

Closing this in favor of #34628