[ia32] Fixes a bug in cmpw.

The opcodes for 'cmpw r/m16, r16' and 'cmpw r16, r/m16' were swapped, causing a few issues when less than/greater than comparison were performed.

Adds a regression test.

BUG=621926

Committed: https://crrev.com/efa7095e3e360fbadbe909d831ac11b268ca26b0
Review-Url: https://codereview.chromium.org/2103713003
Cr-Original-Commit-Position: refs/heads/master@{#37339}
Cr-Commit-Position: refs/heads/master@{#37345}

src/ia32/assembler-ia32.cc

@@ -852,14 +852,14 @@ void Assembler::cmpw(const Operand& op, Immediate imm16) {
 void Assembler::cmpw(Register reg, const Operand& op) {
   EnsureSpace ensure_space(this);
   EMIT(0x66);
-  EMIT(0x39);
+  EMIT(0x3B);
   emit_operand(reg, op);
 }
 
 void Assembler::cmpw(const Operand& op, Register reg) {
   EnsureSpace ensure_space(this);
   EMIT(0x66);
-  EMIT(0x3B);
+  EMIT(0x39);
   emit_operand(reg, op);
 }
 

src/ia32/disasm-ia32.cc

@@ -1622,11 +1622,19 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
         while (*data == 0x66) data++;
         if (*data == 0xf && data[1] == 0x1f) {
           AppendToBuffer("nop");  // 0x66 prefix
-        } else if (*data == 0x90) {
-          AppendToBuffer("nop");  // 0x66 prefix
-        } else if (*data == 0x8B) {
+        } else if (*data == 0x39) {
           data++;
-          data += PrintOperands("mov_w", REG_OPER_OP_ORDER, data);
+          data += PrintOperands("cmpw", OPER_REG_OP_ORDER, data);
+        } else if (*data == 0x3B) {
+          data++;
+          data += PrintOperands("cmpw", REG_OPER_OP_ORDER, data);
+        } else if (*data == 0x81) {
+          data++;
+          AppendToBuffer("cmpw ");
+          data += PrintRightOperand(data);
+          int imm = *reinterpret_cast<int16_t*>(data);
+          AppendToBuffer(",0x%x", imm);
+          data += 2;
         } else if (*data == 0x87) {
           data++;
           int mod, regop, rm;
@@ -1640,6 +1648,11 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
           AppendToBuffer("mov_w ");
           data += PrintRightOperand(data);
           AppendToBuffer(",%s", NameOfCPURegister(regop));
+        } else if (*data == 0x8B) {
+          data++;
+          data += PrintOperands("mov_w", REG_OPER_OP_ORDER, data);
+        } else if (*data == 0x90) {
+          AppendToBuffer("nop");  // 0x66 prefix
         } else if (*data == 0xC7) {
           data++;
           AppendToBuffer("%s ", "mov_w");

test/cctest/test-assembler-ia32.cc

@@ -1497,4 +1497,45 @@ TEST(AssemblerIa32JumpTables2) {
   }
 }
 
+TEST(Regress621926) {
+  // Bug description:
+  // The opcodes for cmpw r/m16, r16 and cmpw r16, r/m16 were swapped.
+  // This was causing non-commutative comparisons to produce the wrong result.
+  CcTest::InitializeVM();
+  Isolate* isolate = reinterpret_cast<Isolate*>(CcTest::isolate());
+  HandleScope scope(isolate);
+  Assembler assm(isolate, nullptr, 0);
+
+  int16_t a = 42;
+
+  Label fail;
+  __ push(ebx);
+  __ mov(ebx, Immediate(reinterpret_cast<intptr_t>(&a)));
+  __ mov(eax, Immediate(41));
+  __ cmpw(eax, Operand(ebx));
+  __ j(above_equal, &fail);
+  __ cmpw(Operand(ebx), eax);
+  __ j(below_equal, &fail);
+  __ mov(eax, 1);
+  __ pop(ebx);
+  __ ret(0);
+  __ bind(&fail);
+  __ mov(eax, 0);
+  __ pop(ebx);
+  __ ret(0);
+
+  CodeDesc desc;
+  assm.GetCode(&desc);
+  Handle<Code> code = isolate->factory()->NewCode(
+      desc, Code::ComputeFlags(Code::STUB), Handle<Code>());
+
+#ifdef OBJECT_PRINT
+  OFStream os(stdout);
+  code->Print(os);
+#endif
+
+  F0 f = FUNCTION_CAST<F0>(code->entry());
+  CHECK_EQ(f(), 1);
+}
+
 #undef __