Now tracking constant values for most operations.

This will make it easier to enable memory access optimizations and hacks.
Not all operations are updating the constant registers, only those that
are likely to participate in constant value building. Constant values are
cleared on every basic block, though this may be too zealous it prevents
weird fall-through cases.

src/xenia/cpu/x64/x64_emitter.cc

@@ -243,6 +243,8 @@ int X64Emitter::MakeFunction(FunctionSymbol* symbol) {
 
   access_bits_.Clear();
 
+  clear_all_constant_gpr_values();
+
   locals_.indirection_target = GpVar();
   locals_.indirection_cia = GpVar();
 
@@ -550,6 +552,13 @@ void X64Emitter::GenerateBasicBlock(FunctionBlock* block) {
     c.comment("bb %.8X - %.8X", block->start_address, block->end_address);
   }
 
+  // Reset constant values.
+  // They may be valid, but it's hard to tell.
+  // If we wanted we could ensure that any incoming edges were immediately
+  // preceeding us and not something that was messing with the values, however
+  // most constant values are set within their own blocks anyway.
+  clear_all_constant_gpr_values();
+
   // This will create a label if it hasn't already been done.
   std::map<uint32_t, Label>::iterator label_it =
       bbs_.find(block->start_address);
@@ -1225,6 +1234,32 @@ void X64Emitter::SpillRegisters() {
   }
 }
 
+bool X64Emitter::get_constant_gpr_value(uint32_t n, uint64_t* value) {
+  if (gpr_values_[n].is_constant) {
+    *value = gpr_values_[n].value;
+    return true;
+  } else {
+    return false;
+  }
+}
+
+void X64Emitter::set_constant_gpr_value(uint32_t n, uint64_t value) {
+  gpr_values_[n].is_constant = true;
+  gpr_values_[n].value = value;
+}
+
+void X64Emitter::clear_constant_gpr_value(uint32_t n) {
+  gpr_values_[n].is_constant = false;
+  gpr_values_[n].value = 0;
+}
+
+void X64Emitter::clear_all_constant_gpr_values() {
+  for (size_t n = 0; n < XECOUNT(gpr_values_); n++) {
+    gpr_values_[n].is_constant = false;
+    gpr_values_[n].value = 0;
+  }
+}
+
 GpVar X64Emitter::xer_value() {
   X86Compiler& c = compiler_;
   if (FLAGS_cache_registers) {