On Fri, 14 Oct 2016, James Greenhalgh wrote:
> +/* If the join of the implicit precision in which the target will compute
> + floating-point values and the standard precision in which the target will
> + compute values is not equal to the standard precision, then the target
> + is either unpredictable, or is a broken configuration in which it claims
> + standards compliance, but doesn't honor that.
> +
> + Effective predictability for __GCC_IEC_559 in flag_iso_mode, means that
> + the implicit precision is not wider, or less predictable than the
> + standard precision.
> +
> + Return TRUE if we have been asked to compile with
> + -fexcess-precision=standard, and following the rules above we are able
> + to guarantee the standards mode. */
> +
> +static bool
> +c_cpp_flt_eval_method_iec_559 (void)
> +{
> + enum flt_eval_method implicit
> + = targetm.c.excess_precision (EXCESS_PRECISION_TYPE_IMPLICIT);
> + enum flt_eval_method standard
> + = targetm.c.excess_precision (EXCESS_PRECISION_TYPE_STANDARD);
> +
> + return (excess_precision_mode_join (implicit, standard) == standard
> + && flag_excess_precision_cmdline == EXCESS_PRECISION_STANDARD);
> +}
> +
> /* Return the value for __GCC_IEC_559. */
> static int
> cpp_iec_559_value (void)
> @@ -775,11 +801,12 @@ cpp_iec_559_value (void)
> applies to unpredictable contraction. For C++, and outside
> strict conformance mode, do not consider these options to mean
> lack of IEEE 754 support. */
> +
> if (flag_iso
> && !c_dialect_cxx ()
> - && TARGET_FLT_EVAL_METHOD != 0
> - && flag_excess_precision_cmdline != EXCESS_PRECISION_STANDARD)
> + && !c_cpp_flt_eval_method_iec_559 ())
> ret = 0;
I'm not convinced by the logic you have here. At least, it seems
different from what we have at present, where -std=c11
-fexcess-precision=fast is not considered unpredictable if the target
doesn't have any implicit excess precision.
That is: I think the right question is whether the combination (front-end
excess precision, implicit back-end excess precision) does the same thing
as just front-end excess precision, regardless of the -fexcess-precision=
option.
--
Joseph S. Myers
jos...@codesourcery.com
