Re: [PATCH] target/ppc: Change VSX instructions behavior to fill with zeros

[Richard Henderson <richard.henderson@linaro.org>]

On 2/5/22 05:19, Víctor Colombo wrote:
> ISA v3.1 changed some VSX instructions behavior by changing what the
> other words/doubleword in the result should contain when the result is
> only one word/doubleword. e.g. xsmaxdp operates on doubleword 0 and
> saves the result also in doubleword 0.
> Before, the second doubleword result was undefined according to the
> ISA, but now it's stated that it should be zeroed.
> 
> Even tough the result was undefined before, hardware implementing these
> instructions already filled these fields with 0s. Changing every ISA
> version in QEMU to this behavior makes the results match what happens
> in hardware.
> 
> Signed-off-by: Víctor Colombo<victor.colombo@eldorado.org.br>
> 
> ---
> 
> This patch is a proposal on a previous RFC I sent on this topic [1].
> I preferred this approach because it makes QEMU behavior closer to the
> real hardware.
> 
> The affected instructions have the following note in the ISA:
> 
> """
> Previous versions of the architecture allowed the
> contents of doubleword 1 of the result register to be
> undefined. However, all processors that support
> this instruction write 0s into doubleword 1 of the
> result register, as is required by this version of the
> architecture.
> """
> 
> This patch is not exhaustive as
> 1. some instructions that had its behavior changed are not 'corrected'
>      by this patch (mostly multiply-add instructions);
> 2. some instructions changed the behavior to also replicate the result
>      in the other word from the doubleword if the result is a single
>      word e.g. xscvdpuxws. So, this patch only focus on the 'zeroing'
>      part, not the replication;
> 
> Best regards,
> 
> -- Víctor
> 
> [1]https://lists.gnu.org/archive/html/qemu-ppc/2021-12/msg00198.html
> ---
>   target/ppc/fpu_helper.c             | 26 +++++++++++++-------------
>   target/ppc/translate/vsx-impl.c.inc |  4 +++-
>   2 files changed, 16 insertions(+), 14 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~