On Tue, Aug 20, 2019 at 12:26:04PM -0500, Paul A. Clarke wrote: > From: "Paul A. Clarke" <p...@us.ibm.com> > > helper_xscvdpspn() uses float64_to_float32() to convert double-precision > floating-point to single-precision. Unfortunately, float64_to_float32() > converts SNAN to QNAN, which should not happen with xscvdpspn. > > float64_to_float32() is also used by other instruction implementations > for conversions which _should_ convert SNAN to QNAN. > > Rather than trying to wedge code to preserve SNAN in float64_to_float32() > just for this this one case, I instead embed an embodiment of the > conversion code outlined in the POWER ISA for xscvdpspn. > > Signed-off-by: Paul A. Clarke <p...@us.ibm.com>
Applied to ppc-for-4.2. I used rth's description rather than the one
above, since I found it clearer.
> ---
> target/ppc/fpu_helper.c | 32 ++++++++++++++++++++++++++++++--
> 1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
> index 07bc905..c8e7192 100644
> --- a/target/ppc/fpu_helper.c
> +++ b/target/ppc/fpu_helper.c
> @@ -2887,12 +2887,40 @@ void helper_xscvqpdp(CPUPPCState *env, uint32_t
> opcode,
>
> uint64_t helper_xscvdpspn(CPUPPCState *env, uint64_t xb)
> {
> - uint64_t result;
> + uint64_t result, sign, exp, frac;
>
> float_status tstat = env->fp_status;
> set_float_exception_flags(0, &tstat);
>
> - result = (uint64_t)float64_to_float32(xb, &tstat);
> + sign = extract64(xb, 63, 1);
> + exp = extract64(xb, 52, 11);
> + frac = extract64(xb, 0, 52) | 0x10000000000000ULL;
> +
> + if (unlikely(exp == 0 && extract64(frac, 0, 52) != 0)) {
> + /* DP denormal operand. */
> + /* Exponent override to DP min exp. */
> + exp = 1;
> + /* Implicit bit override to 0. */
> + frac = deposit64(frac, 53, 1, 0);
> + }
> +
> + if (unlikely(exp < 897 && frac != 0)) {
> + /* SP tiny operand. */
> + if (897 - exp > 63) {
> + frac = 0;
> + } else {
> + /* Denormalize until exp = SP min exp. */
> + frac >>= (897 - exp);
> + }
> + /* Exponent override to SP min exp - 1. */
> + exp = 896;
> + }
> +
> + result = sign << 31;
> + result |= extract64(exp, 10, 1) << 30;
> + result |= extract64(exp, 0, 7) << 23;
> + result |= extract64(frac, 29, 23);
> +
> /* hardware replicates result to both words of the doubleword result. */
> return (result << 32) | result;
> }
--
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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