Messages in this thread | | | Date | Mon, 07 Dec 2009 10:35:56 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/1] X86: use explicit register name for get/put_user |
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On 12/07/2009 04:37 AM, Jiri Slaby wrote: > Is this documented somewhere? Or do we rely on an undocumented feature? > I mean it doesn't refer only to the constraint but also to a concrete > register allocation. As far as I understand it (from the gcc 4.4 > documentation), if one does > "insn %0" : "=r" (out) : "0" (in) > the "0" constraint corresponds to the concrete register allocated for > out, not to any register (which is the constraint "r").
Yes, but it only corresponds to the information that is conveyed in the register selection.
> In the document they write only about the "same location" occupied by in > and out, nothing is said about size (and hence I think we cannot > mismatch size of operands). And I couldn't find any other > restrictions/documentation about inline assembly, hence the patch, > because nothing assured me this cannot change in the future.
There is almost no documentation at all; some of the little documentation there is is in comments in the source code. To a first order of approximation, asm() is defined by behavior, not by a written spec. Trying to play language lawyer with the little bit that is written down is pointless -- the gcc people have been more than happy to break asm() between releases regardless of what is and is not written down.
> Now I tried different compilers (clang, llvm-gcc) and they choke on that: > $ cat c.c > void x(void) > { > unsigned long in; > int out; > asm("insn %0" : "=r" (out) : "0" (in)); > } > $ clang c.c -S -o - > c.c:5:36: error: unsupported inline asm: input with type 'unsigned long' > matching output with type 'int' > asm("insn %0" : "=r" (out) : "0" (in)); > ~~~ ^~ > 1 diagnostic generated. > $ llvm-gcc c.c -S -o - > c.c: In function 'x': > c.c:5: error: unsupported inline asm: input constraint with a matching > output constraint of incompatible type! > > thanks for the review,
gcc is the standard for gcc-style asm()... if they don't comply, that a bug...
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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