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

Jim Grosbach grosbach at apple.com
Wed Mar 13 10:57:20 PDT 2013

On Mar 13, 2013, at 10:15 AM, Weiming Zhao <weimingz at codeaurora.org> wrote:

> Hi Renato,
> 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.

> So it a constraint is a 64-integer type, it *probably* needs paired GPR.

This is not. Consider the Thumb2 encodings of LDRD, LDREXD, etc,, for example. They allow arbitrary Rt and Rt2, not just a register pair. It's only in ARM mode that the instructions are more constrained.


> Weiming
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
> From: Renato Golin [mailto:renato.golin at linaro.org] 
> Sent: Wednesday, March 13, 2013 10:03 AM
> To: Måns Rullgård
> Cc: LLVM Dev; weimingz at codeaurora.org
> Subject: Re: Problems with 64-bit register operands of inline asm on ARM
> On 13 March 2013 13:43, Måns Rullgård <mans at mansr.com> wrote:
> One possible fix, which I have tested, is to look for the specific
> instructions requiring such a pair (LDRD/STRD and LDREXD/STREXD) in
> addition to the 'H' modifier.  However, there are probably other
> creative ways in which inline asm might rely on the specific pairing.
> Hi Mans,
> Either that method is ignoring an inline asm parser or there isn't one, but I agree, we should be able to have something better than just grep for possible extensions for paired registers.
> Thus I believe the safest solution is to always force 64-bit operands
> into even/odd pairs for any inline asm.  In other words, we should
> probably do something like this (untested):
> I tested this, and it fails on other inline assembly tests. I think that the non-paired asm is correctly selected by the table generated parser, but when you pair things that didn't need pairing, the parser goes bad.
> I don't think we should force pairing on every inline assembly either. Maybe someone knows how to parse inline assembly in a better way than it is currently being done? If there isn't any, than possibly creating a function to return "needsPairedRegister()" or something would still be ugly, but better than incrementing it inline.
> cheers,
> --renato
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