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