Eliminate proven array bounds checks

Absolute-CodeGen/src/codegen.cpp

@@ -1,6 +1,157 @@
 #include "codegen_internal.h"
 
+#include <llvm/ADT/SmallVector.h>
+#include <llvm/Analysis/LazyValueInfo.h>
+#include <llvm/Analysis/ScalarEvolution.h>
+#include <llvm/Transforms/Scalar/DCE.h>
+#include <llvm/Transforms/Scalar/SimplifyCFG.h>
+
 namespace Absolute {
+    namespace {
+        bool IsArrayBoundsFailure(const llvm::BasicBlock* block) {
+            return block && block->getName().find("array.bounds.failure") == 0 &&
+                llvm::isa<llvm::UnreachableInst>(block->getTerminator());
+        }
+
+        bool IsKnownIntegerComparison(
+            const llvm::ICmpInst& comparison,
+            bool expected,
+            llvm::Instruction* context,
+            llvm::LazyValueInfo& lazyValues,
+            llvm::ScalarEvolution& scalarEvolution) {
+            const llvm::ICmpInst::Predicate predicate = expected
+                ? comparison.getPredicate()
+                : llvm::CmpInst::getInversePredicate(comparison.getPredicate());
+            llvm::Value* left = comparison.getOperand(0);
+            llvm::Value* right = comparison.getOperand(1);
+
+            if (llvm::isa<llvm::Constant>(left) || llvm::isa<llvm::Constant>(right)) {
+                llvm::Constant* result = lazyValues.getPredicateAt(
+                    predicate, left, right, context, true);
+                if (const auto* known = llvm::dyn_cast_or_null<llvm::ConstantInt>(result))
+                    if (known->isOne()) return true;
+            }
+
+            if (!left->getType()->isIntegerTy() || left->getType() != right->getType())
+                return false;
+            return scalarEvolution.isKnownPredicateAt(
+                predicate,
+                scalarEvolution.getSCEV(left),
+                scalarEvolution.getSCEV(right),
+                context);
+        }
+
+        bool IsKnownBoolean(
+            llvm::Value* value,
+            bool expected,
+            llvm::Instruction* context,
+            llvm::LazyValueInfo& lazyValues,
+            llvm::ScalarEvolution& scalarEvolution,
+            unsigned depth = 0) {
+            if (!value || depth > 12 || !value->getType()->isIntegerTy(1)) return false;
+            if (const auto* constant = llvm::dyn_cast<llvm::ConstantInt>(value))
+                return constant->isOne() == expected;
+
+            llvm::Constant* known = lazyValues.getPredicateAt(
+                llvm::CmpInst::ICMP_EQ,
+                value,
+                llvm::ConstantInt::get(value->getType(), expected),
+                context,
+                true);
+            if (const auto* result = llvm::dyn_cast_or_null<llvm::ConstantInt>(known))
+                if (result->isOne()) return true;
+
+            if (const auto* comparison = llvm::dyn_cast<llvm::ICmpInst>(value)) {
+                if (IsKnownIntegerComparison(
+                    *comparison, expected, context, lazyValues, scalarEvolution)) return true;
+            }
+
+            const auto* binary = llvm::dyn_cast<llvm::BinaryOperator>(value);
+            if (!binary) return false;
+            llvm::Value* left = binary->getOperand(0);
+            llvm::Value* right = binary->getOperand(1);
+            switch (binary->getOpcode()) {
+            case llvm::Instruction::And:
+                if (expected)
+                    return IsKnownBoolean(
+                               left, true, context, lazyValues, scalarEvolution, depth + 1) &&
+                        IsKnownBoolean(
+                               right, true, context, lazyValues, scalarEvolution, depth + 1);
+                return IsKnownBoolean(
+                           left, false, context, lazyValues, scalarEvolution, depth + 1) ||
+                    IsKnownBoolean(
+                           right, false, context, lazyValues, scalarEvolution, depth + 1);
+            case llvm::Instruction::Or:
+                if (expected)
+                    return IsKnownBoolean(
+                               left, true, context, lazyValues, scalarEvolution, depth + 1) ||
+                        IsKnownBoolean(
+                               right, true, context, lazyValues, scalarEvolution, depth + 1);
+                return IsKnownBoolean(
+                           left, false, context, lazyValues, scalarEvolution, depth + 1) &&
+                    IsKnownBoolean(
+                           right, false, context, lazyValues, scalarEvolution, depth + 1);
+            case llvm::Instruction::Xor:
+                if (const auto* constant = llvm::dyn_cast<llvm::ConstantInt>(right)) {
+                    if (constant->isOne())
+                        return IsKnownBoolean(
+                            left, !expected, context, lazyValues, scalarEvolution, depth + 1);
+                    if (constant->isZero())
+                        return IsKnownBoolean(
+                            left, expected, context, lazyValues, scalarEvolution, depth + 1);
+                }
+                return false;
+            default:
+                return false;
+            }
+        }
+
+        class ProvenArrayBoundsPass final
+            : public llvm::PassInfoMixin<ProvenArrayBoundsPass> {
+        public:
+            llvm::PreservedAnalyses run(
+                llvm::Function& function,
+                llvm::FunctionAnalysisManager& analyses) {
+                llvm::LazyValueInfo& lazyValues =
+                    analyses.getResult<llvm::LazyValueAnalysis>(function);
+                llvm::ScalarEvolution& scalarEvolution =
+                    analyses.getResult<llvm::ScalarEvolutionAnalysis>(function);
+                llvm::SmallVector<std::pair<llvm::BranchInst*, llvm::BasicBlock*>, 8> rewrites;
+
+                for (llvm::BasicBlock& block : function) {
+                    auto* branch = llvm::dyn_cast<llvm::BranchInst>(block.getTerminator());
+                    if (!branch || !branch->isConditional()) continue;
+                    llvm::BasicBlock* first = branch->getSuccessor(0);
+                    llvm::BasicBlock* second = branch->getSuccessor(1);
+                    const bool firstFailure = IsArrayBoundsFailure(first);
+                    const bool secondFailure = IsArrayBoundsFailure(second);
+                    if (firstFailure == secondFailure) continue;
+
+                    const bool requiredCondition = secondFailure;
+                    if (!IsKnownBoolean(branch->getCondition(), requiredCondition, branch,
+                            lazyValues, scalarEvolution)) continue;
+                    rewrites.emplace_back(branch, firstFailure ? second : first);
+                }
+
+                for (auto& [branch, success] : rewrites) {
+                    llvm::BranchInst::Create(success, branch);
+                    branch->eraseFromParent();
+                }
+                return rewrites.empty()
+                    ? llvm::PreservedAnalyses::all()
+                    : llvm::PreservedAnalyses::none();
+            }
+        };
+
+        void AddAbsoluteOptimizationPasses(llvm::ModulePassManager& passes) {
+            llvm::FunctionPassManager functions;
+            functions.addPass(ProvenArrayBoundsPass());
+            functions.addPass(llvm::SimplifyCFGPass());
+            functions.addPass(llvm::DCEPass());
+            passes.addPass(llvm::createModuleToFunctionPassAdaptor(std::move(functions)));
+        }
+    }
+
     CodeGenerator::CodeGenerator(const Analyzer* analyzer)
         : impl(std::make_unique<Impl>(*this, analyzer)) {
     }
@@ -13,7 +164,74 @@ namespace Absolute {
 
