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Re: [PATCH, PR59833]: Fix sNaN handling in ARM float to double conversion


On Wed, Jul 20, 2016 at 8:48 AM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> On ARM soft-float, the float to double conversion doesn't convert a sNaN
> to qNaN as the IEEE Std 754 standard mandates:
>
> "Under default exception handling, any operation signaling an invalid
> operation exception and for which a floating-point result is to be
> delivered shall deliver a quiet NaN."
>
> Given the soft float ARM code ignores exceptions and always provides a
> result, a float to double conversion of a signaling NaN should return a
> quiet NaN. Fix this in extendsfdf2.
>
> gcc/ChangeLog:
>
>         PR target/59833
>         * config/arm/ieee754-df.S (extendsfdf2): Convert sNaN to qNaN.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/pr59833.c: New testcase.


Ok - assuming this was tested appropriately with no regressions.

Thanks for following up.

Ramana


> ---
>  gcc/testsuite/gcc.dg/pr59833.c | 18 ++++++++++++++++++
>  libgcc/config/arm/ieee754-df.S | 10 +++++++---
>  2 files changed, 25 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/pr59833.c
>
> diff --git a/gcc/testsuite/gcc.dg/pr59833.c b/gcc/testsuite/gcc.dg/pr59833.c
> new file mode 100644
> index 0000000..e0e4ed5
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr59833.c
> @@ -0,0 +1,18 @@
> +/* { dg-do run { target { *-*-linux* *-*-gnu* } } } */
> +/* { dg-options "-O0 -lm" } */
> +/* { dg-require-effective-target issignaling } */
> +
> +#define _GNU_SOURCE
> +#include <math.h>
> +
> +int main (void)
> +{
> +  float sNaN = __builtin_nansf ("");
> +  double x = (double) sNaN;
> +  if (issignaling(x))
> +  {
> +    __builtin_abort();
> +  }
> +
> +  return 0;
> +}
> diff --git a/libgcc/config/arm/ieee754-df.S b/libgcc/config/arm/ieee754-df.S
> index a2aac70..1ecaa9d 100644
> --- a/libgcc/config/arm/ieee754-df.S
> +++ b/libgcc/config/arm/ieee754-df.S
> @@ -507,11 +507,15 @@ ARM_FUNC_ALIAS aeabi_f2d extendsfdf2
>         eorne   xh, xh, #0x38000000     @ fixup exponent otherwise.
>         RETc(ne)                        @ and return it.
>
> -       teq     r2, #0                  @ if actually 0
> -       do_it   ne, e
> -       teqne   r3, #0xff000000         @ or INF or NAN
> +       bics    r2, r2, #0xff000000     @ isolate mantissa
> +       do_it   eq                      @ if 0, that is ZERO or INF,
>         RETc(eq)                        @ we are done already.
>
> +       teq     r3, #0xff000000         @ check for NAN
> +       do_it   eq, t
> +       orreq   xh, xh, #0x00080000     @ change to quiet NAN
> +       RETc(eq)                        @ and return it.
> +
>         @ value was denormalized.  We can normalize it now.
>         do_push {r4, r5, lr}
>         .cfi_adjust_cfa_offset 12   @ CFA is now sp + previousOffset + 12
>
> --
> Aurelien Jarno                          GPG: 4096R/1DDD8C9B
> aurelien@aurel32.net                 http://www.aurel32.net


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