Re: [PATCH 2/8] target/sparc: Fix VIS fmul8x16au instruction.

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

On 9/24/23 01:03, Nick Bowler wrote:
> On a real UltraSparc II, the fmul8x16au instruction takes two single-
> precision input operands and returns a double-precision result.  For
> the second operand, bits 31:16 are used, and bits 15:0 are ignored.
> 
> However, the emulation is taking two double-precision input operands,
> and furthermore it is using bits 15:0 of the second operand (ignoring
> bits 31:16).  These are unlikely to contain the correct values.
> 
> Even still, the emulator overwrites the second input before all outputs
> are calculated, so even if by chance the data loaded in happens to be
> correct, the results are just garbage except in trivial cases.
> 
> Signed-off-by: Nick Bowler <nbowler@draconx.ca>
> ---
>   target/sparc/helper.h     |  2 +-
>   target/sparc/translate.c  | 19 ++++++++++++++++++-
>   target/sparc/vis_helper.c | 14 +++++++++-----
>   3 files changed, 28 insertions(+), 7 deletions(-)

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

>   #define PMUL(r)                                                 \
> -    tmp = (int32_t)d.VIS_SW64(0) * (int32_t)s.VIS_B64(r);       \
> +    tmp = (int32_t)s2.VIS_SW32(1) * (int32_t)s1.VIS_B64(r);     \
>       if ((tmp & 0xff) > 0x7f) {                                  \
>           tmp += 0x100;                                           \
>       }                                                           \

Belated follow-up suggestion:

-   if ((tmp & 0xff) > 0x7f) {
-       tmp += 0x100;
-   }
+   tmp += 0x80;

7 occurrences throughout vis_helper.c.


r~