     void CodeGenerator::GenerateObject(
         Program& program, const std::string& moduleName, const std::string& outputPath) {
-        impl->GenerateObject(program, moduleName, outputPath);
-    }
+        static std::once_flag initializeTarget;
+        std::call_once(initializeTarget, [] {
+            if (llvm::InitializeNativeTarget())
+                throw std::runtime_error("LLVM codegen: failed to initialize the native target");
+            if (llvm::InitializeNativeTargetAsmPrinter())
+                throw std::runtime_error("LLVM codegen: failed to initialize the native assembly printer");
+        });
+
+        const std::string triple = llvm::sys::getDefaultTargetTriple();
+        std::string targetError;
+        const llvm::Target* target = llvm::TargetRegistry::lookupTarget(triple, targetError);
+        if (!target)
+            throw std::runtime_error("cannot select target '" + triple + "': " + targetError);
+
+        llvm::TargetOptions options;
+        const std::string cpu = llvm::sys::getHostCPUName().str();
+        std::unique_ptr<llvm::TargetMachine> targetMachine(target->createTargetMachine(
+            triple, cpu.empty() ? "generic" : cpu, "", options, llvm::Reloc::PIC_,
+            std::nullopt, llvm::CodeGenOptLevel::Aggressive));
+        if (!targetMachine)
+            throw std::runtime_error("cannot create target machine for '" + triple + "'");
 
+        const std::string dataLayout =
+            targetMachine->createDataLayout().getStringRepresentation();
+        llvm::Module& generatedModule =
+            impl->BuildModule(program, moduleName, triple, dataLayout);
+
+        llvm::LoopAnalysisManager loopAnalyses;
+        llvm::FunctionAnalysisManager functionAnalyses;
+        llvm::CGSCCAnalysisManager cgsccAnalyses;
+        llvm::ModuleAnalysisManager moduleAnalyses;
+        llvm::PassBuilder passBuilder(targetMachine.get());
+        passBuilder.registerModuleAnalyses(moduleAnalyses);
+        passBuilder.registerCGSCCAnalyses(cgsccAnalyses);
+        passBuilder.registerFunctionAnalyses(functionAnalyses);
+        passBuilder.registerLoopAnalyses(loopAnalyses);
+        passBuilder.crossRegisterProxies(
+            loopAnalyses, functionAnalyses, cgsccAnalyses, moduleAnalyses);
+
+        llvm::ModulePassManager optimizationPasses =
+            passBuilder.buildPerModuleDefaultPipeline(llvm::OptimizationLevel::O3);
+        AddAbsoluteOptimizationPasses(optimizationPasses);
+        optimizationPasses.run(generatedModule, moduleAnalyses);
+
+        std::string verifierMessage;
+        llvm::raw_string_ostream verifierStream(verifierMessage);
+        if (llvm::verifyModule(generatedModule, &verifierStream)) {
+            verifierStream.flush();
+            throw std::runtime_error(
+                "optimized module verification failed: " + verifierMessage);
+        }
+
+        std::error_code error;
+        llvm::raw_fd_ostream output(outputPath, error, llvm::sys::fs::OF_None);
+        if (error)
+            throw std::runtime_error(
+                "cannot create object file '" + outputPath + "': " + error.message());
+
+        llvm::legacy::PassManager emissionPasses;
+#if LLVM_VERSION_MAJOR >= 18
+        constexpr llvm::CodeGenFileType fileType = llvm::CodeGenFileType::ObjectFile;
+#else
+        constexpr llvm::CodeGenFileType fileType = llvm::CGFT_ObjectFile;
+#endif
+        if (targetMachine->addPassesToEmitFile(
+            emissionPasses, output, nullptr, fileType))
+            throw std::runtime_error("target cannot emit an object file");
+        emissionPasses.run(generatedModule);
+        output.flush();
+    }
 }