From 28f13bccbe6a100f53519d1f32cbb78e407e2b83 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Sat, 1 Feb 2025 16:39:08 +0000 Subject: fpu: allow flushing of output denormals to be after rounding 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 Reviewed-by: Richard Henderson --- include/fpu/softfloat-types.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'include/fpu/softfloat-types.h') diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h index bba1c397bb..53d5eb8521 100644 --- a/include/fpu/softfloat-types.h +++ b/include/fpu/softfloat-types.h @@ -304,6 +304,22 @@ typedef enum __attribute__((__packed__)) { float_infzeronan_suppress_invalid = (1 << 7), } FloatInfZeroNaNRule; +/* + * When flush_to_zero is set, should we detect denormal results to + * be flushed before or after rounding? For most architectures this + * should be set to match the tininess_before_rounding setting, + * but a few architectures, e.g. MIPS MSA, detect FTZ before + * rounding but tininess after rounding. + * + * This enum is arranged so that the default if the target doesn't + * configure it matches the default for tininess_before_rounding + * (i.e. "after rounding"). + */ +typedef enum __attribute__((__packed__)) { + float_ftz_after_rounding = 0, + float_ftz_before_rounding = 1, +} FloatFTZDetection; + /* * Floating Point Status. Individual architectures may maintain * several versions of float_status for different functions. The @@ -321,6 +337,8 @@ typedef struct float_status { bool tininess_before_rounding; /* should denormalised results go to zero and set output_denormal_flushed? */ bool flush_to_zero; + /* do we detect and flush denormal results before or after rounding? */ + FloatFTZDetection ftz_detection; /* should denormalised inputs go to zero and set input_denormal_flushed? */ bool flush_inputs_to_zero; bool default_nan_mode; -- cgit 1.4.1