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

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Message-ID: <CAFHTzf+O7sdeETp5nsnoQDFeg-zE4q6Kmq9rHPZEr5z2jm2zxg@mail.gmail.com>
Date:	Sun, 14 Jul 2013 19:30:00 +0000
From:	Tim Northover <t.p.northover@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ramkumar Ramachandra <artagnon@...il.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andi Kleen <ak@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	LLVMdev <llvmdev@...uiuc.edu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [LLVMdev] [PATCH] x86/asm: avoid mnemonics without type suffix

> And that is why I think you should just consider "bt $x,y" to be
> trivially the same thing and not at all ambiguous. Because there is
> ABSOLUTELY ZERO ambiguity when people write
>
>    bt $63, mem
>
> Zero. Nada. None. The semantics are *exactly* the same for btl and btq
> in this case, so why would you want the user to specify one or the
> other?

I don't think you've actually tested that, have you? (x86-64)

int main() {
  long val = 0xffffffff;
  char res;

  asm("btl $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
  printf("%d\n", res);

  asm("btq $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
  printf("%d\n", res);
}

Tim.
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