Add UD0_COMPAT decoder mode (#477)

include/Zydis/Decoder.h

@@ -144,11 +144,20 @@ typedef enum ZydisDecoderMode_
      * This mode is enabled by default.
      */
     ZYDIS_DECODER_MODE_IPREFETCH,
+    /**
+     * Enables the `UD0` compatibility mode.
+     *
+     * Some processors decode the `UD0` instruction without a ModR/M byte. Enable this decoder mode
+     * to mimic this behavior.
+     *
+     * This mode is disabled by default.
+     */
+    ZYDIS_DECODER_MODE_UD0_COMPAT,
 
     /**
      * Maximum value of this enum.
      */
-    ZYDIS_DECODER_MODE_MAX_VALUE = ZYDIS_DECODER_MODE_IPREFETCH,
+    ZYDIS_DECODER_MODE_MAX_VALUE = ZYDIS_DECODER_MODE_UD0_COMPAT,
     /**
      * The minimum number of bits required to represent all values of this enum.
      */

include/Zydis/Internal/DecoderData.h

@@ -177,7 +177,11 @@ enum ZydisDecoderTreeNodeTypes
     /**
      * Reference to a IPREFETCH-mode filter.
      */
-    ZYDIS_NODETYPE_FILTER_MODE_IPREFETCH    = 0x1C
+    ZYDIS_NODETYPE_FILTER_MODE_IPREFETCH    = 0x1C,
+    /**
+     * Reference to a UD0_COMPAT-mode filter.
+     */
+    ZYDIS_NODETYPE_FILTER_MODE_UD0_COMPAT   = 0x1D
 };
 
 /* ---------------------------------------------------------------------------------------------- */

src/Decoder.c

@@ -4803,6 +4803,9 @@ static ZyanStatus ZydisDecodeInstruction(ZydisDecoderState* state,
         case ZYDIS_NODETYPE_FILTER_MODE_IPREFETCH:
             index = !!(state->decoder->decoder_mode & (1 << ZYDIS_DECODER_MODE_IPREFETCH));
             break;
+        case ZYDIS_NODETYPE_FILTER_MODE_UD0_COMPAT:
+            index = !!(state->decoder->decoder_mode & (1 << ZYDIS_DECODER_MODE_UD0_COMPAT));
+            break;
         default:
             if (node_type & ZYDIS_NODETYPE_DEFINITION_MASK)
             {
@@ -4899,6 +4902,8 @@ static ZyanStatus ZydisDecodeInstruction(ZydisDecoderState* state,
 ZyanStatus ZydisDecoderInit(ZydisDecoder* decoder, ZydisMachineMode machine_mode,
     ZydisStackWidth stack_width)
 {
+    ZYAN_STATIC_ASSERT(ZYDIS_DECODER_MODE_MAX_VALUE <= 32);
+
     static const ZyanU32 decoder_modes =
 #ifdef ZYDIS_MINIMAL_MODE
         (1 << ZYDIS_DECODER_MODE_MINIMAL) |

src/DecoderData.c

@@ -158,6 +158,9 @@ const ZydisDecoderTreeNode* ZydisDecoderTreeGetChildNode(const ZydisDecoderTreeN
     case ZYDIS_NODETYPE_FILTER_MODE_IPREFETCH:
         ZYAN_ASSERT(index <   2);
         return &FILTERS_MODE_IPREFETCH[parent->value][index];
+    case ZYDIS_NODETYPE_FILTER_MODE_UD0_COMPAT:
+        ZYAN_ASSERT(index <   2);
+        return &FILTERS_MODE_UD0_COMPAT[parent->value][index];
     default:
         ZYAN_UNREACHABLE;
     }