[flang-commits] [flang] [flang] Added LoopInvariantCodeMotion pass for [HL]FIR. (PR #173438)
Tom Eccles via flang-commits
flang-commits at lists.llvm.org
Fri Jan 2 04:35:08 PST 2026
================
@@ -0,0 +1,233 @@
+//===- LoopInvariantCodeMotion.cpp ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// FIR-specific Loop Invariant Code Motion pass.
+/// The pass relies on FIR types and interfaces to prove the safety
+/// of hoisting invariant operations out of loop-like operations.
+/// It may be run on both HLFIR and FIR representations.
+//===----------------------------------------------------------------------===//
+
+#include "flang/Optimizer/Analysis/AliasAnalysis.h"
+#include "flang/Optimizer/Dialect/FortranVariableInterface.h"
+#include "flang/Optimizer/Transforms/Passes.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/LoopInvariantCodeMotionUtils.h"
+#include "llvm/Support/DebugLog.h"
+
+namespace fir {
+#define GEN_PASS_DEF_LOOPINVARIANTCODEMOTION
+#include "flang/Optimizer/Transforms/Passes.h.inc"
+} // namespace fir
+
+#define DEBUG_TYPE "flang-licm"
+
+// Temporary engineering option for triaging LICM.
+static llvm::cl::opt<bool> disableFlangLICM(
+ "disable-flang-licm", llvm::cl::init(false), llvm::cl::Hidden,
+ llvm::cl::desc("Disable Flang's loop invariant code motion"));
+
+namespace {
+
+using namespace mlir;
+
+/// The pass tries to hoist loop invariant operations with only
+/// MemoryEffects::Read effects (MemoryEffects::Write support
+/// may be added later).
+/// The safety of hoisting is proven by:
+/// * Proving that the loop runs at least one iteration.
+/// * Proving that is is always safe to load from this location
+/// (see isSafeToHoistLoad() comments below).
+struct LoopInvariantCodeMotion
+ : fir::impl::LoopInvariantCodeMotionBase<LoopInvariantCodeMotion> {
+ void runOnOperation() override;
+};
+
+} // namespace
+
+/// 'location' is a memory reference used by a memory access.
+/// The type of 'location' defines the data type of the access
+/// (e.g. it is considered to be invalid to access 'i64'
+/// data using '!fir.ref<i32>`).
+/// For the given location, this function returns true iff
+/// the Fortran object being accessed is a scalar that
+/// may not be OPTIONAL.
+///
+/// Note that the '!fir.ref<!fir.box<>>' accesses are considered
+/// to be scalar, even if the underlying data is an array.
+///
+/// Note that an access of '!fir.ref<scalar>' may access
+/// an array object. For example:
+/// real :: x(:)
+/// do i=...
+/// = x(10)
+/// 'x(10)' accesses array 'x', and it may be unsafe to hoist
+/// it without proving that '10' is a valid index for the array.
+/// The fact that 'x' is not OPTIONAL does not allow hoisting
+/// on its own.
+static bool isNonOptionalScalar(Value location) {
+ while (true) {
+ LDBG() << "Checking location:\n" << location;
+ Type dataType = fir::unwrapRefType(location.getType());
+ if (!isa<fir::BaseBoxType>(location.getType()) &&
+ (!dataType ||
+ (!isa<fir::BaseBoxType>(dataType) && !fir::isa_trivial(dataType) &&
+ !fir::isa_derived(dataType)))) {
+ LDBG() << "Failure: data access is not scalar";
+ return false;
+ }
+ Operation *defOp = location.getDefiningOp();
+ if (!defOp) {
+ LDBG() << "Failure: no defining operation";
+ return false;
+ }
+ if (auto varIface = dyn_cast<fir::FortranVariableOpInterface>(defOp)) {
+ bool result = !varIface.isOptional();
+ if (result)
+ LDBG() << "Success: is non optional scalar";
+ else
+ LDBG() << "Failure: is not non optional scalar";
+ return result;
+ }
+ if (auto viewIface = dyn_cast<fir::FortranObjectViewOpInterface>(defOp)) {
+ location = viewIface.getViewSource(cast<OpResult>(location));
+ } else {
+ LDBG() << "Failure: unknown operation:\n" << *defOp;
+ return false;
+ }
+ }
+}
+
+/// Returns true iff it is safe to hoist the given load-like operation 'op',
+/// which access given memory 'locations', out of the operation 'loopLike'.
+/// The current safety conditions are:
+/// * The loop runs at least one iteration, OR
+/// * all the accessed locations are inside scalar non-OPTIONAL
+/// Fortran objects (Fortran descriptors are considered to be scalars).
