Special case disassembler handling of REX.B prefix on NOP instruction to...

Special case disassembler handling of REX.B prefix on NOP instruction to decode as XCHG R8D, EAX instead. Fixes PR10344.

llvm-svn: 140971

llvm/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c

@@ -58,8 +58,8 @@ static InstructionContext contextForAttrs(uint8_t attrMask) {
  * @return            - TRUE if the ModR/M byte is required, FALSE otherwise.
  */
 static int modRMRequired(OpcodeType type,
-                                InstructionContext insnContext,
-                                uint8_t opcode) {
+                         InstructionContext insnContext,
+                         uint8_t opcode) {
   const struct ContextDecision* decision = 0;
   
   switch (type) {
@@ -885,6 +885,43 @@ static int getID(struct InternalInstruction* insn) {
     }
     return 0;
   }
+
+  if (insn->opcodeType == ONEBYTE && insn->opcode == 0x90 &&
+      insn->rexPrefix & 0x01) {
+    /*
+     * NOOP shouldn't decode as NOOP if REX.b is set. Instead
+     * it should decode as XCHG %r8, %eax.
+     */
+
+    const struct InstructionSpecifier *spec;
+    uint16_t instructionIDWithNewOpcode;
+    const struct InstructionSpecifier *specWithNewOpcode;
+
+    spec = specifierForUID(instructionID);
+    
+    // Borrow opcode from one of the other XCHGar opcodes
+    insn->opcode = 0x91;
+   
+    if (getIDWithAttrMask(&instructionIDWithNewOpcode,
+                          insn,
+                          attrMask)) {
+      insn->opcode = 0x90;
+
+      insn->instructionID = instructionID;
+      insn->spec = spec;
+      return 0;
+    }
+
+    specWithNewOpcode = specifierForUID(instructionIDWithNewOpcode);
+
+    // Change back 
+    insn->opcode = 0x90;
+
+    insn->instructionID = instructionIDWithNewOpcode;
+    insn->spec = specWithNewOpcode;
+
+    return 0;
+  }
   
   insn->instructionID = instructionID;
   insn->spec = specifierForUID(insn->instructionID);

llvm/test/MC/Disassembler/X86/simple-tests.txt

@@ -308,3 +308,6 @@
 
 # CHECK: invvpid (%rax), %rax
 0x66 0x0f 0x38 0x81 0x00
+
+# CHECK: xchgl %r8d, %eax
+0x41 0x90