Re: [PATCH 03/19] target/ppc: Zero second doubleword in DFP instructions

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 9/1/22 10:17, Víctor Colombo wrote:
> Starting at PowerISA v3.1, the second doubleword of the registers
> used to store results in DFP instructions are supposed to be zeroed.
> 
>  From the ISA, chapter 7.2.1.1 Floating-Point Registers:
> """
> Chapter 4. Floating-Point Facility provides 32 64-bit
> FPRs. Chapter 5. Decimal Floating-Point also employs
> FPRs in decimal floating-point (DFP) operations. When
> VSX is implemented, the 32 FPRs are mapped to
> doubleword 0 of VSRs 0-31. (...)
> All instructions that operate on an FPR are redefined
> to operate on doubleword element 0 of the
> corresponding VSR. (...)
> and the contents of doubleword element 1 of the
> VSR corresponding to the target FPR or FPR pair for these
> instructions are set to 0.
> """
> 
> Before, the result stored at doubleword 1 was said to be undefined.
> 
> With that, this patch changes the DFP facility to zero doubleword 1
> when using set_dfp64 and set_dfp128. This fixes the behavior for ISA
> 3.1 while keeping the behavior correct for previous ones.
> 
> Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
> ---

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

>   target/ppc/dfp_helper.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/target/ppc/dfp_helper.c b/target/ppc/dfp_helper.c
> index 5ba74b2124..be7aa5357a 100644
> --- a/target/ppc/dfp_helper.c
> +++ b/target/ppc/dfp_helper.c
> @@ -42,13 +42,16 @@ static void get_dfp128(ppc_vsr_t *dst, ppc_fprp_t *dfp)
>   
>   static void set_dfp64(ppc_fprp_t *dfp, ppc_vsr_t *src)
>   {
> -    dfp->VsrD(0) = src->VsrD(1);
> +    dfp[0].VsrD(0) = src->VsrD(1);
> +    dfp[0].VsrD(1) = 0ULL;
>   }
>   
>   static void set_dfp128(ppc_fprp_t *dfp, ppc_vsr_t *src)
>   {
>       dfp[0].VsrD(0) = src->VsrD(0);
>       dfp[1].VsrD(0) = src->VsrD(1);
> +    dfp[0].VsrD(1) = 0ULL;
> +    dfp[1].VsrD(1) = 0ULL;
>   }
>   
>   static void set_dfp128_to_avr(ppc_avr_t *dst, ppc_vsr_t *src)