[x64] Support SysV ABI in emitter

Add support for the SysV ABI in the x64 emitter backend by using the
appropriate registers for argument passing.

Since the SysV ABI has less preserved registers than the Microsoft x64
ABI, reduce by 2 the number of General Purpose Registers (GPR) available
to the emitter, so they can be used for the Context and Membase saved
registers.

Author: amessier

src/xenia/cpu/backend/x64/x64_emitter.cc

@@ -60,8 +60,10 @@ static const size_t kStashOffset = 32;
 
 const uint32_t X64Emitter::gpr_reg_map_[X64Emitter::GPR_COUNT] = {
     Xbyak::Operand::RBX, Xbyak::Operand::R10, Xbyak::Operand::R11,
-    Xbyak::Operand::R12, Xbyak::Operand::R13, Xbyak::Operand::R14,
-    Xbyak::Operand::R15,
+    Xbyak::Operand::R12, Xbyak::Operand::R13,
+#if XE_PLATFORM_WIN32
+    Xbyak::Operand::R14, Xbyak::Operand::R15,
+#endif
 };
 
 const uint32_t X64Emitter::xmm_reg_map_[X64Emitter::XMM_COUNT] = {
@@ -469,7 +471,11 @@ void X64Emitter::CallIndirect(const hir::Instr* instr,
   } else {
     // Old-style resolve.
     // Not too important because indirection table is almost always available.
+#if XE_PLATFORM_WIN32
     mov(edx, reg.cvt32());
+#else
+    mov(esi, reg.cvt32());
+#endif
     mov(rax, reinterpret_cast<uint64_t>(ResolveFunction));
     mov(GetNativeReg(0), GetContextReg());
     call(rax);
@@ -582,6 +588,7 @@ void X64Emitter::SetReturnAddress(uint64_t value) {
 }
 
 Xbyak::Reg64 X64Emitter::GetNativeReg(uint32_t reg) {
+#if XE_PLATFORM_WIN32
   if (reg == 0)
     return rcx;
   else if (reg == 1)
@@ -593,15 +600,41 @@ Xbyak::Reg64 X64Emitter::GetNativeReg(uint32_t reg) {
 
   assert_always();
   return r9;
+#else
+  if (reg == 0)
+    return rdi;
+  else if (reg == 1)
+    return rsi;
+  else if (reg == 2)
+    return rdx;
+  else if (reg == 3)
+    return rcx;
+
+  assert_always();
+  return rcx;
+#endif
 }
 
 Xbyak::Reg64 X64Emitter::GetNativeParam(uint32_t param) {
   return GetNativeReg(param + 1);
 }
 
 // Important: If you change these, you must update the thunks in x64_backend.cc!
-Xbyak::Reg64 X64Emitter::GetContextReg() { return rsi; }
-Xbyak::Reg64 X64Emitter::GetMembaseReg() { return rdi; }
+Xbyak::Reg64 X64Emitter::GetContextReg() {
+#if XE_PLATFORM_WIN32
+  return rsi;
+#else
+  return r14;
+#endif
+}
+
+Xbyak::Reg64 X64Emitter::GetMembaseReg() {
+#if XE_PLATFORM_WIN32
+  return rdi;
+#else
+  return r15;
+#endif
+}
 
 void X64Emitter::ReloadContext() {
   mov(GetContextReg(), qword[rsp + StackLayout::GUEST_CTX_HOME]);

src/xenia/cpu/backend/x64/x64_emitter.h

@@ -161,13 +161,23 @@ class X64Emitter : public Xbyak::CodeGenerator {
             std::vector<SourceMapEntry>* out_source_map);
 
  public:
+#if XE_PLATFORM_WIN32
   // Reserved:  rsp, rsi, rdi
   // Scratch:   rax/rcx/rdx
   //            xmm0-2
   // Available: rbx, r10-r15
   //            xmm4-xmm15 (save to get xmm3)
   static const int GPR_COUNT = 7;
   static const int XMM_COUNT = 12;
+#else
+  // Reserved:  rsp, r14, r15
+  // Scratch:   rax/rdi/rsi/rcx/rdx
+  //            xmm0-2
+  // Available: rbx, r10-r13
+  //            xmm4-xmm15 (save to get xmm3)
+  static const int GPR_COUNT = 5;
+  static const int XMM_COUNT = 12;
+#endif
 
   static void SetupReg(const hir::Value* v, Xbyak::Reg8& r) {
     auto idx = gpr_reg_map_[v->reg.index];