Re: ARM inline assembly usage in Linux kernel

[Andrew Pinski <pinskia@gmail.com>]

On Tue, Feb 18, 2014 at 6:56 PM, Saleem Abdulrasool
<compnerd@compnerd.org> wrote:
> Hello.
>
> I am sending this at the behest of Renato.  I have been working on the ARM
> integrated assembler in LLVM and came across an interesting item in the Linux
> kernel.
>
> I am wondering if this is an unstated covenant between the kernel and GCC or
> simply a clever use of an unintended/undefined behaviour.
>
> The Linux kernel uses the *compiler* as a fancy preprocessor to generate a
> specially crafted assembly file.  This file is then post-processed via sed to
> generate a header containing constants which is shared across assembly and C
> sources.
>
> In order to clarify the question, I am selecting a particular example and
> pulling out the relevant bits of the source code below.
>
> #define DEFINE(sym, val) asm volatile("\n->" #sym " %0 " #val : : "i" (val))
>
> #define __NR_PAGEFLAGS 22
>
> void definitions(void) {
>   DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS);
> }
>
> This is then assembled to generate the following:
>
> ->NR_PAGEFLAGS #22 __NR_PAGEFLAGS
>
> This will later be post-processed to generate:
>
> #define NR_PAGELAGS 22 /* __NR_PAGEFLAGS */
>
> By using the inline assembler to evaluate (constant) expressions into constant
> values and then emit that using a special identifier (->) is a fairly clever
> trick.  This leads to my question: is this just use of an unintentional
> "feature" or something that was worked out between the two projects.
>
> Please explicitly CC me on any response as I am not subscribed to this mailing
> list.

I don't see why this is a bad use of the inline-asm.  GCC does not
know and is not supposed to know what the string inside the inline-asm
is going to be.  In fact if you have a newer assembler than the
compiler, you could use instructions that GCC does not even know
about.  This is the purpose of inline-asm.  I think it was a bad
design decision on LLVM/clang's part that it would check the assembly
code up front.

Thanks,
Andrew Pinski


>
> Thanks.
>
> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org
>