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

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Message-ID: <497A2BC7.5030107@gmail.com>
Date:	Fri, 23 Jan 2009 22:42:47 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	LLVM Developers Mailing List <llvmdev@...uiuc.edu>
Subject: Re: inline asm semantics: output constraint width smaller than input

On 2009-01-23 20:52, Török Edwin wrote:
> On 2009-01-23 20:30, Ingo Molnar wrote:
>   
>> * Török Edwin <edwintorok@...il.com> wrote:
>>
>>   
>>     
>>> Having said that, llvm-gcc is not yet able to compile the full Linux 
>>> kernel on its own [for example the boot code, due to asm(".code16gcc")], 
>>> but with LLVM 2.4 it was possible to build "arch=UM", and "arch=X86" (by 
>>> using gcc to build the bootcode). I'd like LLVM 2.5 to be able to build 
>>> the kernel, so I'll file bugs for llvm/kernel depending on where the 
>>> problem is.
>>>     
>>>       
>> Could we get LLVM folks on the Cc: and see how difficult it would be to 
>> fix this on the LLVM side? Asm constraints are used all around the place 
>> and different input/output types are very common.
>>     
>
>   

Hi Ingo,

Could you describe what are the semantics you need for inline asm
constraints in the kernel?
GCC doesn't document all the corner cases, and defining inline asm =
"whatever gcc accepts" is not very useful for LLVM.

So far we've encountered the problem with input/output operand tied to
same register, but having different widths:
- output wider than input, both integers: do you need this case?
- output narrower than input, both integers: this is the common case, right?
- can it also happen that input is pointer, output is integer of
different width?
- .. any other mismatches?

Could you also describe why put_user/the example from pcbios needs the
different widths?

Best regards,
--Edwin
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