El Fri, May 05, 2017 at 07:50:39PM +0200 Ingo Molnar ha dit:
>
> * Peter Zijlstra <[email protected]> wrote:
>
> > On Fri, May 05, 2017 at 01:11:47AM -0700, tip-bot for Matthias Kaehlcke
> > wrote:
> > > Commit-ID: 121843eb02a6e2fa30aefab64bfe183c97230c75
> > > Gitweb:
> > > http://git.kernel.org/tip/121843eb02a6e2fa30aefab64bfe183c97230c75
> > > Author: Matthias Kaehlcke <[email protected]>
> > > AuthorDate: Mon, 1 May 2017 15:47:41 -0700
> > > Committer: Ingo Molnar <[email protected]>
> > > CommitDate: Fri, 5 May 2017 08:31:05 +0200
> > >
> > > x86/mm/kaslr: Use the _ASM_MUL macro for multiplication to work around
> > > Clang incompatibility
> > >
> > > The constraint "rm" allows the compiler to put mix_const into memory.
> > > When the input operand is a memory location then MUL needs an operand
> > > size suffix, since Clang can't infer the multiplication width from the
> > > operand.
> >
> > *sigh*, this is another shining example of how LLVM is a better, faster
> > moving compiler?
>
> Well, I don't like it - but we already have similar patterns to cover some
> asm
> complications so I didn't mind. Apparently Clang is very close to being able
> to
> build a working Linux kernel, right?
Indeed, I expect 4.12 (with this patch ...) to build with Clang for a
x86 defconfig (with tons of warnings). ARM64 is very close.
> In that sense it would be unfair to expect it to not have various legacies,
> missing features and quirks - just like the kernel has dozens of GCC related
> workarounds.
Also my understanding is that this isn't really a clang issue. In the
context of this code gcc apparently chooses to use a register for
'mix_const', for memory locations it also needs a suffix.
Actually I just tried to build this code from a single C file:
void test() {
unsigned long raw, random;
const unsigned long mix_const = 0x3f39e593UL;
asm("MUL %3"
: "=a" (random), "=d" (raw)
: "a" (random), "rm" (mix_const));
}
gcc -c /tmp/test.c
/tmp/test.c: Assembler messages:
/tmp/test.c:6: Error: no instruction mnemonic suffix given and no
register operands; can't size instruction
gcc version 4.9.x 20150123
Cheers
Matthias
