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Merged
googlewalt merged 2 commits into from
Dec 30, 2025
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Revert "[mlir][scf] Fold away scf.for iter args cycles (#173436)" #173991

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f45ec52
Revert "[mlir][SCF] Fold unused `index_switch` results (#173560)"
googlewalt Dec 30, 2025
0bb1e0f
Revert "[mlir][scf] Fold away `scf.for` iter args cycles (#173436)"
googlewalt Dec 30, 2025
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164 changes: 14 additions & 150 deletions mlir/lib/Dialect/SCF/IR/SCF.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -990,8 +990,9 @@ mlir::scf::replaceAndCastForOpIterArg(RewriterBase &rewriter, scf::ForOp forOp,

namespace {
// Fold away ForOp iter arguments when:
// 1) The argument's corresponding outer region iterators (inputs) are yielded.
// 2) The iter arguments have no use and the corresponding (operation) results
// 1) The op yields the iter arguments.
// 2) The argument's corresponding outer region iterators (inputs) are yielded.
// 3) The iter arguments have no use and the corresponding (operation) results
// have no use.
//
// These arguments must be defined outside of the ForOp region and can just be
Expand All @@ -1000,7 +1001,7 @@ namespace {
// The implementation uses `inlineBlockBefore` to steal the content of the
// original ForOp and avoid cloning.
struct ForOpIterArgsFolder : public OpRewritePattern<scf::ForOp> {
using Base::Base;
using OpRewritePattern<scf::ForOp>::OpRewritePattern;

LogicalResult matchAndRewrite(scf::ForOp forOp,
PatternRewriter &rewriter) const final {
Expand Down Expand Up @@ -1029,11 +1030,12 @@ struct ForOpIterArgsFolder : public OpRewritePattern<scf::ForOp> {
forOp.getYieldedValues() // iter yield
)) {
// Forwarded is `true` when:
// 1) The region `iter` argument the corresponding input is yielded.
// 2) The region `iter` argument has no use, and the corresponding op
// 1) The region `iter` argument is yielded.
// 2) The region `iter` argument the corresponding input is yielded.
// 3) The region `iter` argument has no use, and the corresponding op
// result has no use.
bool forwarded =
(init == yielded) || (arg.use_empty() && result.use_empty());
bool forwarded = (arg == yielded) || (init == yielded) ||
(arg.use_empty() && result.use_empty());
if (forwarded) {
canonicalize = true;
keepMask.push_back(false);
Expand Down Expand Up @@ -1131,7 +1133,7 @@ struct ForOpIterArgsFolder : public OpRewritePattern<scf::ForOp> {
/// single-iteration loops with their bodies, and removes empty loops that
/// iterate at least once and only return values defined outside of the loop.
struct SimplifyTrivialLoops : public OpRewritePattern<ForOp> {
using Base::Base;
using OpRewritePattern<ForOp>::OpRewritePattern;

LogicalResult matchAndRewrite(ForOp op,
PatternRewriter &rewriter) const override {
Expand Down Expand Up @@ -1202,7 +1204,7 @@ struct SimplifyTrivialLoops : public OpRewritePattern<ForOp> {
/// use_of(%1)
/// ```
struct ForOpTensorCastFolder : public OpRewritePattern<ForOp> {
using Base::Base;
using OpRewritePattern<ForOp>::OpRewritePattern;

LogicalResult matchAndRewrite(ForOp op,
PatternRewriter &rewriter) const override {
Expand Down Expand Up @@ -1234,100 +1236,12 @@ struct ForOpTensorCastFolder : public OpRewritePattern<ForOp> {
}
};

/// Rewriting pattern that folds away cycles in the yield of a scf.for op.
///
/// ```
/// %res:2 = scf.for ... iter_args(%arg0 = %init, %arg1 = %init) {
/// ...
/// use %arg0, %arg1
/// scf.yield %arg1, %arg0
/// }
/// return %res#0, %res#1
/// ```
///
/// folds into:
///
/// ```
/// scf.for ... iter_args() {
/// ...
/// use %init, %init
/// scf.yield
/// }
/// return %init, %init
/// ```
struct ForOpYieldCyclesFolder : public OpRewritePattern<ForOp> {
using Base::Base;

