A recent C++ Interop pull broke cross-compiling the stdlib with an alternate sysroot for SwiftGlibc platforms

[finagolfin]

Describe the bug
My Android CI builds each new snapshot tag of the stdlib with the Android NDK, but since #59846 it cannot cross-compile the stdlib:

swift/stdlib/public/Platform/Glibc.swift.gyb:13:19: error: no such module 'SwiftGlibc'
@_exported import SwiftGlibc // Clang module

Steps To Reproduce
Steps to reproduce the behavior:

1. Cross-compile the stdlib with an alternate libc sysroot.

Expected behavior
Previously, the Swift compiler would look in the -sdk and -resource-dir flags passed in for a Swift resource directory and use the glibc.modulemap from there. Now, it only remaps any Include directories the clang Driver is aware of, which works fine if using the system libc sysroot, but not for alternate cross-compilation sysroots.

Environment

• OS: Ubuntu 20.04 x86_64 building for Android API 24 with LTS NDK 23c

Additional context
@3405691582 has also noted that a mistaken check from that pull broke building for non-Linux platforms like BSD.

@egorzhdan, let me know if you have a fix in mind.

[finagolfin]

@colemancda, maybe you can reproduce with your Linux armv7 sysroot, if you build with trunk?

[egorzhdan]

I think it's possible that we're not passing -target-triple to Clang driver when querying the include dirs.

[colemancda]

@buttaface I'll test sometime this week with my Debian 11 (armhf) cross compilation scripts.

[finagolfin]

I think it's possible that we're not passing -target-triple to Clang driver when querying the include dirs.

What difference do you believe that would make?

I just dumped parsedIncludeArgs natively on Android, where your injection pull works fine, and it listed five Include directories: the first was the newly built clang resource directory at llvm-android-aarch64/lib/clang/13.0.0/include and the remaining four were directories in the Termux sysroot at /data/data/com.termux/files/. That's only possible because I build the Swift clang with that Termux default sysroot passed in when clang is built, so the clang Driver finds the Termux sysroot fine.

Obviously, that's not going to work for arbitrary libc sysroots that are passed in when cross-compiling the Swift stdlib with a prebuilt Swift compiler.

[finagolfin]

@colemancda, try it yet?

This also broke the community Android CI. I'll try dumping what paths the pull that broke this is using when cross-compiling from linux to Android.

[3405691582]

I'm also noticing this now I'm starting to run some cross-compiling experiments.

[finagolfin]

It still worked with the June 26 trunk snapshot before #59846 was merged, so I pinned my Android CI to that tag. I'm going to experiment with a fix for this issue in the next couple days, but anyone can feel free to beat me to it.

[finagolfin]

I built the Aug. 6 trunk source snapshot on linux x86_64 last week and dumped parsedIncludeArgs there too and just as I suspected it is only searching the default / sysroot for libc headers, even though the Android stdlib cross-compilation build passes in the NDK sysroot to -sdk.

I just looked and I had not noticed that Egor had attempted to pass such alternative sysroot paths to the clang Driver, but for whatever reason that is obviously not working. I will dump some more of those paths and try to figure out why.

[colemancda]

@buttaface Sorry, I did try but was not able to compile due to other cross compilation issues (specific to my scripts).