[MLIR][SCF] Canonicalize redundant scf.if from scf.while before region into after region #169892
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Member
|
@llvm/pr-subscribers-mlir-scf @llvm/pr-subscribers-mlir Author: Ming Yan (NexMing) ChangesWhen a Full diff: https://github.com/llvm/llvm-project/pull/169892.diff 2 Files Affected:
diff --git a/mlir/lib/Dialect/SCF/IR/SCF.cpp b/mlir/lib/Dialect/SCF/IR/SCF.cpp
index 881e256a8797b..79dcf562db993 100644
--- a/mlir/lib/Dialect/SCF/IR/SCF.cpp
+++ b/mlir/lib/Dialect/SCF/IR/SCF.cpp
@@ -26,6 +26,7 @@
#include "mlir/Interfaces/ParallelCombiningOpInterface.h"
#include "mlir/Interfaces/ValueBoundsOpInterface.h"
#include "mlir/Transforms/InliningUtils.h"
+#include "mlir/Transforms/RegionUtils.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
@@ -3687,6 +3688,127 @@ LogicalResult scf::WhileOp::verify() {
}
namespace {
+/// Move a scf.if op that is directly before the scf.condition op in the while
+/// before region, and whose condition matches the condition of the
+/// scf.condition op, down into the while after region.
+///
+/// scf.while (..) : (...) -> ... {
+/// %additional_used_values = ...
+/// %cond = ...
+/// ...
+/// %res = scf.if %cond -> (...) {
+/// use(%additional_used_values)
+/// ... // then block
+/// scf.yield %then_value
+/// } else {
+/// scf.yield %else_value
+/// }
+/// scf.condition(%cond) %res, ...
+/// } do {
+/// ^bb0(%res_arg, ...):
+/// use(%res_arg)
+/// ...
+///
+/// becomes
+/// scf.while (..) : (...) -> ... {
+/// %additional_used_values = ...
+/// %cond = ...
+/// ...
+/// scf.condition(%cond) %else_value, ..., %additional_used_values
+/// } do {
+/// ^bb0(%res_arg ..., %additional_args): :
+/// use(%additional_args)
+/// ... // if then block
+/// use(%then_value)
+/// ...
+struct WhileMoveIfDown : public OpRewritePattern<scf::WhileOp> {
+ using OpRewritePattern<scf::WhileOp>::OpRewritePattern;
+
+ LogicalResult matchAndRewrite(scf::WhileOp op,
+ PatternRewriter &rewriter) const override {
+ auto conditionOp =
+ cast<scf::ConditionOp>(op.getBeforeBody()->getTerminator());
+ auto ifOp = dyn_cast_or_null<scf::IfOp>(conditionOp->getPrevNode());
+
+ // Check that the ifOp is directly before the conditionOp and that it
+ // matches the condition of the conditionOp. Also ensure that the ifOp has
+ // no else block with content, as that would complicate the transformation.
+ // TODO: support else blocks with content.
+ if (!ifOp || ifOp.getCondition() != conditionOp.getCondition() ||
+ (ifOp.elseBlock() && !ifOp.elseBlock()->without_terminator().empty()))
+ return failure();
+
+ assert(ifOp->use_empty() || (llvm::all_equal(ifOp->getUsers()) &&
+ *ifOp->user_begin() == conditionOp) &&
+ "ifOp has unexpected uses");
+
+ Location loc = op.getLoc();
+
+ // Replace uses of ifOp results in the conditionOp with the yielded values
+ // from the ifOp branches.
+ for (auto [idx, arg] : llvm::enumerate(conditionOp.getArgs())) {
+ auto it = llvm::find(ifOp->getResults(), arg);
+ if (it != ifOp->getResults().end()) {
+ size_t ifOpIdx = it.getIndex();
+ Value thenValue = ifOp.thenYield()->getOperand(ifOpIdx);
+ Value elseValue = ifOp.elseYield()->getOperand(ifOpIdx);
+
+ rewriter.replaceAllUsesWith(ifOp->getResults()[ifOpIdx], elseValue);
+ rewriter.replaceAllUsesWith(op.getAfterArguments()[idx], thenValue);
+ }
+ }
+
+ // Collect additional used values from before region.
+ SetVector<Value> additionalUsedValues;
+ visitUsedValuesDefinedAbove(ifOp.getThenRegion(), [&](OpOperand *operand) {
+ if (op.getBefore().isAncestor(operand->get().getParentRegion()))
+ additionalUsedValues.insert(operand->get());
+ });
+
+ // Create new whileOp with additional used values as results.
+ auto additionalValueTypes = llvm::map_to_vector(
+ additionalUsedValues, [](Value val) { return val.getType(); });
+ size_t additionalValueSize = additionalUsedValues.size();
+ SmallVector<Type> newResultTypes(op.getResultTypes());
+ newResultTypes.append(additionalValueTypes);
+
+ auto newWhileOp =
+ scf::WhileOp::create(rewriter, loc, newResultTypes, op.getInits());
+
+ newWhileOp.getBefore().takeBody(op.getBefore());
+ newWhileOp.getAfter().takeBody(op.getAfter());
+ newWhileOp.getAfter().addArguments(
+ additionalValueTypes, SmallVector<Location>(additionalValueSize, loc));
+
+ SmallVector<Value> conditionArgs = conditionOp.getArgs();
+ llvm::append_range(conditionArgs, additionalUsedValues);
+
+ // Update conditionOp inside new whileOp before region.
