target/arm: Diagnose writeback register in list for LDM for v7

Prior to v7, for the A32 encoding, this operation wrote an UNKNOWN
value back to the base register.  Starting in v7 this is UNPREDICTABLE.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190904193059.26202-30-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/translate.c

@@ -9997,6 +9997,15 @@ static bool do_ldm(DisasContext *s, arg_ldst_block *a)
 
 static bool trans_LDM_a32(DisasContext *s, arg_ldst_block *a)
 {
+    /*
+     * Writeback register in register list is UNPREDICTABLE
+     * for ArchVersion() >= 7.  Prior to v7, A32 would write
+     * an UNKNOWN value to the base register.
+     */
+    if (ENABLE_ARCH_7 && a->w && (a->list & (1 << a->rn))) {
+        unallocated_encoding(s);
+        return true;
+    }
     return do_ldm(s, a);
 }