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

diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
index 0122b35008a901ff6664d456d9cba1b30563c1b9..1d09f066c5d83853bd4467ba5027b377df571754 100644 (file)
--- a/fpu/softfloat-parts.c.inc
+++ b/fpu/softfloat-parts.c.inc
@@ -334,7 +334,8 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
             p->frac_lo &= ~round_mask;
         }
         frac_shr(p, frac_shift);
-    } else if (s->flush_to_zero) {
+    } else if (s->flush_to_zero &&
+               s->ftz_detection == float_ftz_before_rounding) {
         flags |= float_flag_output_denormal_flushed;
         p->cls = float_class_zero;
         exp = 0;
@@ -381,11 +382,19 @@ static void partsN(uncanon_normal)(FloatPartsN *p, float_status *s,
         exp = (p->frac_hi & DECOMPOSED_IMPLICIT_BIT) && !fmt->m68k_denormal;
         frac_shr(p, frac_shift);
 
-        if (is_tiny && (flags & float_flag_inexact)) {
-            flags |= float_flag_underflow;
-        }
-        if (exp == 0 && frac_eqz(p)) {
-            p->cls = float_class_zero;
+        if (is_tiny) {
+            if (s->flush_to_zero) {
+                assert(s->ftz_detection == float_ftz_after_rounding);
+                flags |= float_flag_output_denormal_flushed;
+                p->cls = float_class_zero;
+                exp = 0;
+                frac_clear(p);
+            } else if (flags & float_flag_inexact) {
+                flags |= float_flag_underflow;
+            }
+            if (exp == 0 && frac_eqz(p)) {
+                p->cls = float_class_zero;
+            }
         }
     }
     p->exp = exp;

diff --git a/include/fpu/softfloat-helpers.h b/include/fpu/softfloat-helpers.h
index 4cb30a48220f99d3c86b464030f38d7cb425337c..8983c2748ecd85e924597c1a1b9675487b2562c3 100644 (file)
--- a/include/fpu/softfloat-helpers.h
+++ b/include/fpu/softfloat-helpers.h
@@ -109,6 +109,12 @@ static inline void set_flush_inputs_to_zero(bool val, float_status *status)
     status->flush_inputs_to_zero = val;
 }
 
+static inline void set_float_ftz_detection(FloatFTZDetection d,
+                                           float_status *status)
+{
+    status->ftz_detection = d;
+}
+
 static inline void set_default_nan_mode(bool val, float_status *status)
 {
     status->default_nan_mode = val;
@@ -183,4 +189,9 @@ static inline bool get_default_nan_mode(const float_status *status)
     return status->default_nan_mode;
 }
 
+static inline FloatFTZDetection get_float_ftz_detection(const float_status *status)
+{
+    return status->ftz_detection;
+}
+
 #endif /* SOFTFLOAT_HELPERS_H */

diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
index bba1c397bb7ce4793dfe432f3f660d6fc5555704..53d5eb85210e2470b31139f410b12b214d8020d7 100644 (file)
--- 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;

diff --git a/target/alpha/cpu.c b/target/alpha/cpu.c
index e1b898e57556d48102c3cfe894059d503f595ad1..f5dd744987618974d32cb36feab8957d54960b79 100644 (file)
--- a/target/alpha/cpu.c
+++ b/target/alpha/cpu.c
@@ -202,6 +202,13 @@ static void alpha_cpu_initfn(Object *obj)
     set_float_2nan_prop_rule(float_2nan_prop_x87, &env->fp_status);
     /* Default NaN: sign bit clear, msb frac bit set */
     set_float_default_nan_pattern(0b01000000, &env->fp_status);
+    /*
+     * TODO: this is incorrect. The Alpha Architecture Handbook version 4
+     * section 4.7.7.11 says that we flush to zero for underflow cases, so
+     * this should be float_ftz_after_rounding to match the
+     * tininess_after_rounding (which is specified in section 4.7.5).
+     */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
 #if defined(CONFIG_USER_ONLY)
     env->flags = ENV_FLAG_PS_USER | ENV_FLAG_FEN;
     cpu_alpha_store_fpcr(env, (uint64_t)(FPCR_INVD | FPCR_DZED | FPCR_OVFD

