linux-kernel - Re: [PATCH] x86/asm: avoid mnemonics without type suffix

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Date:	Sun, 14 Jul 2013 23:56:10 +0530
From:	Ramkumar Ramachandra <artagnon@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eli Friedman <eli.friedman@...il.com>,
	Jim Grosbach <grosbach@...le.com>,
	Stephen Checkoway <s@...tak.org>, LLVMdev <llvmdev@...uiuc.edu>
Subject: Re: [PATCH] x86/asm: avoid mnemonics without type suffix

Linus Torvalds wrote:
>>   btrl  $1, 0
>>   btr   $1, 0
>>   btsl  $1, 0
>>   bts   $1, 0
>
> What the heck is that supposed to show?

I was trying to show a reduced case where gas doesn't complain, but
llvm-mc does.  Try compiling this with llvm-mc, and you'll get:

        .text
        btrl    $1, 0
in.s:2:1: error: ambiguous instructions require an explicit suffix
      (could be 'btrw', 'btrl', or 'btrq')
btr     $1, 0
^
        btsl    $1, 0
in.s:4:1: error: ambiguous instructions require an explicit suffix
      (could be 'btsw', 'btsl', or 'btsq')
bts     $1, 0
^

Obviously, I misunderstood something major and screwed up the commit message.

> int main(int argc, char **argv)
> {
>   asm("bt %1,%0":"=m" (**argv): "a" (argc));
>   asm("bt %1,%0":"=m" (**argv): "a" ((unsigned long)(argc)));
> }

Right, so in:

int main(int argc, char **argv)
{
  asm("bts %1,%0":"=m" (**argv): "r" (argc));
  asm("btsl %1,%0":"=m" (**argv): "r" (argc));
  asm("btr %1,%0":"=m" (**argv): "r" ((unsigned long)(argc)));
  asm("btrq %1,%0":"=m" (**argv): "r" ((unsigned long)(argc)));
}

bts disambiguates to btsl, and btr disambiguates to btrq, as
advertised.  Is it dependent on whether I have a 32-bit machine or
64-bit machine, or just on the operand lengths?

Either way, this is not a very enlightening example, because clang
also compiles this fine, and doesn't complain about any ambiguity.  To
see the ambiguity I'm talking about, try to compile linux.git with
clang; I'll paste one error:

arch/x86/include/asm/bitops.h:129:15: error: ambiguous instructions
      require an explicit suffix (could be 'btrw', 'btrl', or 'btrq')
        asm volatile("btr %1,%0" : ADDR : "Ir" (nr));
                     ^
<inline asm>:1:2: note: instantiated into assembly here
        btr $0,(%rsi)
        ^

Since nr is an int, and ADDR is *(volatile long *), this should
disambiguate to btrl, right?  Any clue why clang is complaining?
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