+static bool isSafeToHoistLoad(Operation *op, ArrayRef<Value> locations,
+ LoopLikeOpInterface loopLike,
+ AliasAnalysis &aliasAnalysis) {
+ for (Value location : locations)
+ if (aliasAnalysis.getModRef(loopLike.getOperation(), location)
+ .isModAndRef()) {
+ LDBG() << "Failure: reads location:\n"
+ << location << "\nwhich is modified inside the loop";
+ return false;
+ }
+
+ // Check that it is safe to read from all the locations before the loop.
+ std::optional<llvm::APInt> tripCount = loopLike.getStaticTripCount();
+ if (tripCount && !tripCount->isZero()) {
+ // Loop executes at least one iteration, so it is safe to hoist.
+ LDBG() << "Success: loop has non-zero iterations";
+ return true;
+ }
+
+ // Check whether the access must always be valid.
+ return llvm::all_of(
+ locations, [&](Value location) { return isNonOptionalScalar(location); });
+ // TODO: consider hoisting under condition of the loop's trip count
+ // being non-zero.
+}
+
+/// Returns true iff the given 'op' is a load-like operation,
+/// and it can be hoisted out of 'loopLike' operation.
+static bool canHoistLoad(Operation *op, LoopLikeOpInterface loopLike,
+ AliasAnalysis &aliasAnalysis) {
+ LDBG() << "Checking operation:\n" << *op;
+ if (auto effectInterface = dyn_cast<MemoryEffectOpInterface>(op)) {
+ SmallVector<MemoryEffects::EffectInstance> effects;
+ effectInterface.getEffects(effects);
+ if (effects.empty()) {
+ LDBG() << "Failure: not a load";
+ return false;
+ }
+ llvm::SetVector<Value> locations;
+ for (const MemoryEffects::EffectInstance &effect : effects) {
+ Value location = effect.getValue();
+ if (!isa<MemoryEffects::Read>(effect.getEffect())) {
+ LDBG() << "Failure: has unsupported effects";
+ return false;
+ } else if (!location) {
+ LDBG() << "Failure: reads from unknown location";
+ return false;
+ }
+ locations.insert(location);
+ }
+ return isSafeToHoistLoad(op, locations.getArrayRef(), loopLike,
+ aliasAnalysis);
+ }
+ LDBG() << "Failure: has unknown effects";
+ return false;
+}
+
+void LoopInvariantCodeMotion::runOnOperation() {
+ if (disableFlangLICM) {
+ LDBG() << "Skipping [HL]FIR LoopInvariantCodeMotion()";
+ return;
+ }
+
+ LDBG() << "Enter [HL]FIR LoopInvariantCodeMotion()";
+
+ auto &aliasAnalysis = getAnalysis<AliasAnalysis>();
+ aliasAnalysis.addAnalysisImplementation(fir::AliasAnalysis{});
+
+ std::function<bool(Operation *, LoopLikeOpInterface loopLike)>
+ shouldMoveOutOfLoop = [&](Operation *op, LoopLikeOpInterface loopLike) {
+ if (isPure(op)) {
+ LDBG() << "Pure operation: " << *op;
+ return true;
+ }
+
+ // Handle RecursivelySpeculatable operations that have
+ // RecursiveMemoryEffects by checking if all their
+ // nested operations can be hoisted.
+ auto iface = dyn_cast<ConditionallySpeculatable>(op);
+ if (iface && iface.getSpeculatability() ==
+ Speculation::RecursivelySpeculatable) {
+ if (op->hasTrait<OpTrait::HasRecursiveMemoryEffects>()) {
+ LDBG() << "Checking recursive operation:\n" << *op;
+ llvm::SmallVector<Operation *> nestedOps;
+ for (Region ®ion : op->getRegions())
+ for (Block &block : region)
+ for (Operation &nestedOp : block)
+ nestedOps.push_back(&nestedOp);
+
+ bool result = llvm::all_of(nestedOps, [&](Operation *nestedOp) {
+ return shouldMoveOutOfLoop(nestedOp, loopLike);
+ });
+ LDBG() << "Recursive operation can" << (result ? "" : "not")
+ << " be hoisted";
+
+ // If nested operations cannot be hoisted, there is nothing
+ // else to check. Also if the operation itself does not have
+ // any memory effects, we can return the result now.
+ // Otherwise, we have to check the operation itself below.
+ if (!result || !isa<MemoryEffectOpInterface>(op))
+ return result;
+ }
+ }
+ return canHoistLoad(op, loopLike, aliasAnalysis);
----------------
tblah wrote:
Could be another PR, but looking at https://github.com/llvm/llvm-project/issues/62419, I think it could be useful to hoist operations with allocation effects too.
https://github.com/llvm/llvm-project/pull/173438
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