[LLVMdev] Problem with __inline keyword

[Eli Friedman]

On Fri, Jul 6, 2012 at 3:39 PM, Matt Fischer <mattfischer84 at gmail.com> wrote:
> I'm attempting to use clang/llvm to compile some legacy C code, and am
> running into a strange issue.  There are several functions in my
> codebase which make use of the __inline keyword, but do not include
> the customary static keyword along with it.  When I compile these
> files, the generated code appears not to have any of the function
> definitions in it--just undefined references to external symbols.
>
> Specifically, if I have the following test program:
>
> __inline int foo()
> {
>   return 5;
> }
>
> int bar()
> {
>   return foo();
> }
>
> Compiling for ARM with no optimizations yields this:
>
> 00000000 <bar>:
>    0:   e92d4800        push    {fp, lr}
>    4:   e1a0b00d        mov     fp, sp
>    8:   e24dd008        sub     sp, sp, #8
>    c:   ebfffffe        bl      0 <foo>
>   10:   e58d0004        str     r0, [sp, #4]
>   14:   e1a0d00b        mov     sp, fp
>   18:   e8bd4800        pop     {fp, lr}
>   1c:   e12fff1e        bx      lr
>
> As you can see, no inlining took place, so there is still a reference
> to foo().  However, no definition for foo() is actually present.
>
> If I instead define foo as "static __inline", then the definition of
> foo() is retained:
>
> 00000000 <bar>:
>    0:   e92d4800        push    {fp, lr}
>    4:   e1a0b00d        mov     fp, sp
>    8:   e24dd008        sub     sp, sp, #8
>    c:   eb000003        bl      20 <foo>
>   10:   e58d0004        str     r0, [sp, #4]
>   14:   e1a0d00b        mov     sp, fp
>   18:   e8bd4800        pop     {fp, lr}
>   1c:   e12fff1e        bx      lr
>
> 00000020 <foo>:
>   20:   e3a00005        mov     r0, #5
>   24:   e12fff1e        bx      lr
>
> Finally, if I compile with -O2, then in either case, inlining takes
> place and there's no problem:
>
> 00000000 <bar>:
>    0:   e3a00005        mov     r0, #5
>    4:   e12fff1e        bx      lr
>
> I know there are some weird semantics among different compilers with
> the inline/__inline keywords in C when you don't pair them with
> static, but I can't imagine that this is correct.  Is there a bug
> here, or am I missing something?

No bug; those are the rules according to the C99 standard.  You can
force the old GNU inline semantics with "-std=gnu89", if that's what
your code is expecting...

-Eli