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Re: [Qemu-devel] [PATCH v2] ppc: Fix emulated single to double denormali
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH v2] ppc: Fix emulated single to double denormalized conversions |
Date: |
Tue, 20 Aug 2019 11:00:48 +1000 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
On Mon, Aug 19, 2019 at 04:42:16PM -0500, Paul A. Clarke wrote:
> From: "Paul A. Clarke" <address@hidden>
>
> helper_todouble() was not properly converting any denormalized 32 bit
> float to 64 bit double.
>
> Fix-suggested-by: Richard Henderson <address@hidden>
> Signed-off-by: Paul A. Clarke <address@hidden>
>
> 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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