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 32dc7c1e692e98bf7bff421926423a7f26eeedbe..80377443008098590347032e50951b02c8874b05 100644 (file)
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -185,6 +185,7 @@ void arm_register_el_change_hook(ARMCPU *cpu, ARMELChangeHookFn *hook,
 static void arm_set_default_fp_behaviours(float_status *s)
 {
     set_float_detect_tininess(float_tininess_before_rounding, s);
+    set_float_ftz_detection(float_ftz_before_rounding, s);
     set_float_2nan_prop_rule(float_2nan_prop_s_ab, s);
     set_float_3nan_prop_rule(float_3nan_prop_s_cab, s);
     set_float_infzeronan_rule(float_infzeronan_dnan_if_qnan, s);

diff --git a/target/hppa/fpu_helper.c b/target/hppa/fpu_helper.c
index 239c027ec528bcbabbd1faea017a3168deb9d2b9..8ff4b4480490c09fdcfeef36b6c412c65cb17da6 100644 (file)
--- a/target/hppa/fpu_helper.c
+++ b/target/hppa/fpu_helper.c
@@ -67,6 +67,17 @@ void HELPER(loaded_fr0)(CPUHPPAState *env)
     set_float_infzeronan_rule(float_infzeronan_dnan_never, &env->fp_status);
     /* Default NaN: sign bit clear, msb-1 frac bit set */
     set_float_default_nan_pattern(0b00100000, &env->fp_status);
+    /*
+     * "PA-RISC 2.0 Architecture" says it is IMPDEF whether the flushing
+     * enabled by FPSR.D happens before or after rounding. We pick "before"
+     * for consistency with tininess detection.
+     */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
+    /*
+     * TODO: "PA-RISC 2.0 Architecture" chapter 10 says that we should
+     * detect tininess before rounding, but we don't set that here so we
+     * get the default tininess after rounding.
+     */
 }
 
 void cpu_hppa_loaded_fr0(CPUHPPAState *env)

diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
index de6d0b252ec831a462f7d74952217325ab47e8b0..f112c6c673768bb6619832ca7746341cc2f6cf1d 100644 (file)
--- a/target/i386/tcg/fpu_helper.c
+++ b/target/i386/tcg/fpu_helper.c
@@ -188,6 +188,14 @@ void cpu_init_fp_statuses(CPUX86State *env)
     set_float_default_nan_pattern(0b11000000, &env->fp_status);
     set_float_default_nan_pattern(0b11000000, &env->mmx_status);
     set_float_default_nan_pattern(0b11000000, &env->sse_status);
+    /*
+     * TODO: x86 does flush-to-zero detection after rounding (the SDM
+     * section 10.2.3.3 on the FTZ bit of MXCSR says that we flush
+     * when we detect underflow, which x86 does after rounding).
+     */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
+    set_float_ftz_detection(float_ftz_before_rounding, &env->mmx_status);
+    set_float_ftz_detection(float_ftz_before_rounding, &env->sse_status);
 }
 
 static inline uint8_t save_exception_flags(CPUX86State *env)

diff --git a/target/mips/fpu_helper.h b/target/mips/fpu_helper.h
index 6ad1e466cfde0ec8d1e4ff2b1835d5b23b9f3697..08fb4093904a26f1aa6e89c3d7e3db126beef630 100644 (file)
--- a/target/mips/fpu_helper.h
+++ b/target/mips/fpu_helper.h
@@ -84,6 +84,12 @@ static inline void fp_reset(CPUMIPSState *env)
      */
     set_float_2nan_prop_rule(float_2nan_prop_s_ab,
                              &env->active_fpu.fp_status);
+    /*
+     * TODO: the spec does't say clearly whether FTZ happens before
+     * or after rounding for normal FPU operations.
+     */
+    set_float_ftz_detection(float_ftz_before_rounding,
+                            &env->active_fpu.fp_status);
 }
 
 /* MSA */

diff --git a/target/mips/msa.c b/target/mips/msa.c
index fc77bfc7b9a36b1af7a7293fa037bb6d4858779a..32c6acbcc56868ff561537db672822cd5796c9e7 100644 (file)
--- a/target/mips/msa.c
+++ b/target/mips/msa.c
@@ -48,6 +48,15 @@ void msa_reset(CPUMIPSState *env)
     /* tininess detected after rounding.*/
     set_float_detect_tininess(float_tininess_after_rounding,
                               &env->active_tc.msa_fp_status);
+    /*
+     * MSACSR.FS detects tiny results to flush to zero before rounding
+     * (per "MIPS Architecture for Programmers Volume IV-j: The MIPS64 SIMD
+     * Architecture Module, Revision 1.1" section 3.5.4), even though it
+     * detects tininess after rounding for underflow purposes (section 3.4.2
+     * table 3.3).
+     */
+    set_float_ftz_detection(float_ftz_before_rounding,
+                            &env->active_tc.msa_fp_status);
 
