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[mlir][affine] Add fold logic when the affine.yield has IV as operand in the AffineForEmptyLoopFolder #165639

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[mlir][affine] Add fold logic when the affine.yield has IV as operand in the AffineForEmptyLoopFolder #165639

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a4e8857
add loop yield iv logic into AffineForEmptyLoopFolder.
linuxlonelyeagle Oct 18, 2025
0fa939c
Update mlir/lib/Dialect/Affine/IR/AffineOps.cpp
linuxlonelyeagle Oct 18, 2025
e4ebc2b
update.
linuxlonelyeagle Oct 18, 2025
b0ce9ff
Process trip count equals zero.
linuxlonelyeagle Oct 18, 2025
c816d09
rebase main and update code.
linuxlonelyeagle Oct 28, 2025
86c0c0b
make replacements from SmallVector<OpFoldResult,4> to SmallVector<OpF…
linuxlonelyeagle Oct 30, 2025
22b96ba
use llvm::to_vector return replacements.
linuxlonelyeagle Oct 30, 2025
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29 changes: 24 additions & 5 deletions mlir/lib/Dialect/Affine/IR/AffineOps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2610,6 +2610,21 @@ static std::optional<uint64_t> getTrivialConstantTripCount(AffineForOp forOp) {
return ub - lb <= 0 ? 0 : (ub - lb + step - 1) / step;
}

/// Calculate the constant value of the loop's induction variable for its last
/// trip.
static std::optional<int64_t>
getConstantInductionVarForLastTrip(AffineForOp forOp) {
std::optional<uint64_t> tripCount = getTrivialConstantTripCount(forOp);
if (!tripCount.has_value())
return std::nullopt;
if (tripCount.value() == 0)
return std::nullopt;
int64_t lb = forOp.getConstantLowerBound();
int64_t step = forOp.getStepAsInt();
int64_t lastTripIv = lb + (tripCount.value() - 1) * step;
return lastTripIv;
}

/// Fold the empty loop.
static SmallVector<OpFoldResult> AffineForEmptyLoopFolder(AffineForOp forOp) {
if (!llvm::hasSingleElement(*forOp.getBody()))
Expand All @@ -2622,18 +2637,22 @@ static SmallVector<OpFoldResult> AffineForEmptyLoopFolder(AffineForOp forOp) {
// results.
return forOp.getInits();
}
SmallVector<Value, 4> replacements;
SmallVector<OpFoldResult> replacements;
auto yieldOp = cast<AffineYieldOp>(forOp.getBody()->getTerminator());
auto iterArgs = forOp.getRegionIterArgs();
bool hasValDefinedOutsideLoop = false;
bool iterArgsNotInOrder = false;
for (unsigned i = 0, e = yieldOp->getNumOperands(); i < e; ++i) {
Value val = yieldOp.getOperand(i);
BlockArgument *iterArgIt = llvm::find(iterArgs, val);
// TODO: It should be possible to perform a replacement by computing the
// last value of the IV based on the bounds and the step.
if (val == forOp.getInductionVar())
if (val == forOp.getInductionVar()) {
if (auto lastTripIv = getConstantInductionVarForLastTrip(forOp)) {
replacements.push_back(IntegerAttr::get(
IndexType::get(forOp.getContext()), lastTripIv.value()));
continue;
}
return {};
}
if (iterArgIt == iterArgs.end()) {
// `val` is defined outside of the loop.
assert(forOp.isDefinedOutsideOfLoop(val) &&
Expand All @@ -2656,7 +2675,7 @@ static SmallVector<OpFoldResult> AffineForEmptyLoopFolder(AffineForOp forOp) {
// out of order.
if (tripCount.has_value() && tripCount.value() >= 2 && iterArgsNotInOrder)
return {};
return llvm::to_vector_of<OpFoldResult>(replacements);
return replacements;
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@kuhar kuhar Oct 30, 2025

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I don't think this is any different than the previous version -- to_vector_of<T> should pick the same small size as SmallVector<T>

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@kuhar kuhar Oct 30, 2025

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llvm-project/llvm/include/llvm/ADT/SmallVector.h

Lines 1318 to 1319 in e63f0f5

template <typename Out, typename R> SmallVector<Out> to_vector_of(R &&Range) {
return {adl_begin(Range), adl_end(Range)};

}

/// Canonicalize the bounds of the given loop.
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13 changes: 13 additions & 0 deletions mlir/test/Dialect/Affine/canonicalize.mlir
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Original file line number Diff line number Diff line change
Expand Up @@ -609,6 +609,19 @@ func.func @fold_zero_iter_loops(%in : index) -> index {

// -----

// CHECK-LABEL: func @fold_empty_loop_iv
// CHECK-SAME: %[[INIT:.*]]: index
func.func @fold_empty_loop_iv(%init: index) -> (index, index) {
%res:2 = affine.for %i = 0 to 10 step 1 iter_args(%arg0 = %init, %arg1 = %init) -> (index, index) {
affine.yield %i, %arg1 : index, index
}
// CHECK: %[[C9:.*]] = arith.constant 9 : index
// CHECK: return %[[C9]], %[[INIT]] : index, index
return %res#0, %res#1 : index, index
}

// -----

// CHECK-DAG: #[[$SET:.*]] = affine_set<(d0, d1)[s0] : (d0 >= 0, -d0 + 1022 >= 0, d1 >= 0, -d1 + s0 - 2 >= 0)>

// CHECK-LABEL: func @canonicalize_affine_if
Expand Down