registerSpill.md

ChatGPT prompt: How would you write an LLVM optimization pass to lint for register spilling?

An LLVM optimization pass is a way to analyze or transform LLVM Intermediate Representation (IR) code. To write an LLVM optimization pass that lints for register spilling, you would need to analyze the register allocation process and identify cases where register spilling occurs. Here's a high-level outline of the steps you'd need to take:

Set up an LLVM pass: First, you need to set up a new LLVM pass. To do this, create a new class that inherits from llvm::FunctionPass. In this class, you'll implement the runOnFunction method, which will be called for each function in the module you're analyzing.

#include "llvm/Pass.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"

using namespace llvm;

namespace {
  struct RegisterSpillingLintPass : public FunctionPass {
    static char ID;
    RegisterSpillingLintPass() : FunctionPass(ID) {}

    bool runOnFunction(Function &F) override {
      // Your implementation goes here.
    }
  };
}

char RegisterSpillingLintPass::ID = 0;
static RegisterPass<RegisterSpillingLintPass> X("lint-register-spilling", "Register Spilling Lint Pass", false, false);

Analyze the register allocation process: To detect register spilling, you need to analyze the machine-level representation of your code, as LLVM IR does not directly expose register allocation information. You'll need to use LLVM's MachineFunctionPass to analyze the machine-level representation of your code.

#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"

using namespace llvm;

namespace {
  struct RegisterSpillingLintPass : public MachineFunctionPass {
    static char ID;
    RegisterSpillingLintPass() : MachineFunctionPass(ID) {}

    bool runOnMachineFunction(MachineFunction &MF) override {
      // Your implementation goes here.
    }
  };
}

char RegisterSpillingLintPass::ID = 0;
static RegisterPass<RegisterSpillingLintPass> X("lint-register-spilling", "Register Spilling Lint Pass", false, false);

Detect register spills: To detect register spills, iterate through each basic block and its instructions in the MachineFunction object. Analyze each instruction and check for any that represent spills or reloads. You can identify spills and reloads by looking for instructions that store and load registers to and from the stack.

bool runOnMachineFunction(MachineFunction &MF) override {
  for (auto &MBB : MF) {
    for (auto &MI : MBB) {
      // Check if MI represents a spill or reload operation.
      if (isSpillInstruction(MI) || isReloadInstruction(MI)) {
        // Report the spill or reload operation.
        reportSpillOrReload(MI);
      }
    }
  }
  return false;
}

Implement helper functions: Implement the helper functions isSpillInstruction, isReloadInstruction, and reportSpillOrReload to detect spill/reload operations and report them. Build and use the pass: Finally, build your LLVM pass and use it with the opt tool or as part of an LLVM-based compiler to analyze your code and detect register spills. Please note that the code snippets provided here are for illustration purposes only and may need adjustments to work correctly in your specific LLVM environment.