Trying to port GCC specific asm goto to Clang

Question

I've been trying to turn a bit of GNU extensions in to actual standard C so it'll run on clang, knowing standard C and not GNU extensions, I'm at a bit of a loss.

    __asm__ (goto("1:"
            STATIC_KEY_INITIAL_NOP
            ".pushsection __jump_table,  \"aw\" \n\t"
            _ASM_ALIGN "\n\t"
            _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
            ".popsection \n\t"
            : :  "i" (key) : : l_yes););

I've tried to turn this in to actual asm, but have yet to be successful.

If you're curious, this is part of a kernel I've just about got to build on clang, besides that one section.

Answer 1

You seem to be having a problem compiling arch/x86/include/asm/jump_label.h. The entire code-snippet is to enable support for "jump label patching". A new feature quite useful to allow debugging(print logs etc.) with have near-zero overhead when debugging is disabled.

The implementation you encounter depends on gcc(v4.5) which adds a new asm goto statement that allows branching to a label.

It appears that Clang/LLVM < 9.0.0 does NOT support asm goto.

As a quick fix to get your Linux kernel compiling properly, you can disable CONFIG_JUMP_LABEL in your kernel configuration. This config option is used to disable the optimisation when the compiler does NOT support asm goto properly.

---

Update: Initial support for asm goto was added to Clang in v9.0.0.

Initial support for asm goto statements (a GNU C extension) has been added for control flow from inline assembly to labels. The main consumers of this construct are the Linux kernel (CONFIG_JUMP_LABEL=y) and glib. There are still a few unsupported corner cases in Clang's integrated assembler and IfConverter. Please file bugs for any issues you run into.