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[MLIR] Fix arbitrary checks in affine LICM #116469

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Nov 23, 2024
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[MLIR] Fix arbitrary checks in affine LICM #116469

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142 changes: 50 additions & 92 deletions mlir/lib/Dialect/Affine/Transforms/AffineLoopInvariantCodeMotion.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -12,25 +12,9 @@

#include "mlir/Dialect/Affine/Passes.h"

#include "mlir/Analysis/SliceAnalysis.h"
#include "mlir/Dialect/Affine/Analysis/AffineAnalysis.h"
#include "mlir/Dialect/Affine/Analysis/AffineStructures.h"
#include "mlir/Dialect/Affine/Analysis/LoopAnalysis.h"
#include "mlir/Dialect/Affine/Analysis/Utils.h"
#include "mlir/Dialect/Affine/IR/AffineOps.h"
#include "mlir/Dialect/Affine/LoopUtils.h"
#include "mlir/Dialect/Affine/Utils.h"
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/Dialect/Func/IR/FuncOps.h"
#include "mlir/IR/AffineExpr.h"
#include "mlir/IR/AffineMap.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/Matchers.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

Expand All @@ -41,17 +25,21 @@ namespace affine {
} // namespace affine
} // namespace mlir

#define DEBUG_TYPE "licm"
#define DEBUG_TYPE "affine-licm"

using namespace mlir;
using namespace mlir::affine;

namespace {

/// Affine loop invariant code motion (LICM) pass.
/// TODO: The pass is missing zero-trip tests.
/// TODO: This code should be removed once the new LICM pass can handle its
/// uses.
/// TODO: The pass is missing zero tripcount tests.
/// TODO: When compared to the other standard LICM pass, this pass
/// has some special handling for affine read/write ops but such handling
/// requires aliasing to be sound, and as such this pass is unsound. In
/// addition, this handling is nothing particular to affine memory ops but would
/// apply to any memory read/write effect ops. Either aliasing should be handled
/// or this pass can be removed and the standard LICM can be used.
struct LoopInvariantCodeMotion
: public affine::impl::AffineLoopInvariantCodeMotionBase<
LoopInvariantCodeMotion> {
Expand All @@ -61,100 +49,80 @@ struct LoopInvariantCodeMotion
} // namespace

static bool
checkInvarianceOfNestedIfOps(AffineIfOp ifOp, Value indVar, ValueRange iterArgs,
checkInvarianceOfNestedIfOps(AffineIfOp ifOp, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist);
static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
static bool isOpLoopInvariant(Operation &op, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist);

static bool
areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,
ValueRange iterArgs,
areAllOpsInTheBlockListInvariant(Region &blockList, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist);

// Returns true if the individual op is loop invariant.
static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,
/// Returns true if `op` is invariant on `loop`.
static bool isOpLoopInvariant(Operation &op, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist) {
LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
Value iv = loop.getInductionVar();

