3 files changed, 72 insertions, 12 deletions
diff --git a/compiler/utils/x86/assembler_x86.cc b/compiler/utils/x86/assembler_x86.cc
index adc7bcc0f4..b6708de81c 100644
--- a/compiler/utils/x86/assembler_x86.cc
+++ b/compiler/utils/x86/assembler_x86.cc
@@ -2887,9 +2887,22 @@ void X86Assembler::fprem() {
 }
 
 
-void X86Assembler::xchgb(Register reg, const Address& address) {
-  // For testing purpose
-  xchgb(static_cast<ByteRegister>(reg), address);
+bool X86Assembler::try_xchg_eax(Register dst, Register src) {
+  if (src != EAX && dst != EAX) {
+    return false;
+  }
+  if (dst == EAX) {
+    std::swap(src, dst);
+  }
+  EmitUint8(0x90 + dst);
+  return true;
+}
+
+
+void X86Assembler::xchgb(ByteRegister dst, ByteRegister src) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x86);
+  EmitRegisterOperand(dst, src);
 }
 
 
@@ -2900,6 +2913,29 @@ void X86Assembler::xchgb(ByteRegister reg, const Address& address) {
 }
 
 
+void X86Assembler::xchgb(Register dst, Register src) {
+  xchgb(static_cast<ByteRegister>(dst), static_cast<ByteRegister>(src));
+}
+
+
+void X86Assembler::xchgb(Register reg, const Address& address) {
+  xchgb(static_cast<ByteRegister>(reg), address);
+}
+
+
+void X86Assembler::xchgw(Register dst, Register src) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitOperandSizeOverride();
+  if (try_xchg_eax(dst, src)) {
+    // A short version for AX.
+    return;
+  }
+  // General case.
+  EmitUint8(0x87);
+  EmitRegisterOperand(dst, src);
+}
+
+
 void X86Assembler::xchgw(Register reg, const Address& address) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitOperandSizeOverride();
@@ -2910,6 +2946,11 @@ void X86Assembler::xchgw(Register reg, const Address& address) {
 
 void X86Assembler::xchgl(Register dst, Register src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  if (try_xchg_eax(dst, src)) {
+    // A short version for EAX.
+    return;
+  }
+  // General case.
   EmitUint8(0x87);
   EmitRegisterOperand(dst, src);
 }
diff --git a/compiler/utils/x86/assembler_x86.h b/compiler/utils/x86/assembler_x86.h
index db92a80a4e..f6e7fbc8cd 100644
--- a/compiler/utils/x86/assembler_x86.h
+++ b/compiler/utils/x86/assembler_x86.h
@@ -736,8 +736,14 @@ class X86Assembler final : public Assembler {
   void fptan();
   void fprem();
 
+  void xchgb(ByteRegister dst, ByteRegister src);
   void xchgb(ByteRegister reg, const Address& address);
+
+  // Wrappers for `xchgb` that accept `Register` instead of `ByteRegister` (used for testing).
+  void xchgb(Register dst, Register src);
   void xchgb(Register reg, const Address& address);
+
+  void xchgw(Register dst, Register src);
   void xchgw(Register reg, const Address& address);
 
   void xchgl(Register dst, Register src);
@@ -1044,6 +1050,10 @@ class X86Assembler final : public Assembler {
   uint8_t EmitVexPrefixByteTwo(bool W,
                                int SET_VEX_L,
                                int SET_VEX_PP);
+
+  // Helper function to emit a shorter variant of XCHG for when at least one operand is EAX/AX.
+  bool try_xchg_eax(Register dst, Register src);
+
   ConstantArea constant_area_;
   bool has_AVX_;     // x86 256bit SIMD AVX.
   bool has_AVX2_;    // x86 256bit SIMD AVX 2.0.
diff --git a/compiler/utils/x86/assembler_x86_test.cc b/compiler/utils/x86/assembler_x86_test.cc
index b7b1d33a74..92ce788f11 100644
--- a/compiler/utils/x86/assembler_x86_test.cc
+++ b/compiler/utils/x86/assembler_x86_test.cc
@@ -331,19 +331,28 @@ TEST_F(AssemblerX86Test, LoadLongConstant) {
   DriverStr(expected, "LoadLongConstant");
 }
 
-TEST_F(AssemblerX86Test, Xchgb) {
-  DriverStr(RepeatwA(&x86::X86Assembler::xchgb,
-                     "xchgb {mem}, %{reg}"), "xchgb");
+TEST_F(AssemblerX86Test, XchgbReg) {
+  DriverStr(Repeatww(&x86::X86Assembler::xchgb, "xchgb %{reg2}, %{reg1}"), "xchgb");
 }
 
-TEST_F(AssemblerX86Test, Xchgw) {
-  DriverStr(RepeatrA(&x86::X86Assembler::xchgw,
-                     "xchgw {mem}, %{reg}"), "xchgw");
+TEST_F(AssemblerX86Test, XchgbMem) {
+  DriverStr(RepeatwA(&x86::X86Assembler::xchgb, "xchgb {mem}, %{reg}"), "xchgb");
 }
 
-TEST_F(AssemblerX86Test, Xchgl) {
-  DriverStr(RepeatRA(&x86::X86Assembler::xchgl,
-                     "xchgl {mem}, %{reg}"), "xchgl");
+TEST_F(AssemblerX86Test, XchgwReg) {
+  DriverStr(Repeatrr(&x86::X86Assembler::xchgw, "xchgw %{reg2}, %{reg1}"), "xchgw");
+}
+
+TEST_F(AssemblerX86Test, XchgwMem) {
+  DriverStr(RepeatrA(&x86::X86Assembler::xchgw, "xchgw {mem}, %{reg}"), "xchgw");
+}
+
+TEST_F(AssemblerX86Test, XchglReg) {
+  DriverStr(RepeatRR(&x86::X86Assembler::xchgl, "xchgl %{reg2}, %{reg1}"), "xchgl");
+}
+
+TEST_F(AssemblerX86Test, XchglMem) {
+  DriverStr(RepeatRA(&x86::X86Assembler::xchgl, "xchgl {mem}, %{reg}"), "xchgl");
 }
 
 TEST_F(AssemblerX86Test, Cmpxchgb) {