linux-kernel - Re: [LLVMdev] inline asm semantics: output constraint width smaller than input

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Message-ID: <f73f7ab80901280528m6a4f123axbaff9ec284bebe32@mail.gmail.com>
Date:	Wed, 28 Jan 2009 08:28:33 -0500
From:	Kyle Moffett <kyle@...fetthome.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Duncan Sands <baldrick@...e.fr>, llvmdev@...uiuc.edu,
	Ingo Molnar <mingo@...e.hu>,
	Török Edwin <edwintorok@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [LLVMdev] inline asm semantics: output constraint width smaller 
	than input

On Tue, Jan 27, 2009 at 8:56 PM, H. Peter Anvin <hpa@...or.com> wrote:
> Kyle Moffett wrote:
>> Actually, PPC64 boxes basically don't care... the usable GPRs are all
>> either 32-bit (for PPC32) or 64-bit (for PPC64), the <=32-bit
>> instructions are identical across both, they just
>> truncate/sign-extend/etc based on the lower 32-bits of the register.
>> Also, you would only do a right-shift if you were going all the way
>> out to memory as 64-bit and all the way back in as 32-bit... within a
>> single register it's kept coherent.
>
> Think about a 64-bit integer on ppc32.  It will by necessity kept in two
> registers.  On gcc I believe it will always be a consecutive pair of
> registers (AFAIK that's a hard-coded assumption in gcc, with the result that
> gcc has a nonstandard internal register numbering for x86 since the commonly
> used dx:ax pair is actually registers 2:0 in the hardware numbering.)

Even in the 64-bit-integer on 32-bit-CPU case, you still end up with
the lower 32-bits in a standard integer GPR, and it's trivial to just
ignore the "upper" register.  You also would not need to do any kind
of bit-shift, so long as your inline assembly initializes both GPRs
and puts the halves of the result where they belong.

Cheers,
Kyle Moffett
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