target/arm: Use asimd_imm_const for A64 decode

The A64 AdvSIMD modified-immediate grouping uses almost the same
constant encoding that A32 Neon does; reuse asimd_imm_const() (to
which we add the AArch64-specific case for cmode 15 op 1) instead of
reimplementing it all.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210628135835.6690-5-peter.maydell@linaro.org

1 files changed, 15 insertions, 2 deletions

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 95ceb24ec3..66b24ab56e 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;