Re: [PATCH v2 05/69] fpu: allow flushing of output denormals to be after rounding

[Richard Henderson <richard.henderson@linaro.org>]

On 2/1/25 08:39, Peter Maydell wrote:
> Currently we handle flushing of output denormals in uncanon_normal
> always before we deal with rounding.  This works for architectures
> that detect tininess before rounding, but is usually not the right
> place when the architecture detects tininess after rounding.  For
> example, for x86 the SDM states that the MXCSR FTZ control bit causes
> outputs to be flushed to zero "when it detects a floating-point
> underflow condition".  This means that we mustn't flush to zero if
> the input is such that after rounding it is no longer tiny.
> 
> At least one of our guest architectures does underflow detection
> after rounding but flushing of denormals before rounding (MIPS MSA);
> this means we need to have a config knob for this that is separate
> from our existing tininess_before_rounding setting.
> 
> Add an ftz_detection flag.  For consistency with
> tininess_before_rounding, we make it default to "detect ftz after
> rounding"; this means that we need to explicitly set the flag to
> "detect ftz before rounding" on every existing architecture that sets
> flush_to_zero, so that this commit has no behaviour change.
> (This means more code change here but for the long term a less
> confusing API.)
> 
> For several architectures the current behaviour is either
> definitely or possibly wrong; annotate those with TODO comments.
> These architectures are definitely wrong (and should detect
> ftz after rounding):
>   * x86
>   * Alpha
> 
> For these architectures the spec is unclear:
>   * MIPS (for non-MSA)
>   * RX
>   * SH4
> 
> PA-RISC makes ftz detection IMPDEF, but we aren't setting the
> "tininess before rounding" setting that we ought to.
> 
> Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> ---
>   include/fpu/softfloat-helpers.h | 11 +++++++++++
>   include/fpu/softfloat-types.h   | 18 ++++++++++++++++++
>   target/mips/fpu_helper.h        |  6 ++++++
>   target/alpha/cpu.c              |  7 +++++++
>   target/arm/cpu.c                |  1 +
>   target/hppa/fpu_helper.c        | 11 +++++++++++
>   target/i386/tcg/fpu_helper.c    |  8 ++++++++
>   target/mips/msa.c               |  9 +++++++++
>   target/ppc/cpu_init.c           |  3 +++
>   target/rx/cpu.c                 |  8 ++++++++
>   target/sh4/cpu.c                |  8 ++++++++
>   target/tricore/helper.c         |  1 +
>   tests/fp/fp-bench.c             |  1 +
>   fpu/softfloat-parts.c.inc       | 21 +++++++++++++++------
>   14 files changed, 107 insertions(+), 6 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~