target/arm: AArch32 VCVT fixed-point to float is always round-to-nearest

For AArch32, unlike the VCVT of integer to float, which honours the
rounding mode specified by the FPSCR, VCVT of fixed-point to float is
always round-to-nearest. (AArch64 fixed-point-to-float conversions
always honour the FPCR rounding mode.)

Implement this by providing _round_to_nearest versions of the
relevant helpers which set the rounding mode temporarily when making
the call to the underlying softfloat function.

We only need to change the VFP VCVT instructions, because the
standard- FPSCR value used by the Neon VCVT is always set to
round-to-nearest, so we don't need to do the extra work of saving
and restoring the rounding mode.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201013103532.13391-1-peter.maydell@linaro.org

3 files changed, 47 insertions, 13 deletions

diff --git a/target/arm/helper.h b/target/arm/helper.h
index 8defd7c801..774d2cddb5 100644
--- a/target/arm/helper.h
+++ b/target/arm/helper.h
@@ -213,6 +213,19 @@ DEF_HELPER_3(vfp_ultoh, f16, i32, i32, ptr)
 DEF_HELPER_3(vfp_sqtoh, f16, i64, i32, ptr)
 DEF_HELPER_3(vfp_uqtoh, f16, i64, i32, ptr)
 
+DEF_HELPER_3(vfp_shtos_round_to_nearest, f32, i32, i32, ptr)
+DEF_HELPER_3(vfp_sltos_round_to_nearest, f32, i32, i32, ptr)
+DEF_HELPER_3(vfp_uhtos_round_to_nearest, f32, i32, i32, ptr)
+DEF_HELPER_3(vfp_ultos_round_to_nearest, f32, i32, i32, ptr)
+DEF_HELPER_3(vfp_shtod_round_to_nearest, f64, i64, i32, ptr)
+DEF_HELPER_3(vfp_sltod_round_to_nearest, f64, i64, i32, ptr)
+DEF_HELPER_3(vfp_uhtod_round_to_nearest, f64, i64, i32, ptr)
+DEF_HELPER_3(vfp_ultod_round_to_nearest, f64, i64, i32, ptr)
+DEF_HELPER_3(vfp_shtoh_round_to_nearest, f16, i32, i32, ptr)
+DEF_HELPER_3(vfp_uhtoh_round_to_nearest, f16, i32, i32, ptr)
+DEF_HELPER_3(vfp_sltoh_round_to_nearest, f16, i32, i32, ptr)
+DEF_HELPER_3(vfp_ultoh_round_to_nearest, f16, i32, i32, ptr)
+
 DEF_HELPER_FLAGS_2(set_rmode, TCG_CALL_NO_RWG, i32, i32, ptr)
 
