[Exegesis] Implemented strategy for load operation

This fix helps to map operand memory to destination registers.
If instruction is load, we can self-alias it in case when instruction
overrides whole address register. For that we use provided scratch memory.

Author: AnastasiyaChernikova

llvm/tools/llvm-exegesis/lib/CodeTemplate.h

@@ -132,6 +132,9 @@ struct CodeTemplate {
   // If the template uses the provided scratch memory, the register in which
   // the pointer to this memory is passed in to the function.
   unsigned ScratchSpacePointerInReg = 0;
+  // Require to pre-store value of a given register (fisrt)
+  // to scratch memory with given offset (second)
+  SmallVector<std::pair<unsigned, unsigned>, 2> PreinitScratchMemory;
 
 #if defined(__GNUC__) && (defined(__clang__) || LLVM_GNUC_PREREQ(8, 0, 0))
   // FIXME: GCC7 bug workaround. Drop #if after GCC7 no longer supported.

llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp

@@ -104,7 +104,51 @@ static void appendCodeTemplates(const LLVMState &State,
   }
   case ExecutionMode::SERIAL_VIA_MEMORY_INSTR: {
     // Select back-to-back memory instruction.
-    // TODO: Implement me.
+
+    auto &I = Variant.getInstr();
+    if (I.Description.mayLoad()) {
+      // If instruction is load, we can self-alias it in case when instruction
+      // overrides whole address register. For that we use provided scratch
+      // memory.
+
+      // TODO: now it is not checked if load writes the whole register.
+
+      auto DefOpIt = find_if(I.Operands, [](Operand const &op) {
+        return op.isDef() && op.isReg();
+      });
+
+      if (DefOpIt == I.Operands.end())
+        return;
+
+      const Operand &DefOp = *DefOpIt;
+      auto &ET = State.getExegesisTarget();
+      auto ScratchMemoryRegister = ET.getScratchMemoryRegister(
+          State.getTargetMachine().getTargetTriple());
+      auto &RegClass =
+          State.getTargetMachine().getMCRegisterInfo()->getRegClass(
+              DefOp.getExplicitOperandInfo().RegClass);
+
+      // Register classes of def operand and memory operand must be the same
+      // to perform aliasing.
+      if (!RegClass.contains(ScratchMemoryRegister))
+        return;
+
+      ET.fillMemoryOperands(Variant, ScratchMemoryRegister, 0);
+      Variant.getValueFor(DefOp) = MCOperand::createReg(ScratchMemoryRegister);
+
+      CodeTemplate CT;
+      CT.Execution = ExecutionModeBit;
+      if (CT.ScratchSpacePointerInReg == 0)
+        CT.ScratchSpacePointerInReg = ScratchMemoryRegister;
+
+      CT.Info = std::string(ExecutionClassDescription);
+      CT.Instructions.push_back(std::move(Variant));
+      CT.PreinitScratchMemory.emplace_back(ScratchMemoryRegister,
+                                           /* Offset */ 0);
+      CodeTemplates.push_back(std::move(CT));
+    }
+
+    // TODO: implement more cases
     return;
   }
   case ExecutionMode::SERIAL_VIA_EXPLICIT_REGS: {