Re: [Qemu-devel] [PATCH v2] ppc: Fix emulated single to double denormalized conversions

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Aug 19, 2019 at 04:42:16PM -0500, Paul A. Clarke wrote:
> From: "Paul A. Clarke" <pc@us.ibm.com>
> 
> helper_todouble() was not properly converting any denormalized 32 bit
> float to 64 bit double.
> 
> Fix-suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Paul A. Clarke <pc@us.ibm.com>
> 
> v2:
> - Splitting patch "ppc: Three floating point fixes"; this is just one part.
> - Original suggested "fix" was likely flawed.  v2 is rewritten by
>   Richard Henderson (Thanks, Richard!); I reformatted the comments in a
>   couple of places, compiled, and tested.

Applied to ppc-for-4.2, thanks.

> ---
>  target/ppc/fpu_helper.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
> index 52bcda2..07bc905 100644
> --- a/target/ppc/fpu_helper.c
> +++ b/target/ppc/fpu_helper.c
> @@ -73,11 +73,20 @@ uint64_t helper_todouble(uint32_t arg)
>          /* Zero or Denormalized operand.  */
>          ret = (uint64_t)extract32(arg, 31, 1) << 63;
>          if (unlikely(abs_arg != 0)) {
> -            /* Denormalized operand.  */
> -            int shift = clz32(abs_arg) - 9;
> -            int exp = -126 - shift + 1023;
> +            /*
> +             * Denormalized operand.
> +             * Shift fraction so that the msb is in the implicit bit position.
> +             * Thus, shift is in the range [1:23].
> +             */
> +            int shift = clz32(abs_arg) - 8;
> +            /*
> +             * The first 3 terms compute the float64 exponent.  We then bias
> +             * this result by -1 so that we can swallow the implicit bit below.
> +             */
> +            int exp = -126 - shift + 1023 - 1;
> +
>              ret |= (uint64_t)exp << 52;
> -            ret |= abs_arg << (shift + 29);
> +            ret += (uint64_t)abs_arg << (52 - 23 + shift);
>          }
>      }
>      return ret;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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