Re: target/ppc: bug in optimised vsl/vsr implementation?

[Aleksandar Markovic]

26.09.2019. 20.14, "Mark Cave-Ayland" <address@hidden> је написао/ла:
>
> As part of the investigation into the DFP number issue reported at
> https://bugs.launchpad.net/qemu/+bug/1841990 it appears that there may also be a bug
> introduced by the new optimised vsl/vsr implementation:
>
> commit 4e6d0920e7547e6af4bbac5ffe9adfe6ea621822
> Author: Stefan Brankovic <address@hidden>
> Date: Mon Jul 15 16:22:48 2019 +0200
>
>     target/ppc: Optimize emulation of vsl and vsr instructions
>
>    

(sorry in advance if the format of this message looks odd, I have some problems with mail settings related to recent qemu-devel mailer settings changes; I will adjust my mail settings in next few days)

Mark and Paul (and Stefan),

Thanks for spotting this and pinpointing the culprit commit. I guess Stefan is going to respond soon, but, in the meantime, I took a look at the commit in question:

https://github.com/qemu/qemu/commit/4e6d0920e7547e6af4bbac5ffe9adfe6ea621822

I don't have at the moment any dev/test environment handy, but I did manual inspection of the code, and here is what I found (in order of importance, perceived by me):

1. The code will not work correctly if the shift ammount (variable 'sh') is 0. This is because, in that case, one of succeeding invocations of TCG shift functions will be required to shift a 64-bit TCG variable by 64 bits, and the result of such TCG operation is undefined (shift amount must be 63 or less) - see https://github.com/qemu/qemu/blob/master/tcg/README.

2. Variable naming is better in the old helper than in the new translator. In that light, I would advise Stefan to change 'sh' to 'shift', and 'shifted' to 'carry'.

3. Lines

tcg_gen_andi_i64(shifted, shifted, 0x7fULL);

and

tcg_gen_andi_i64(shifted, shifted, 0xfe00000000000000ULL);

appear to be spurious (albait in a harmless way). Therefore, they should be deleted, or, alternatevely, a justification for them should be provided.

4. In the commit message, variable names were used without quotation mark, resulting in puzzling and unclear wording.

5. (a question for Mark) After all recent changes, does get_avr64(..., ..., true) always (for any endian configuration) return the "high" half of an Altivec register, and get_avr64(..., ..., false) the "low" one?

Given all these circumstances, perhaps the most reasonable solution would be to revert the commit in question, and allow Stefan enough dev and test time to hopefully submit a new, better, version later on.

Sincerely,
Aleksandar