if (auto ifOp = dyn_cast<AffineIfOp>(op)) {
if (!checkInvarianceOfNestedIfOps(ifOp, indVar, iterArgs, opsWithUsers,
opsToHoist))
if (!checkInvarianceOfNestedIfOps(ifOp, loop, opsWithUsers, opsToHoist))
return false;
} else if (auto forOp = dyn_cast<AffineForOp>(op)) {
if (!areAllOpsInTheBlockListInvariant(forOp.getRegion(), indVar, iterArgs,
opsWithUsers, opsToHoist))
if (!areAllOpsInTheBlockListInvariant(forOp.getRegion(), loop, opsWithUsers,
opsToHoist))
return false;
} else if (auto parOp = dyn_cast<AffineParallelOp>(op)) {
if (!areAllOpsInTheBlockListInvariant(parOp.getRegion(), indVar, iterArgs,
opsWithUsers, opsToHoist))
if (!areAllOpsInTheBlockListInvariant(parOp.getRegion(), loop, opsWithUsers,
opsToHoist))
return false;
} else if (!isMemoryEffectFree(&op) &&
!isa<AffineReadOpInterface, AffineWriteOpInterface,
AffinePrefetchOp>(&op)) {
!isa<AffineReadOpInterface, AffineWriteOpInterface>(&op)) {
// Check for side-effecting ops. Affine read/write ops are handled
// separately below.
return false;
} else if (!matchPattern(&op, m_Constant())) {
} else if (isa<AffineReadOpInterface, AffineWriteOpInterface>(op)) {
// Register op in the set of ops that have users.
opsWithUsers.insert(&op);
if (isa<AffineReadOpInterface, AffineWriteOpInterface>(op)) {
auto read = dyn_cast<AffineReadOpInterface>(op);
Value memref = read ? read.getMemRef()
: cast<AffineWriteOpInterface>(op).getMemRef();
for (auto *user : memref.getUsers()) {
// If this memref has a user that is a DMA, give up because these
// operations write to this memref.
if (isa<AffineDmaStartOp, AffineDmaWaitOp>(user))
SmallVector<AffineForOp, 8> userIVs;
auto read = dyn_cast<AffineReadOpInterface>(op);
Value memref =
read ? read.getMemRef() : cast<AffineWriteOpInterface>(op).getMemRef();
for (auto *user : memref.getUsers()) {
// If the memref used by the load/store is used in a store elsewhere in
// the loop nest, we do not hoist. Similarly, if the memref used in a
// load is also being stored too, we do not hoist the load.
// FIXME: This is missing checking aliases.
if (&op == user)
continue;
if (hasEffect<MemoryEffects::Write>(user, memref) ||
(hasEffect<MemoryEffects::Read>(user, memref) &&
isa<AffineWriteOpInterface>(op))) {
userIVs.clear();
getAffineForIVs(*user, &userIVs);
// Check that userIVs don't contain the for loop around the op.
if (llvm::is_contained(userIVs, loop))
return false;
// If the memref used by the load/store is used in a store elsewhere in
// the loop nest, we do not hoist. Similarly, if the memref used in a
// load is also being stored too, we do not hoist the load.
if (isa<AffineWriteOpInterface>(user) ||
(isa<AffineReadOpInterface>(user) &&
isa<AffineWriteOpInterface>(op))) {
if (&op != user) {
SmallVector<AffineForOp, 8> userIVs;
getAffineForIVs(*user, &userIVs);
// Check that userIVs don't contain the for loop around the op.
if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar)))
return false;
}
}
}
}

if (op.getNumOperands() == 0 && !isa<AffineYieldOp>(op)) {
LLVM_DEBUG(llvm::dbgs() << "Non-constant op with 0 operands\n");
return false;
}
}

// Check operands.
ValueRange iterArgs = loop.getRegionIterArgs();
for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
auto *operandSrc = op.getOperand(i).getDefiningOp();

LLVM_DEBUG(
op.getOperand(i).print(llvm::dbgs() << "Iterating on operand\n"));

// If the loop IV is the operand, this op isn't loop invariant.
if (indVar == op.getOperand(i)) {
LLVM_DEBUG(llvm::dbgs() << "Loop IV is the operand\n");
if (iv == op.getOperand(i))
return false;
}

// If the one of the iter_args is the operand, this op isn't loop invariant.
if (llvm::is_contained(iterArgs, op.getOperand(i))) {
LLVM_DEBUG(llvm::dbgs() << "One of the iter_args is the operand\n");
if (llvm::is_contained(iterArgs, op.getOperand(i)))
return false;
}

if (operandSrc) {
LLVM_DEBUG(llvm::dbgs() << *operandSrc << "Iterating on operand src\n");

// If the value was defined in the loop (outside of the if/else region),
// and that operation itself wasn't meant to be hoisted, then mark this
// operation loop dependent.
Expand All @@ -170,14 +138,13 @@ static bool isOpLoopInvariant(Operation &op, Value indVar, ValueRange iterArgs,

// Checks if all ops in a region (i.e. list of blocks) are loop invariant.
static bool
areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,
ValueRange iterArgs,
areAllOpsInTheBlockListInvariant(Region &blockList, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist) {

for (auto &b : blockList) {
for (auto &op : b) {
if (!isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist))
if (!isOpLoopInvariant(op, loop, opsWithUsers, opsToHoist))
return false;
}
}
Expand All @@ -187,40 +154,36 @@ areAllOpsInTheBlockListInvariant(Region &blockList, Value indVar,

