Special case disassembler handling of REX.B prefix on NOP instruction to decode as XCHG R8D, EAX instead. Fixes PR10344.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140971 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c b/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
index ed2cb3f..8a88955 100644
--- a/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
+++ b/lib/Target/X86/Disassembler/X86DisassemblerDecoder.c
@@ -58,8 +58,8 @@
* @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 @@
}
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);
diff --git a/test/MC/Disassembler/X86/simple-tests.txt b/test/MC/Disassembler/X86/simple-tests.txt
index 7687435..fc64d64 100644
--- a/test/MC/Disassembler/X86/simple-tests.txt
+++ b/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