LogicalResult matchAndRewrite(ForOp op,
PatternRewriter &rewriter) const override {
ValueRange yieldedValues = op.getYieldedValues();
ValueRange initArgs = op.getInitArgs();
ValueRange results = op.getResults();
ValueRange regionIterArgs = op.getRegionIterArgs();
Block *body = op.getBody();

unsigned numYieldedValues = op.getNumRegionIterArgs();

bool changed = false;
SmallVector<unsigned> cycle;
llvm::SmallBitVector visited(numYieldedValues, false);

// Go through all possible start points for the cycle.
for (auto start : llvm::seq(numYieldedValues)) {
if (visited[start])
continue;

cycle.clear();
unsigned current = start;
bool validCycle = true;
Value initValue = initArgs[start];
// Go through yield -> block arg -> yield cycles and check if all values
// are always equal to the init.
while (!visited[current]) {
cycle.push_back(current);
visited[current] = true;

// Find whether this yield is from a region iter arg.
auto yieldedValue = yieldedValues[current];
if (auto arg = dyn_cast<BlockArgument>(yieldedValue);
!arg || arg.getOwner() != body) {
validCycle = false;
break;
}

// Next yield position.
current = cast<BlockArgument>(yieldedValue).getArgNumber() -
op.getNumInductionVars();

// Check if next position has the same init value.
if (initArgs[current] != initValue) {
validCycle = false;
break;
}
}

// If we found a valid cycle (yielding own iter arg forms cycle of length
// 1), all values in it are always equal to initValue.
if (validCycle) {
changed = true;
for (unsigned idx : cycle) {
// This will leave region args and results dead so other
// canonicalization patterns can clean them up.
rewriter.replaceAllUsesWith(regionIterArgs[idx], initValue);
rewriter.replaceAllUsesWith(results[idx], initValue);
}
}
}
return success(changed);
}
};

} // namespace

void ForOp::getCanonicalizationPatterns(RewritePatternSet &results,
MLIRContext *context) {
results.add<ForOpIterArgsFolder, SimplifyTrivialLoops, ForOpTensorCastFolder,
ForOpYieldCyclesFolder>(context);
results.add<ForOpIterArgsFolder, SimplifyTrivialLoops, ForOpTensorCastFolder>(
context);
}

std::optional<APInt> ForOp::getConstantStep() {
Expand Down Expand Up @@ -4797,59 +4711,9 @@ struct FoldConstantCase : OpRewritePattern<scf::IndexSwitchOp> {
}
};

/// Canonicalization patterns that folds away dead results of
/// "scf.index_switch" ops.
struct FoldUnusedIndexSwitchResults : OpRewritePattern<IndexSwitchOp> {
using OpRewritePattern<IndexSwitchOp>::OpRewritePattern;

LogicalResult matchAndRewrite(IndexSwitchOp op,
PatternRewriter &rewriter) const override {
// Find dead results.
BitVector deadResults(op.getNumResults(), false);
SmallVector<Type> newResultTypes;
for (auto [idx, result] : llvm::enumerate(op.getResults())) {
if (!result.use_empty()) {
newResultTypes.push_back(result.getType());
} else {
deadResults[idx] = true;
}
}
if (!deadResults.any())
return rewriter.notifyMatchFailure(op, "no dead results to fold");

// Create new op without dead results and inline case regions.
auto newOp = IndexSwitchOp::create(rewriter, op.getLoc(), newResultTypes,
op.getArg(), op.getCases(),
op.getCaseRegions().size());
auto inlineCaseRegion = [&](Region &oldRegion, Region &newRegion) {
rewriter.inlineRegionBefore(oldRegion, newRegion, newRegion.begin());
// Remove respective operands from yield op.
Operation *terminator = newRegion.front().getTerminator();
assert(isa<YieldOp>(terminator) && "expected yield op");
rewriter.modifyOpInPlace(
terminator, [&]() { terminator->eraseOperands(deadResults); });
};
for (auto [oldRegion, newRegion] :
llvm::zip_equal(op.getCaseRegions(), newOp.getCaseRegions()))
inlineCaseRegion(oldRegion, newRegion);
inlineCaseRegion(op.getDefaultRegion(), newOp.getDefaultRegion());

