llvm dev - Nov 2015 - [RFC] Strategies for Bootstrapping Compiler-RT builtins

> On Nov 2, 2015, at 10:10 AM, Anton Korobeynikov <anton at
korobeynikov.info> wrote:
> 
>> Today building this toolchain with CMake is impossible because you
cannot configure the cross-compiled builtins. The failure is a result of CMake’s
try_compile function always testing a full compile then link operation. When
bootstrapping a cross-compiler this will always fail because linking even the
simplest applications fails when you don’t have libclang_rt prebuilt.
> The situation is much worse, because it requires not only compiler-rt,
> but also all the runtime libraries (libc, libc++, libc++abi) to be
> available.
> 
>> This is the most complicated approach, but I also think it is the best
approach.
> This won't work due to dependencies on libc / libc++ / libc++abi.
That is not entirely correct. Only some of the builtins have dependencies on
libc, and none of them should have dependencies on libc++ or libc++abi. I
believe we can construct a working partial builtin library without libc, to make
bootstrapping work.

-Chris
> 
> I think either (1) should be implemented. Or (4) - make the libraries
> just to be "shimmed" on fly - proving empty libs for the
configure
> time just to pacify cmake.
> 
> -- 
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University

> That is not entirely correct. Only some of the builtins have dependencies
on libc, and none of them should have dependencies on libc++ or libc++abi. I
believe we can construct a working partial builtin library without libc, to make
bootstrapping work.
It's not that the builtins depend on them, but cmake depends on them.
Requiring the full compile & link cycle effectively requires the .a /
.so . /.dylib files to be present.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

> On Nov 2, 2015, at 10:48 AM, Anton Korobeynikov <anton at
korobeynikov.info> wrote:
> 
>> That is not entirely correct. Only some of the builtins have
dependencies on libc, and none of them should have dependencies on libc++ or
libc++abi. I believe we can construct a working partial builtin library without
libc, to make bootstrapping work.
> It's not that the builtins depend on them, but cmake depends on them.
> Requiring the full compile & link cycle effectively requires the .a /
> .so . /.dylib files to be present.
The checks that cause this issue can be bypassed by setting
CMAKE_C_COMPILER_WORKS=On and CMAKE_CXX_COMPILER_WORKS=On. That isn’t ideal, and
I’m currently talking with members of the CMake development community to figure
out a proper solution.

-Chris
> 
> -- 
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University