[llvm] [Exegesis] Implemented strategy for load operation (PR #113458)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-tools-llvm-exegesis

Author: None (AnastasiyaChernikova)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/113458.diff


1 Files Affected:

- (modified) llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp (+45-1) 


``````````diff
diff --git a/llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp b/llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp
index 7100b51bbb7298..ac4881f298618d 100644
--- a/llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp
+++ b/llvm/tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp
@@ -113,7 +113,51 @@ 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: {

``````````

</details>


https://github.com/llvm/llvm-project/pull/113458