[LLVMdev] Problems with 64-bit register operands of inline asm on ARM

[Weiming Zhao]

Currently, there are possible issues in the inlineasm pair patch. I see two unit test fails when I enable paired reg for all ARM mode inline asm.
I'm looking into it.

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-----Original Message-----
From: Måns Rullgård [mailto:mans at mansr.com] 
Sent: Wednesday, March 13, 2013 1:04 PM
To: Jim Grosbach
Cc: Måns Rullgård; Renato Golin; Zhao; LLVM Dev
Subject: Re: [LLVMdev] Problems with 64-bit register operands of inline asm on ARM

Jim Grosbach <grosbach at apple.com> writes:

> On Mar 13, 2013, at 11:21 AM, Måns Rullgård <mans at mansr.com> wrote:
>
>> Jim Grosbach <grosbach at apple.com> writes:
>> 
>>> On Mar 13, 2013, at 11:01 AM, Renato Golin <renato.golin at linaro.org>
>>> wrote:
>>> 
>>>> On 13 March 2013 17:57, Jim Grosbach <grosbach at apple.com> wrote:
>>>>> It seems to me that LLVM doesn’t parse the inline asm body. It 
>>>>> just checks the constraints, (ie. Input/output interface). During 
>>>>> ASM writing, it then binding those constraints to placeholders 
>>>>> like %0, %1.
>>>> This is correct.
>>>> 
>>>> Ok, so maybe checking all possible ways to require paired registers 
>>>> is not such a bad idea after all.
>>>> 
>>> 
>>> The constraints are the right way to do it. There shouldn't be any 
>>> magic beyond that.
>> 
>> Since there is no special operand constraint for a register pair, 
>> there is no way to tell at that level.
>> 
>> GCC has (implicitly) defined 64-bit register operands as residing in 
>> even/odd pairs, thus leaving inline asm free to make all manner of 
>> assumptions based on this.  The only way I see to guarantee 
>> compatibility is to mimic the gcc behaviour here.  It may be slightly 
>> suboptimal in a few cases, but it's the safe choice.
>
> Sure, that's fine for ARM mode. No realistic other option there. So 
> long as Thumb2 code can get the more expressive syntax for the more 
> relaxed regalloc availability, it's all good. This basically falls 
> into "using the constraints to figure it out."

So let's at least make this pair allocation unconditional in ARM mode.
A lot of existing inline asm doesn't work in Thumb mode anyway, so if that fails, it's less of an issue.

--
Måns Rullgård
mans at mansr.com