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

Måns Rullgård mans at mansr.com
Wed Mar 13 13:04:12 PDT 2013

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

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