     /*
      * According to MIPS specifications, if one of the two operands is

diff --git a/target/ppc/cpu_init.c b/target/ppc/cpu_init.c
index 8e49051254bd6db0269a38d51e8e08a92585de8b..062a6e85fbaecf980dc90ffe4a5e9ac1e9a96265 100644 (file)
--- a/target/ppc/cpu_init.c
+++ b/target/ppc/cpu_init.c
@@ -7262,6 +7262,9 @@ static void ppc_cpu_reset_hold(Object *obj, ResetType type)
     /* tininess for underflow is detected before rounding */
     set_float_detect_tininess(float_tininess_before_rounding,
                               &env->fp_status);
+    /* Similarly for flush-to-zero */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
+
     /*
      * PowerPC propagation rules:
      *  1. A if it sNaN or qNaN

diff --git a/target/rx/cpu.c b/target/rx/cpu.c
index 8c50c7a1bc81c206d8139ba5b952a4cc25ad4dd5..37a6fdd569b37c1769fd54ee0513c24fddff01a7 100644 (file)
--- a/target/rx/cpu.c
+++ b/target/rx/cpu.c
@@ -103,6 +103,14 @@ static void rx_cpu_reset_hold(Object *obj, ResetType type)
     set_float_2nan_prop_rule(float_2nan_prop_x87, &env->fp_status);
     /* Default NaN value: sign bit clear, set frac msb */
     set_float_default_nan_pattern(0b01000000, &env->fp_status);
+    /*
+     * TODO: "RX Family RXv1 Instruction Set Architecture" is not 100% clear
+     * on whether flush-to-zero should happen before or after rounding, but
+     * section 1.3.2 says that it happens when underflow is detected, and
+     * implies that underflow is detected after rounding. So this may not
+     * be the correct setting.
+     */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
 }
 
 static ObjectClass *rx_cpu_class_by_name(const char *cpu_model)

diff --git a/target/sh4/cpu.c b/target/sh4/cpu.c
index 24a22724c617b0fe38ef48ce9de41f5b488b1d69..4ac693d99bd4560a9dcc0c48bd3d2cfb57e959ec 100644 (file)
--- a/target/sh4/cpu.c
+++ b/target/sh4/cpu.c
@@ -130,6 +130,14 @@ static void superh_cpu_reset_hold(Object *obj, ResetType type)
     set_default_nan_mode(1, &env->fp_status);
     /* sign bit clear, set all frac bits other than msb */
     set_float_default_nan_pattern(0b00111111, &env->fp_status);
+    /*
+     * TODO: "SH-4 CPU Core Architecture ADCS 7182230F" doesn't say whether
+     * it detects tininess before or after rounding. Section 6.4 is clear
+     * that flush-to-zero happens when the result underflows, though, so
+     * either this should be "detect ftz after rounding" or else we should
+     * be setting "detect tininess before rounding".
+     */
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
 }
 
 static void superh_cpu_disas_set_info(CPUState *cpu, disassemble_info *info)

diff --git a/target/tricore/helper.c b/target/tricore/helper.c
index e8b0ec51611040e99cbeb2efb8babdf66a477cf6..9898752eb00eb63df513cde6d7645a0484fc58c0 100644 (file)
--- a/target/tricore/helper.c
+++ b/target/tricore/helper.c
@@ -116,6 +116,7 @@ void fpu_set_state(CPUTriCoreState *env)
     set_flush_inputs_to_zero(1, &env->fp_status);
     set_flush_to_zero(1, &env->fp_status);
     set_float_detect_tininess(float_tininess_before_rounding, &env->fp_status);
+    set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
     set_default_nan_mode(1, &env->fp_status);
     /* Default NaN pattern: sign bit clear, frac msb set */
     set_float_default_nan_pattern(0b01000000, &env->fp_status);

diff --git a/tests/fp/fp-bench.c b/tests/fp/fp-bench.c
index eacb39b99cbc9bb27603096def2ad692778193f6..d90f542ea25778523c397e659df8284f4b392e84 100644 (file)
--- a/tests/fp/fp-bench.c
+++ b/tests/fp/fp-bench.c
@@ -496,6 +496,7 @@ static void run_bench(void)
     set_float_3nan_prop_rule(float_3nan_prop_s_cab, &soft_status);
     set_float_infzeronan_rule(float_infzeronan_dnan_if_qnan, &soft_status);
     set_float_default_nan_pattern(0b01000000, &soft_status);
+    set_float_ftz_detection(float_ftz_before_rounding, &soft_status);
 
     f = bench_funcs[operation][precision];
     g_assert(f);