target-arm: Fix VMLA, VMLS, VNMLS, VNMLA handling of NaNs

Correct handling of NaNs for VFP VMLA, VMLS, VNMLS and VNMLA requires that
we implement the set of negations and additions specified by the ARM ARM;
plausible looking simplifications like turning (-A + B) into (B - A) or
computing (A + B) rather than (B + A) result in selecting the wrong NaN or
returning a NaN with the wrong sign bit.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/translate.c

@@ -909,6 +909,26 @@ VFP_OP2(div)
 
 #undef VFP_OP2
 
+static inline void gen_vfp_F1_mul(int dp)
+{
+    /* Like gen_vfp_mul() but put result in F1 */
+    if (dp) {
+        gen_helper_vfp_muld(cpu_F1d, cpu_F0d, cpu_F1d, cpu_env);
+    } else {
+        gen_helper_vfp_muls(cpu_F1s, cpu_F0s, cpu_F1s, cpu_env);
+    }
+}
+
+static inline void gen_vfp_F1_neg(int dp)
+{
+    /* Like gen_vfp_neg() but put result in F1 */
+    if (dp) {
+        gen_helper_vfp_negd(cpu_F1d, cpu_F0d);
+    } else {
+        gen_helper_vfp_negs(cpu_F1s, cpu_F0s);
+    }
+}
+
 static inline void gen_vfp_abs(int dp)
 {
     if (dp)
@@ -3021,27 +3041,34 @@ static int disas_vfp_insn(CPUState * env, DisasContext *s, uint32_t insn)
             for (;;) {
                 /* Perform the calculation.  */
                 switch (op) {
-                case 0: /* mac: fd + (fn * fm) */
-                    gen_vfp_mul(dp);
-                    gen_mov_F1_vreg(dp, rd);
+                case 0: /* VMLA: fd + (fn * fm) */
+                    /* Note that order of inputs to the add matters for NaNs */
+                    gen_vfp_F1_mul(dp);
+                    gen_mov_F0_vreg(dp, rd);
                     gen_vfp_add(dp);
                     break;
-                case 1: /* nmac: fd - (fn * fm) */
+                case 1: /* VMLS: fd + -(fn * fm) */
                     gen_vfp_mul(dp);
-                    gen_vfp_neg(dp);
-                    gen_mov_F1_vreg(dp, rd);
+                    gen_vfp_F1_neg(dp);
+                    gen_mov_F0_vreg(dp, rd);
                     gen_vfp_add(dp);
                     break;
-                case 2: /* msc: -fd + (fn * fm) */
-                    gen_vfp_mul(dp);
-                    gen_mov_F1_vreg(dp, rd);
-                    gen_vfp_sub(dp);
+                case 2: /* VNMLS: -fd + (fn * fm) */
+                    /* Note that it isn't valid to replace (-A + B) with (B - A)
+                     * or similar plausible looking simplifications
+                     * because this will give wrong results for NaNs.
+                     */
+                    gen_vfp_F1_mul(dp);
+                    gen_mov_F0_vreg(dp, rd);
+                    gen_vfp_neg(dp);
+                    gen_vfp_add(dp);
                     break;
-                case 3: /* nmsc: -fd - (fn * fm)  */
+                case 3: /* VNMLA: -fd + -(fn * fm) */
                     gen_vfp_mul(dp);
+                    gen_vfp_F1_neg(dp);
+                    gen_mov_F0_vreg(dp, rd);
                     gen_vfp_neg(dp);
-                    gen_mov_F1_vreg(dp, rd);
-                    gen_vfp_sub(dp);
+                    gen_vfp_add(dp);
                     break;
                 case 4: /* mul: fn * fm */
                     gen_vfp_mul(dp);