x86: Add missing variants for XCHG and tests for this instruction.

Add XCHG variants when both operands are registers. Handle the special
case when at least one of the register operands is EAX/AX. Add tests for
the new variants and for other variants that previously weren't tested.

Bug: 65872996
Test: m test-art-host-gtest
Change-Id: I1e80d1b86f3d2e804b90c2a181e4eabded30d8ae

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) {