[llvm-dev] TargetTriple issue: LC_VERSION_MIN_MACOSX: Darwin kernel version vs SDK version

[Benoit Belley via llvm-dev]

Hi everyone,

I just reported the following issue: https://llvm.org/bugs/show_bug.cgi?id=24927


Using the Xcode 7 linker, one gets messages such as the following when linking objects generated using llvm:

ld: warning: object file (foo.o) was built for newer OS X version (14.5) than being linked (10.9)

The issue is the following:

a) In lib/Support/Unix/Host.inc, sys::getDefaultTargetTriple() sets the OS name to darwin14.5 when running on MaxOS X 10.10. 14.5 is the Darwin kernel version as reported by uname.

b) In lib/CodeGen/AsmPrinter.cpp, AsmPrinter::doInitialization() writes out this version number to the LC_VERSION_MIN_MACOSX of the generated object.

c) The XCode 7 linker seems to expect that LC_VERSION_MIN_MACOSX contains the minimum version number of the MacOS X SDK (10.9, 10.11, ...), NOT the Darwin kernel version number!

AFAICT, LLVM had this wrong behaviour for a long time. The only new thing with Xcode 7 is that the linker is now reporting a warning for the version mismatch.

As a workaround, we are temporarily stripping out the darwin kernel version number from the triple in our project code. Something like:

// FIXME: LLVM confuses Darwin Kernel version numbers with MacOS X SDK
// version. By stripping out the kernel version number, we prevent the
// LC_VERSION_MIN_MACOSX directive from being emitted. Hopefully, that
// will get fixed by LLVM 3.8...
//
// See LLVM bug 24927: https://llvm.org/bugs/show_bug.cgi?id=24927#if defined(OSX)
#if (LLVM_VERSION_MAJOR < 3) || ((LLVM_VERSION_MAJOR == 3) && (LLVM_VERSION_MINOR < 8 ))
        triple = Triple(triple.getArchName(),
                        triple.getVendorName(),
                        "darwin",
                        triple.getEnvironmentName());
#endif
#endif

This prevents the LC_VERSION_MIN_MACOSX directives from being emitted into the LLVM generated object files.

Any suggestions how to go about fixing LLVM to generate the correct  LC_VERSION_MIN_MACOSX minimum SDK version number ? There seems to be no way to pass this information correctly through the Triple!

Thanks,
Benoit
Benoit Belley
Sr Principal Developer
M&E-Product Development Group

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