On Tue, Feb 18, 2014 at 6:56 PM, Saleem Abdulrasool <compn...@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 >