// Replace op with new op.
SmallVector<Value> newResults(op.getNumResults(), Value());
unsigned nextNewResult = 0;
for (unsigned idx = 0; idx < op.getNumResults(); ++idx) {
if (deadResults[idx])
continue;
newResults[idx] = newOp.getResult(nextNewResult++);
}
rewriter.replaceOp(op, newResults);
return success();
}
};

void IndexSwitchOp::getCanonicalizationPatterns(RewritePatternSet &results,
MLIRContext *context) {
results.add<FoldConstantCase, FoldUnusedIndexSwitchResults>(context);
results.add<FoldConstantCase>(context);
}

//===----------------------------------------------------------------------===//
Expand Down
115 changes: 24 additions & 91 deletions mlir/test/Dialect/SCF/canonicalize.mlir
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Original file line number Diff line number Diff line change
Expand Up @@ -1665,11 +1665,11 @@ func.func @func_execute_region_inline_multi_yield() {
module {
func.func private @foo()->()
func.func private @execute_region_yeilding_external_value() -> memref<1x60xui8> {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%1 = scf.execute_region -> memref<1x60xui8> no_inline {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%1 = scf.execute_region -> memref<1x60xui8> no_inline {
func.call @foo():()->()
scf.yield %alloc: memref<1x60xui8>
}
}
return %1 : memref<1x60xui8>
}
}
Expand All @@ -1688,12 +1688,12 @@ func.func private @execute_region_yeilding_external_value() -> memref<1x60xui8>
module {
func.func private @foo()->()
func.func private @execute_region_yeilding_external_and_local_values() -> (memref<1x60xui8>, memref<1x120xui8>) {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%1, %2 = scf.execute_region -> (memref<1x60xui8>, memref<1x120xui8>) no_inline {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%1, %2 = scf.execute_region -> (memref<1x60xui8>, memref<1x120xui8>) no_inline {
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
func.call @foo():()->()
scf.yield %alloc, %alloc_1: memref<1x60xui8>, memref<1x120xui8>
}
}
return %1, %2 : memref<1x60xui8>, memref<1x120xui8>
}
}
Expand All @@ -1716,18 +1716,18 @@ func.func private @execute_region_yeilding_external_and_local_values() -> (memre
module {
func.func private @foo()->()
func.func private @execute_region_multiple_yields_same_operands() -> (memref<1x60xui8>, memref<1x120xui8>) {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%1, %2 = scf.execute_region -> (memref<1x60xui8>, memref<1x120xui8>) no_inline {
%c = "test.cmp"() : () -> i1
cf.cond_br %c, ^bb2, ^bb3
^bb2:
^bb2:
func.call @foo():()->()
scf.yield %alloc, %alloc_1 : memref<1x60xui8>, memref<1x120xui8>
^bb3:
func.call @foo():()->()
^bb3:
func.call @foo():()->()
scf.yield %alloc, %alloc_1 : memref<1x60xui8>, memref<1x120xui8>
}
}
return %1, %2 : memref<1x60xui8>, memref<1x120xui8>
}
}
Expand All @@ -1746,19 +1746,19 @@ module {
module {
func.func private @foo()->()
func.func private @execute_region_multiple_yields_different_operands() -> (memref<1x60xui8>, memref<1x120xui8>) {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%alloc_2 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%alloc_2 = memref.alloc() {alignment = 64 : i64} : memref<1x120xui8>
%1, %2 = scf.execute_region -> (memref<1x60xui8>, memref<1x120xui8>) no_inline {
%c = "test.cmp"() : () -> i1
cf.cond_br %c, ^bb2, ^bb3
^bb2:
^bb2:
func.call @foo():()->()
scf.yield %alloc, %alloc_1 : memref<1x60xui8>, memref<1x120xui8>
^bb3:
func.call @foo():()->()
^bb3:
func.call @foo():()->()
scf.yield %alloc, %alloc_2 : memref<1x60xui8>, memref<1x120xui8>
}
}
return %1, %2 : memref<1x60xui8>, memref<1x120xui8>
}
}
Expand All @@ -1778,18 +1778,18 @@ module {
module {
func.func private @foo()->()
func.func private @execute_region_multiple_yields_different_operands() -> (memref<1x60xui8>) {
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%alloc_1 = memref.alloc() {alignment = 64 : i64} : memref<1x60xui8>
%1 = scf.execute_region -> (memref<1x60xui8>) no_inline {
%c = "test.cmp"() : () -> i1
cf.cond_br %c, ^bb2, ^bb3
^bb2:
^bb2:
func.call @foo():()->()
scf.yield %alloc : memref<1x60xui8>
^bb3:
^bb3:
func.call @foo():()->()
scf.yield %alloc_1 : memref<1x60xui8>
}
}
return %1 : memref<1x60xui8>
}
}
Expand Down Expand Up @@ -2171,70 +2171,3 @@ func.func @scf_for_all_step_size_0() {
}
return
}

