diff --git a/target/arm/translate.h b/target/arm/translate.h
index 6c8d5f6ede1..e2f056c32c2 100644
--- a/target/arm/translate.h
+++ b/target/arm/translate.h
@@ -540,7 +540,8 @@ static inline MemOp finalize_memop(DisasContext *s, MemOp opc)
  * VMVN and VBIC (when cmode < 14 && op == 1).
  *
  * The combination cmode == 15 op == 1 is a reserved encoding for AArch32;
- * callers must catch this.
+ * callers must catch this; we return the 64-bit constant value defined
+ * for AArch64.
  *
  * cmode = 2,3,4,5,6,7,10,11,12,13 imm=0 was UNPREDICTABLE in v7A but
  * is either not unpredictable or merely CONSTRAINED UNPREDICTABLE in v8A;
diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
index 7f74d0e81a9..97b4a41a0c7 100644
--- a/target/arm/translate-a64.c
+++ b/target/arm/translate-a64.c
@@ -8199,8 +8199,6 @@ static void disas_simd_mod_imm(DisasContext *s, uint32_t insn)
 {
     int rd = extract32(insn, 0, 5);
     int cmode = extract32(insn, 12, 4);
-    int cmode_3_1 = extract32(cmode, 1, 3);
-    int cmode_0 = extract32(cmode, 0, 1);
     int o2 = extract32(insn, 11, 1);
     uint64_t abcdefgh = extract32(insn, 5, 5) | (extract32(insn, 16, 3) << 5);
     bool is_neg = extract32(insn, 29, 1);
@@ -8219,83 +8217,13 @@ static void disas_simd_mod_imm(DisasContext *s, uint32_t insn)
         return;
     }
 
-    /* See AdvSIMDExpandImm() in ARM ARM */
-    switch (cmode_3_1) {
-    case 0: /* Replicate(Zeros(24):imm8, 2) */
-    case 1: /* Replicate(Zeros(16):imm8:Zeros(8), 2) */
-    case 2: /* Replicate(Zeros(8):imm8:Zeros(16), 2) */
-    case 3: /* Replicate(imm8:Zeros(24), 2) */
-    {
-        int shift = cmode_3_1 * 8;
-        imm = bitfield_replicate(abcdefgh << shift, 32);
-        break;
-    }
-    case 4: /* Replicate(Zeros(8):imm8, 4) */
-    case 5: /* Replicate(imm8:Zeros(8), 4) */
-    {
-        int shift = (cmode_3_1 & 0x1) * 8;
-        imm = bitfield_replicate(abcdefgh << shift, 16);
-        break;
-    }
-    case 6:
-        if (cmode_0) {
-            /* Replicate(Zeros(8):imm8:Ones(16), 2) */
-            imm = (abcdefgh << 16) | 0xffff;
-        } else {
-            /* Replicate(Zeros(16):imm8:Ones(8), 2) */
-            imm = (abcdefgh << 8) | 0xff;
-        }
-        imm = bitfield_replicate(imm, 32);
-        break;
-    case 7:
-        if (!cmode_0 && !is_neg) {
-            imm = bitfield_replicate(abcdefgh, 8);
-        } else if (!cmode_0 && is_neg) {
-            int i;
-            imm = 0;
-            for (i = 0; i < 8; i++) {
-                if ((abcdefgh) & (1 << i)) {
-                    imm |= 0xffULL << (i * 8);
-                }
-            }
-        } else if (cmode_0) {
-            if (is_neg) {
-                imm = (abcdefgh & 0x3f) << 48;
-                if (abcdefgh & 0x80) {
-                    imm |= 0x8000000000000000ULL;
-                }
-                if (abcdefgh & 0x40) {
-                    imm |= 0x3fc0000000000000ULL;
-                } else {
-                    imm |= 0x4000000000000000ULL;
-                }
-            } else {
-                if (o2) {
-                    /* FMOV (vector, immediate) - half-precision */
-                    imm = vfp_expand_imm(MO_16, abcdefgh);
-                    /* now duplicate across the lanes */
-                    imm = bitfield_replicate(imm, 16);
-                } else {
-                    imm = (abcdefgh & 0x3f) << 19;
-                    if (abcdefgh & 0x80) {
-                        imm |= 0x80000000;
-                    }
-                    if (abcdefgh & 0x40) {
-                        imm |= 0x3e000000;
-                    } else {
-                        imm |= 0x40000000;
-                    }
-                    imm |= (imm << 32);
-                }
-            }
-        }
-        break;
-    default:
-        g_assert_not_reached();
-    }
-
-    if (cmode_3_1 != 7 && is_neg) {
-        imm = ~imm;
+    if (cmode == 15 && o2 && !is_neg) {
+        /* FMOV (vector, immediate) - half-precision */
+        imm = vfp_expand_imm(MO_16, abcdefgh);
+        /* now duplicate across the lanes */
+        imm = bitfield_replicate(imm, 16);
+    } else {
+        imm = asimd_imm_const(abcdefgh, cmode, is_neg);
     }
 
     if (!((cmode & 0x9) == 0x1 || (cmode & 0xd) == 0x9)) {
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 97eea935433..3cb9996a509 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -121,8 +121,8 @@ uint64_t asimd_imm_const(uint32_t imm, int cmode, int op)
     case 14:
         if (op) {
             /*
-             * This is the only case where the top and bottom 32 bits
-             * of the encoded constant differ.
+             * This and cmode == 15 op == 1 are the only cases where
+             * the top and bottom 32 bits of the encoded constant differ.
              */
             uint64_t imm64 = 0;
             int n;
@@ -137,6 +137,19 @@ uint64_t asimd_imm_const(uint32_t imm, int cmode, int op)
         imm |= (imm << 8) | (imm << 16) | (imm << 24);
         break;
     case 15:
+        if (op) {
+            /* Reserved encoding for AArch32; valid for AArch64 */
+            uint64_t imm64 = (uint64_t)(imm & 0x3f) << 48;
+            if (imm & 0x80) {
+                imm64 |= 0x8000000000000000ULL;
+            }
+            if (imm & 0x40) {
+                imm64 |= 0x3fc0000000000000ULL;
+            } else {
+                imm64 |= 0x4000000000000000ULL;
+            }
+            return imm64;
+        }
         imm = ((imm & 0x80) << 24) | ((imm & 0x3f) << 19)
             | ((imm & 0x40) ? (0x1f << 25) : (1 << 30));
         break;
