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

Renato Golin renato.golin at linaro.org
Wed Mar 13 10:02:58 PDT 2013

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.

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