 DEF_HELPER_FLAGS_3(vfp_fcvt_f16_to_f32, TCG_CALL_NO_RWG, f32, f16, ptr, i32)
diff --git a/target/arm/translate-vfp.c.inc b/target/arm/translate-vfp.c.inc
index 28e0dba5f1..9b11b81f80 100644
--- a/target/arm/translate-vfp.c.inc
+++ b/target/arm/translate-vfp.c.inc
@@ -3141,16 +3141,16 @@ static bool trans_VCVT_fix_hp(DisasContext *s, arg_VCVT_fix_sp *a)
     /* Switch on op:U:sx bits */
     switch (a->opc) {
     case 0:
-        gen_helper_vfp_shtoh(vd, vd, shift, fpst);
+        gen_helper_vfp_shtoh_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 1:
-        gen_helper_vfp_sltoh(vd, vd, shift, fpst);
+        gen_helper_vfp_sltoh_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 2:
-        gen_helper_vfp_uhtoh(vd, vd, shift, fpst);
+        gen_helper_vfp_uhtoh_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 3:
-        gen_helper_vfp_ultoh(vd, vd, shift, fpst);
+        gen_helper_vfp_ultoh_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 4:
         gen_helper_vfp_toshh_round_to_zero(vd, vd, shift, fpst);
@@ -3200,16 +3200,16 @@ static bool trans_VCVT_fix_sp(DisasContext *s, arg_VCVT_fix_sp *a)
     /* Switch on op:U:sx bits */
     switch (a->opc) {
     case 0:
-        gen_helper_vfp_shtos(vd, vd, shift, fpst);
+        gen_helper_vfp_shtos_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 1:
-        gen_helper_vfp_sltos(vd, vd, shift, fpst);
+        gen_helper_vfp_sltos_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 2:
-        gen_helper_vfp_uhtos(vd, vd, shift, fpst);
+        gen_helper_vfp_uhtos_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 3:
-        gen_helper_vfp_ultos(vd, vd, shift, fpst);
+        gen_helper_vfp_ultos_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 4:
         gen_helper_vfp_toshs_round_to_zero(vd, vd, shift, fpst);
@@ -3265,16 +3265,16 @@ static bool trans_VCVT_fix_dp(DisasContext *s, arg_VCVT_fix_dp *a)
     /* Switch on op:U:sx bits */
     switch (a->opc) {
     case 0:
-        gen_helper_vfp_shtod(vd, vd, shift, fpst);
+        gen_helper_vfp_shtod_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 1:
-        gen_helper_vfp_sltod(vd, vd, shift, fpst);
+        gen_helper_vfp_sltod_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 2:
-        gen_helper_vfp_uhtod(vd, vd, shift, fpst);
+        gen_helper_vfp_uhtod_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 3:
-        gen_helper_vfp_ultod(vd, vd, shift, fpst);
+        gen_helper_vfp_ultod_round_to_nearest(vd, vd, shift, fpst);
         break;
     case 4:
         gen_helper_vfp_toshd_round_to_zero(vd, vd, shift, fpst);
diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
index 5666393ef7..abfdb6a8e2 100644
--- a/target/arm/vfp_helper.c
+++ b/target/arm/vfp_helper.c
@@ -393,12 +393,32 @@ float32 VFP_HELPER(fcvts, d)(float64 x, CPUARMState *env)
     return float64_to_float32(x, &env->vfp.fp_status);
 }
 
-/* VFP3 fixed point conversion.  */
+/*
+ * VFP3 fixed point conversion. The AArch32 versions of fix-to-float
+ * must always round-to-nearest; the AArch64 ones honour the FPSCR
+ * rounding mode. (For AArch32 Neon the standard-FPSCR is set to
+ * round-to-nearest so either helper will work.) AArch32 float-to-fix
+ * must round-to-zero.
+ */
 #define VFP_CONV_FIX_FLOAT(name, p, fsz, ftype, isz, itype)            \
 ftype HELPER(vfp_##name##to##p)(uint##isz##_t  x, uint32_t shift,      \
                                      void *fpstp) \
 { return itype##_to_##float##fsz##_scalbn(x, -shift, fpstp); }
 
+#define VFP_CONV_FIX_FLOAT_ROUND(name, p, fsz, ftype, isz, itype)      \
+    ftype HELPER(vfp_##name##to##p##_round_to_nearest)(uint##isz##_t  x, \
+                                                     uint32_t shift,   \
+                                                     void *fpstp)      \
+    {                                                                  \
+        ftype ret;                                                     \
+        float_status *fpst = fpstp;                                    \
+        FloatRoundMode oldmode = fpst->float_rounding_mode;            \
+        fpst->float_rounding_mode = float_round_nearest_even;          \
+        ret = itype##_to_##float##fsz##_scalbn(x, -shift, fpstp);      \
+        fpst->float_rounding_mode = oldmode;                           \
+        return ret;                                                    \
+    }
+
 #define VFP_CONV_FLOAT_FIX_ROUND(name, p, fsz, ftype, isz, itype, ROUND, suff) \
 uint##isz##_t HELPER(vfp_to##name##p##suff)(ftype x, uint32_t shift,      \
                                             void *fpst)                   \
@@ -412,6 +432,7 @@ uint##isz##_t HELPER(vfp_to##name##p##suff)(ftype x, uint32_t shift,      \
 
 #define VFP_CONV_FIX(name, p, fsz, ftype, isz, itype)            \
 VFP_CONV_FIX_FLOAT(name, p, fsz, ftype, isz, itype)              \
+VFP_CONV_FIX_FLOAT_ROUND(name, p, fsz, ftype, isz, itype)        \
 VFP_CONV_FLOAT_FIX_ROUND(name, p, fsz, ftype, isz, itype,        \
                          float_round_to_zero, _round_to_zero)    \
 VFP_CONV_FLOAT_FIX_ROUND(name, p, fsz, ftype, isz, itype,        \