From 9df6ac17886cd8923ea775636b93d6f02d5c59f4 Mon Sep 17 00:00:00 2001
From: Alexis Engelke <alexis.engelke@googlemail.com>
Date: Sun, 8 Nov 2020 11:20:03 +0100
Subject: [PATCH] decode: Replace T8+T72 with T16+T8E for R/M value

---
 decode.c       | 15 +++++++--------
 parseinstrs.py | 30 ++++++++++++++++++++----------
 2 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/decode.c b/decode.c
index 66f17db..d4dab48 100644
--- a/decode.c
+++ b/decode.c
@@ -30,8 +30,8 @@ typedef enum DecodeMode DecodeMode;
 #define ENTRY_NONE 0
 #define ENTRY_INSTR 1
 #define ENTRY_TABLE256 2
-#define ENTRY_TABLE8 3
-#define ENTRY_TABLE72 4
+#define ENTRY_TABLE16 3
+#define ENTRY_TABLE8E 4
 #define ENTRY_TABLE_PREFIX 5
 #define ENTRY_TABLE_VEX 6
 #define ENTRY_TABLE_ROOT 8
@@ -409,13 +409,12 @@ fd_decode(const uint8_t* buffer, size_t len_sz, int mode_int, uintptr_t address,
     }
 
     // Then, walk through ModR/M-encoded opcode extensions.
-    if ((kind == ENTRY_TABLE8 || kind == ENTRY_TABLE72) && LIKELY(off < len))
-    {
-        if (kind == ENTRY_TABLE72 && (buffer[off] & 0xc0) == 0xc0)
-            ENTRY_UNPACK(table, kind, table[buffer[off++] - 0xb8]);
-        else
-            ENTRY_UNPACK(table, kind, table[(buffer[off] >> 3) & 7]);
+    if (kind == ENTRY_TABLE16 && LIKELY(off < len)) {
+        unsigned isreg = (buffer[off] & 0xc0) == 0xc0 ? 8 : 0;
+        ENTRY_UNPACK(table, kind, table[((buffer[off] >> 3) & 7) | isreg]);
     }
+    if (kind == ENTRY_TABLE8E && LIKELY(off < len))
+        ENTRY_UNPACK(table, kind, table[buffer[off++] & 7]);
 
     // For VEX prefix, we have to distinguish between VEX.W and VEX.L which may
     // be part of the opcode.
diff --git a/parseinstrs.py b/parseinstrs.py
index 3239407..c70e16e 100644
--- a/parseinstrs.py
+++ b/parseinstrs.py
@@ -202,8 +202,8 @@ class EntryKind(Enum):
     NONE = 0
     INSTR = 1
     TABLE256 = 2
-    TABLE8 = 3
-    TABLE72 = 4
+    TABLE16 = 3
+    TABLE8E = 4
     TABLE_PREFIX = 5
     TABLE_VEX = 6
     TABLE_ROOT = -1
@@ -215,8 +215,8 @@ class TrieEntry(NamedTuple):
 
     TABLE_LENGTH = {
         EntryKind.TABLE256: 256,
-        EntryKind.TABLE8: 8,
-        EntryKind.TABLE72: 72,
+        EntryKind.TABLE16: 16,
+        EntryKind.TABLE8E: 8,
         EntryKind.TABLE_PREFIX: 4,
         EntryKind.TABLE_VEX: 4,
         EntryKind.TABLE_ROOT: 8,
@@ -296,10 +296,18 @@ class Opcode(NamedTuple):
                 prefix_val = ["NP", "66", "F3", "F2"].index(self.prefix)
                 opcode.append((EntryKind.TABLE_PREFIX, [prefix_val]))
         if self.opcext:
-            opcext_kind = [EntryKind.TABLE8, EntryKind.TABLE72][self.opcext[0]]
-            opcext_val = self.opcext[1] - (0 if self.opcext[1] < 8 else 0xb8)
-            opcode.append((opcext_kind, [opcext_val]))
-        if self.extended:
+            opcext_val = self.opcext[1]
+            if not self.opcext[0]:
+                opcode.append((EntryKind.TABLE16, [opcext_val, opcext_val | 8]))
+            elif opcext_val < 8:
+                opcode.append((EntryKind.TABLE16, [opcext_val]))
+            else:
+                opcode.append((EntryKind.TABLE16, [((opcext_val - 0xc0) >> 3) | 8]))
+                if not self.extended:
+                    opcode.append((EntryKind.TABLE8E, [opcext_val & 7]))
+                else:
+                    opcode.append((EntryKind.TABLE8E, list(range(8))))
+        if self.extended and not self.opcext:
             last_type, last_indices = opcode[-1]
             opcode[-1] = last_type, [last_indices[0] + i for i in range(8)]
         if self.vexl in ("0", "1") or self.rexw in ("0", "1"):
@@ -320,8 +328,10 @@ def format_opcode(opcode):
             prefix += ["", "VEX."][byte >> 2]
         elif kind == EntryKind.TABLE256:
             opcode_string += "{:02x}".format(byte)
-        elif kind in (EntryKind.TABLE8, EntryKind.TABLE72):
-            opcode_string += "/{:x}".format(byte)
+        elif kind == EntryKind.TABLE16:
+            opcode_string += "/{:x}{}".format(byte & 7, "mr"[byte >> 3])
+        elif kind == EntryKind.TABLE8E:
+            opcode_string += "+rm={:x}".format(byte)
         elif kind == EntryKind.TABLE_PREFIX:
             if byte & 4:
                 prefix += "VEX."