llvm dev - Nov 2020 - Building an LLVM cross-compiler

You’ve hit on one of the odd historical messes in LLVM. Compiler-RT can’t really
be treated as a monolithic set of libraries when you are bootstrapping. In order
to even configure the LLVM sanitizer runtimes you need libc and many other OS
interfaces, so when bootstrapping you need to build the compiler-rt builtins
library separately from the sanitizers.

This can be done either by using the `LLVM_ENABLE_RUNTIMES` CMake option (see
DistributionExample-stage2.cmake in clang/cmake/caches), or by treating
compiler-rt/lib/builtins as the top-level CMake file.

I would strongly suggest that you start with one of the CMake cache files under
clang/cmake/caches as an example because setting up the CMake options to build a
toolchain that can cross-compile is sadly quite complicated.

-Chris
> On Nov 6, 2020, at 4:13 PM, Cág via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi everyone,
> 
> I recently sent a message that may or may not have made it to the list,
> on problems building an LLVM cross-compiler.  Now the issue seems clear
> to me and is irrelevant at this point, I would like to discuss the general
> issue.
> 
> Suppose you have a host system, a Linux distribution on an x86_64 machine,
> and want to build a cross-compiler for aarch64 (or any platform supported
> by LLVM, other than an AMD-/Intel-based machine) to cross compile a Linux
> distribution.
> 
> The process, in my opinion, should go like this:
> 1. Get the sources (llvm, lld, compiler-rt, libunwind, libcxx...).
> 2. Build an LLVM cross-compiler toolchain using native distribution's
> compiler (i.e. build an x86_64 clang executable that targets aarch64).
> 3. Cross-compile libc and other libraries/dependencies to run the
> userland.
> 4. Cross-compile the userland.
> 
> With LLVM it doesn't work:
> 1. You got the sources.
> 2. Built clang targetting, among others or only, aarch64/mips/etc.
> 3. Clang requires compiler-rt but you need to cross-compile compiler-rt
> for the target platform. *You don't have a cross-compiler*.
> 
> It stops after that.  To build compiler-rt you need C headers, libc
> runtimes, *and libclang_rt.a*.  You can't cross-compile libc because
you
> don't have compiler-rt because you don't have libc.  Chicken or
egg.
> 
> I.e. you need a cross-compiler to build a cross-compiler.
> 
> So, how do you build an LLVM-based cross-compiler?
> 
> I hope I am wrong in my assumptions and if it's so, please forgive me.
> I also hope I made it clear.
> 
> Thanks for taking time and have a nice weekend!
> 
> -- 
> caóc
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Hello Chris,

Chris Bieneman wrote:
> You’ve hit on one of the odd historical messes in LLVM. Compiler-RT
> can’t really be treated as a monolithic set of libraries when you are
> bootstrapping. In order to even configure the LLVM sanitizer runtimes
> you need libc and many other OS interfaces, so when bootstrapping you
> need to build the compiler-rt builtins library separately from the
> sanitizers.
> This can be done either by using the `LLVM_ENABLE_RUNTIMES` CMake option
> (see DistributionExample-stage2.cmake in clang/cmake/caches), or by
> treating compiler-rt/lib/builtins as the top-level CMake file.
> 
> I would strongly suggest that you start with one of the CMake cache
> files under clang/cmake/caches as an example because setting up the
> CMake options to build a toolchain that can cross-compile is sadly
> quite complicated.
Thanks for the input.  I will now be trying this cache file to see.  At
the moment I am simply experimenting with different CMake options to see
if I can get it right.  LLVM_ENABLE_RUNTIMES causes the freshly-built
Clang to be used as the compiler for the runtimes set, here it runs into
including host system headers (no libc exists yet).
Only BUILTINS are set to ON, no sanitizers or xrays.

It looks like a quite common request/question.  Has a fix been proposed or
a reviews.llvm.org page exists?  I'd follow it.  Hacking CMake is fun
until you want to get the job done.

Cheers

-- 
caóc