[llvm-dev] Should the compiler-rt builtins be configured with CMAKE_TRY_COMPILE_TARGET_TYPE? (original) (raw)

Martin Storsjö via llvm-dev llvm-dev at lists.llvm.org
Tue Mar 16 03:46:17 PDT 2021

Hi,

On Mon, 15 Mar 2021, Petr Hosek wrote:

I've been thinking about this a bit more and doing some experiments. I'm no longer sure if setting CMAKETRYCOMPILETARGETTYPE to STATICLIBRARY is the best solution.

Yes, it's a bit problematic in some cases too... But for passing the initial CMake compiler sanity check, it's either that, or passing CMAKE_CXX_COMPILER_WORKS.

While it could simplify checks in projects that only ever produce static libraries like builtins or crt, using it more broadly might be a problem because it breaks checks like checkfunctionexists and checklibraryexists.

Yeah it can't be used straight as-is in other projects

The idea I'm considering would be to instead roll out our own checks. The way checks like checkfunctionexists and checklibraryexists work is that CMake generates a little snippet which it then tries to compile. For example when checking if dlopen in libdl exists, CMake would generate:

char dlopen(void); int main(int ac, char* av[]) { dlopen(); } It'd then try to compile this file with -ldl and see if the command succeeds or fails. The problem with this snippet is that it implicitly uses libc and other libraries like crt or builtins, which in the case of the runtimes build, are libraries we're going to build introducing a cycle. The alternative I came up with would be to generate a custom snippet that looks like this: char dlopen(void); void start() { dlopen(); } When invoking the compiler, we would additionally pass -nostdlib. That way, we would still check if libdl is available without any implicit dependencies. The only downside to this approach I can see is that we would need to maintain our own set of checks, but since we could rely on CMake's trycompile to do most of the heavy lifting, I don't think it'd be too much of a maintenance overhead.

That sounds workable to me (at least on first thought, without trying it out), but it'd require a bit of platform specifics, as the entry point function name differs a bit between platforms (on Windows, it's "mainCRTStartup", not "_start").

// Martin

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