[llvm-dev] [SPARC]: leon2 and leon3: not respecting delayed-write to Y-register

[James Y Knight via llvm-dev]

On Wed, Nov 16, 2016 at 5:29 AM, Patrick Boettcher via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> in section B.29. (Write State Register Instructions) of 'The SPARC
> Architecture Manual Version 8' it is said that the "The write state
> register instructions are delayed-write instructions."
>
> The Y-register is a state-register.
>
> Furthermore in the B.29-secion there is a programming note saying:
>
>   MULScc, RDY, SDIV, SDIVcc, UDIV, and UDIVcc implicitly read the Y
>   register. If any of these instructions execute within three
>   instructions after a WRY which changed the contents of the Y
>   register, its results are undefined.
>
> This currently is not respected with LLVM. I'm using 3.9 and checked
> whether any commit on the master-branch handles this case, I haven't
> seen anything - but I might be wrong.
>

Yep, this definitely isn't done. However, it appears that on the hardware
I've looked at, it didn't appear to actually be necessary (as one might
expect from sane hardware...), so I'd not gotten around to fixing it.

Are you looking at this because you have a program which is mis-executing
on some hardware you have, or just because you were looking at the assembly
output and noticed the spec non-compliance?
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