[x86] Fix 16-bit disassembly of JCXZ/JECXZ

llvm-svn: 199653

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c b/llvm/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
index 66badd95c32..48c16977133 100644
--- a/llvm/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
+++ b/llvm/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
@@ -987,6 +987,25 @@ static int getID(struct InternalInstruction* insn, const void *miiArg) {
   if (getIDWithAttrMask(&instructionID, insn, attrMask))
     return -1;
 
+  /*
+   * JCXZ/JECXZ need special handling for 16-bit mode because the meaning
+   * of the AdSize prefix is inverted w.r.t. 32-bit mode.
+   */
+  if (insn->mode == MODE_16BIT && insn->opcode == 0xE3) {
+    const struct InstructionSpecifier *spec;
+    spec = specifierForUID(instructionID);
+
+    /*
+     * Check for Ii8PCRel instructions. We could alternatively do a
+     * string-compare on the names, but this is probably cheaper.
+     */
+    if (x86OperandSets[spec->operands][0].type == TYPE_REL8) {
+      attrMask ^= ATTR_ADSIZE;
+      if (getIDWithAttrMask(&instructionID, insn, attrMask))
+        return -1;
+    }
+  }
+
   /* The following clauses compensate for limitations of the tables. */
 
   if ((insn->mode == MODE_16BIT || insn->prefixPresent[0x66]) &&