[swift-dev] Building swift-llvm fails when using build-script to cross compile due to multiple -isysroot arguments being in CFLAGS

[ChrisBieneman]

> On Mar 19, 2016, at 4:30 PM, Vedant Kumar <vsk at apple.com> wrote:
> 
> 
>> On Mar 19, 2016, at 12:05 PM, ChrisBieneman <beanz at apple.com> wrote:
>> 
>> [Adding Vedant directly]
>> 
>> I'm not super familiar with the swift build scripts, but I have a theory of what is going wrong. I think the problem is that compiler-rt is being built during the cross build, and it really shouldn't be.
>> 
>> The compiler-rt build system has some significant flaws, and it is being reworked to resolve them. The problem causing this is that we have a hacked up CMake build process that generates multiple cross-targeted binaries from a single build configuration. I believe that compiler-rt's built-in hacky cross-targeting is tripping over however swift is supporting cross-targeting, and it is falling over.
>> 
>> With the current state of compiler-rt the right way to build it for Darwin is to treat it as host content, and it will build the Darwin-cross libraries too. Then you can pull the cross-libraries out and put them into the device build.
> 
> Could you elaborate on what it means to treat it as host content? 

I don't know the exact details of how swift's build scripts work, but I assume it isn't that different from how you generally would cross-compile a compiler and runtimes.

For example, if I wanted to build clang for iOS, the first thing I would do is build a host-capable clang that could target iOS. That clang is then be used to build the cross-targeted clang and corresponding runtimes. I assume swift does something similar to solve version incompatibilities between the host compiler and the target.

Compiler-RT's build system is really screwy for a lot of reasons. On Darwin when you configure compiler-rt it tries to generate a single build tree that can target every Darwin-based platform. The logic around that gets really tripped up if you start trying to treat it as a cross-compile (even though it really is). This is something I'm working to fix.

The correct way to build compiler-rt on Darwin is to build it in-tree as part of the host LLVM. If you're building a full host clang you can use the LLVM_BUILD_EXTERNAL_COMPILER_RT option to enable using the just-built clang. You can set COMPILER_RT_ENABLE_{platform} and it should be fine.

When you build the cross-targeted tools and libraries, you'll want to disable building compiler-rt by setting LLVM_TOOL_COMPILER_RT_BUILD=Off. After the cross-build completes you'll need to pull the clang_rt libraries out of the host build directory into the target compiler's build directory.

Hopefully I'll have this all re-worked in LLVM sometime in the next couple months, then it will all change all over again :-).

-Chris

> 
> Is setting COMPILER_RT_HOST_TRIPLE and disabling COMPILER_RT_ENABLE_{I,TV,WATCH}OS the right thing to do?
> 
> 
>> Vedant, if you have questions feel free to swing by my office on Monday.
> 
> Will do.
> 
> 
> thanks,
> vedant
> 
>> -Chris
>> 
>>> On Mar 19, 2016, at 10:11 AM, Andrew Hyatt via swift-dev <swift-dev at swift.org> wrote:
>>> 
>>> (From /u/thegreatbeanz on reddit.com/r/swift)
>>> 
>>> So the problem here is actually that something has gone horribly wrong when configuring compiler- rt.
>>> You'll notice in the clang commands that it is building in this directory:
>>> clang_rt.builtins_armv7_10.4.dir
>>> What that means is it is building the clang_rt.10.4 archive which provides back ported library functions to OS X 10.4. Obviously 10.4 didn't support armv7 as an architecture, something has gone really wrong here.
>>> If you're on the latest swift it is probably a regression caused by 53f48f8. You need to email swift-dev to get support from Dmitri Gribenko and Vedant Kumar.
>>> Source: I wrote most of the compiler-rt builtin build system for Darwin.
>>> 
>>> Andrew Hyatt
>>> ahyattdev at icloud.com
>>> 
>>>> On Mar 19, 2016, at 9:26 AM, Andrew Hyatt via swift-dev <swift-dev at swift.org> wrote:
>>>> 
>>>> Building swift-llvm fails when using build-script to cross compile because build-script wrongly gives duplicate -isysroot arguments.
>>>> 
>>>> The full arguments that are given to clang by build-script while building LLVM are here: https://gist.github.com/ahyattdev/8ecc5d3808b9ee543a60
>>>> 
>>>> The command I use to build the compiler, using the latest sources from GitHub
>>>> 
>>>> utils/build-script -R -i -- --cross-compile-tools-deployment-targets "iphoneos-arm64 iphoneos-armv7 iphoneos-armv7s" --user-config-args="-DLLVM_ENABLE_BACKTRACES=Off"
>>>> 
>>>> This error occurs after the phase of building llvm for the host machine, and occurs during the cross compilation phase.
>>>> 
>>>> Andrew Hyatt
>>>> andythehyatt at gmail.com
>>>> 
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