// Returns true if the affine.if op can be hoisted.
static bool
checkInvarianceOfNestedIfOps(AffineIfOp ifOp, Value indVar, ValueRange iterArgs,
checkInvarianceOfNestedIfOps(AffineIfOp ifOp, AffineForOp loop,
SmallPtrSetImpl<Operation *> &opsWithUsers,
SmallPtrSetImpl<Operation *> &opsToHoist) {
if (!areAllOpsInTheBlockListInvariant(ifOp.getThenRegion(), indVar, iterArgs,
if (!areAllOpsInTheBlockListInvariant(ifOp.getThenRegion(), loop,
opsWithUsers, opsToHoist))
return false;

if (!areAllOpsInTheBlockListInvariant(ifOp.getElseRegion(), indVar, iterArgs,
if (!areAllOpsInTheBlockListInvariant(ifOp.getElseRegion(), loop,
opsWithUsers, opsToHoist))
return false;

return true;
}

void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
auto *loopBody = forOp.getBody();
auto indVar = forOp.getInductionVar();
ValueRange iterArgs = forOp.getRegionIterArgs();

// This is the place where hoisted instructions would reside.
OpBuilder b(forOp.getOperation());

SmallPtrSet<Operation *, 8> opsToHoist;
SmallVector<Operation *, 8> opsToMove;
SmallPtrSet<Operation *, 8> opsWithUsers;

for (auto &op : *loopBody) {
for (Operation &op : *forOp.getBody()) {
// Register op in the set of ops that have users. This set is used
// to prevent hoisting ops that depend on these ops that are
// not being hoisted.
if (!op.use_empty())
opsWithUsers.insert(&op);
if (!isa<AffineYieldOp>(op)) {
if (isOpLoopInvariant(op, indVar, iterArgs, opsWithUsers, opsToHoist)) {
if (isOpLoopInvariant(op, forOp, opsWithUsers, opsToHoist)) {
opsToMove.push_back(&op);
}
}
Expand All @@ -231,18 +194,13 @@ void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
for (auto *op : opsToMove) {
op->moveBefore(forOp);
}

LLVM_DEBUG(forOp->print(llvm::dbgs() << "Modified loop\n"));
}

void LoopInvariantCodeMotion::runOnOperation() {
// Walk through all loops in a function in innermost-loop-first order. This
// way, we first LICM from the inner loop, and place the ops in
// the outer loop, which in turn can be further LICM'ed.
getOperation().walk([&](AffineForOp op) {
LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
runOnAffineForOp(op);
});
getOperation().walk([&](AffineForOp op) { runOnAffineForOp(op); });
}

std::unique_ptr<OperationPass<func::FuncOp>>
Expand Down
5 changes: 3 additions & 2 deletions mlir/test/Dialect/Affine/affine-loop-invariant-code-motion.mlir
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Original file line number Diff line number Diff line change
Expand Up @@ -855,15 +855,16 @@ func.func @affine_prefetch_invariant() {
affine.for %i0 = 0 to 10 {
affine.for %i1 = 0 to 10 {
%1 = affine.load %0[%i0, %i1] : memref<10x10xf32>
// A prefetch shouldn't be hoisted.
affine.prefetch %0[%i0, %i0], write, locality<0>, data : memref<10x10xf32>
}
}

// CHECK: memref.alloc() : memref<10x10xf32>
// CHECK-NEXT: affine.for %{{.*}} = 0 to 10 {
// CHECK-NEXT: affine.prefetch
// CHECK-NEXT: affine.for %{{.*}} = 0 to 10 {
// CHECK-NEXT: %{{.*}} = affine.load %{{.*}}[%{{.*}} : memref<10x10xf32>
// CHECK-NEXT: affine.load %{{.*}}[%{{.*}} : memref<10x10xf32>
// CHECK-NEXT: affine.prefetch
// CHECK-NEXT: }
// CHECK-NEXT: }
return
Expand Down
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