// -----

func.func private @side_effect()

// CHECK-LABEL: func @iter_args_cycles
// CHECK-SAME: (%[[LB:.*]]: index, %[[UB:.*]]: index, %[[STEP:.*]]: index, %[[A:.*]]: i32, %[[B:.*]]: i64, %[[C:.*]]: f32)
// CHECK: scf.for %[[IV:.*]] = %[[LB]] to %[[UB]] step %[[STEP]] {
// CHECK: func.call @side_effect()
// CHECK-NOT: yield
// CHECK: return %[[A]], %[[B]], %[[A]], %[[B]], %[[B]], %[[C]] : i32, i64, i32, i64, i64, f32
func.func @iter_args_cycles(%lb : index, %ub : index, %step : index, %a : i32, %b : i64, %c : f32) -> (i32, i64, i32, i64, i64, f32) {
%res:6 = scf.for %i = %lb to %ub step %step iter_args(%0 = %a, %1 = %b, %2 = %a, %3 = %b, %4 = %b, %5 = %c) -> (i32, i64, i32, i64, i64, f32) {
func.call @side_effect() : () -> ()
scf.yield %2, %4, %0, %1, %3, %5 : i32, i64, i32, i64, i64, f32
}
return %res#0, %res#1, %res#2, %res#3, %res#4, %res#5 : i32, i64, i32, i64, i64, f32
}

// -----

func.func private @side_effect(i32)

// CHECK-LABEL: func @iter_args_cycles_non_cycle_start
// CHECK-SAME: (%[[LB:.*]]: index, %[[UB:.*]]: index, %[[STEP:.*]]: index, %[[A:.*]]: i32, %[[B:.*]]: i32)
// CHECK: %[[RES:.*]] = scf.for %[[IV:.*]] = %[[LB]] to %[[UB]] step %[[STEP]] iter_args(%[[ITER_ARG:.*]] = %[[A]]) -> (i32) {
// CHECK: func.call @side_effect(%[[ITER_ARG]])
// CHECK: yield %[[B]] : i32
// CHECK: return %[[RES]], %[[B]], %[[B]] : i32, i32, i32
func.func @iter_args_cycles_non_cycle_start(%lb : index, %ub : index, %step : index, %a : i32, %b : i32) -> (i32, i32, i32) {
%res:3 = scf.for %i = %lb to %ub step %step iter_args(%0 = %a, %1 = %b, %2 = %b) -> (i32, i32, i32) {
func.call @side_effect(%0) : (i32) -> ()
scf.yield %1, %2, %1 : i32, i32, i32
}
return %res#0, %res#1, %res#2 : i32, i32, i32
}

// -----

// CHECK-LABEL: func @dead_index_switch_result(
// CHECK-SAME: %[[arg0:.*]]: index
// CHECK-DAG: %[[c10:.*]] = arith.constant 10
// CHECK-DAG: %[[c11:.*]] = arith.constant 11
// CHECK: %[[switch:.*]] = scf.index_switch %[[arg0]] -> index
// CHECK: case 1 {
// CHECK: memref.store %[[c10]]
// CHECK: scf.yield %[[arg0]] : index
// CHECK: }
// CHECK: default {
// CHECK: memref.store %[[c11]]
// CHECK: scf.yield %[[arg0]] : index
// CHECK: }
// CHECK: return %[[switch]]
func.func @dead_index_switch_result(%arg0 : index, %arg1 : memref<i32>) -> index {
%non_live, %live = scf.index_switch %arg0 -> i32, index
case 1 {
%c10 = arith.constant 10 : i32
memref.store %c10, %arg1[] : memref<i32>
scf.yield %c10, %arg0 : i32, index
}
default {
%c11 = arith.constant 11 : i32
memref.store %c11, %arg1[] : memref<i32>
scf.yield %c11, %arg0 : i32, index
}
return %live : index
}