+ rewriter.setInsertionPoint(conditionOp);
+ rewriter.replaceOpWithNewOp<scf::ConditionOp>(
+ conditionOp, conditionOp.getCondition(), conditionArgs);
+
+ // Replace uses of additional used values inside the ifOp then region with
+ // the whileOp after region arguments.
+ rewriter.replaceUsesWithIf(
+ additionalUsedValues.takeVector(),
+ newWhileOp.getAfterArguments().take_back(additionalValueSize),
+ [&](OpOperand &use) {
+ return ifOp.getThenRegion().isAncestor(
+ use.getOwner()->getParentRegion());
+ });
+
+ // Inline ifOp then region into new whileOp after region.
+ rewriter.eraseOp(ifOp.thenYield());
+ rewriter.inlineBlockBefore(ifOp.thenBlock(), newWhileOp.getAfterBody(),
+ newWhileOp.getAfterBody()->begin());
+ rewriter.eraseOp(ifOp);
+ rewriter.replaceOp(op,
+ newWhileOp->getResults().drop_back(additionalValueSize));
+ return success();
+ }
+};
+
/// Replace uses of the condition within the do block with true, since otherwise
/// the block would not be evaluated.
///
@@ -4399,7 +4521,8 @@ void WhileOp::getCanonicalizationPatterns(RewritePatternSet &results,
results.add<RemoveLoopInvariantArgsFromBeforeBlock,
RemoveLoopInvariantValueYielded, WhileConditionTruth,
WhileCmpCond, WhileUnusedResult, WhileRemoveDuplicatedResults,
- WhileRemoveUnusedArgs, WhileOpAlignBeforeArgs>(context);
+ WhileRemoveUnusedArgs, WhileOpAlignBeforeArgs, WhileMoveIfDown>(
+ context);
}
//===----------------------------------------------------------------------===//
diff --git a/mlir/test/Dialect/SCF/canonicalize.mlir b/mlir/test/Dialect/SCF/canonicalize.mlir
index 084c3fc065de3..b02cbc07880b9 100644
--- a/mlir/test/Dialect/SCF/canonicalize.mlir
+++ b/mlir/test/Dialect/SCF/canonicalize.mlir
@@ -974,6 +974,43 @@ func.func @replace_if_with_cond3(%arg0 : i1, %arg2: i64) -> (i32, i64) {
// -----
+// CHECK-LABEL: @while_move_if_down
+func.func @while_move_if_down() -> i32 {
+ %0 = scf.while () : () -> (i32) {
+ %additional_used_value = "test.get_some_value1" () : () -> (i32)
+ %else_value = "test.get_some_value2" () : () -> (i32)
+ %condition = "test.condition"() : () -> i1
+ %res = scf.if %condition -> (i32) {
+ "test.use1" (%additional_used_value) : (i32) -> ()
+ %then_value = "test.get_some_value3" () : () -> (i32)
+ scf.yield %then_value : i32
+ } else {
+ scf.yield %else_value : i32
+ }
+ scf.condition(%condition) %res : i32
+ } do {
+ ^bb0(%res_arg: i32):
+ "test.use2" (%res_arg) : (i32) -> ()
+ scf.yield
+ }
+ return %0 : i32
+}
+// CHECK-NEXT: %[[WHILE_0:.*]]:2 = scf.while : () -> (i32, i32) {
+// CHECK-NEXT: %[[VAL_0:.*]] = "test.get_some_value1"() : () -> i32
+// CHECK-NEXT: %[[VAL_1:.*]] = "test.get_some_value2"() : () -> i32
+// CHECK-NEXT: %[[VAL_2:.*]] = "test.condition"() : () -> i1
+// CHECK-NEXT: scf.condition(%[[VAL_2]]) %[[VAL_1]], %[[VAL_0]] : i32, i32
+// CHECK-NEXT: } do {
+// CHECK-NEXT: ^bb0(%[[VAL_3:.*]]: i32, %[[VAL_4:.*]]: i32):
+// CHECK-NEXT: "test.use1"(%[[VAL_4]]) : (i32) -> ()
+// CHECK-NEXT: %[[VAL_5:.*]] = "test.get_some_value3"() : () -> i32
+// CHECK-NEXT: "test.use2"(%[[VAL_5]]) : (i32) -> ()
+// CHECK-NEXT: scf.yield
+// CHECK-NEXT: }
+// CHECK-NEXT: return %[[VAL_6:.*]]#0 : i32
+
+// -----
+
// CHECK-LABEL: @while_cond_true
func.func @while_cond_true() -> i1 {
%0 = scf.while () : () -> i1 {
|
joker-eph
reviewed
Nov 28, 2025
joker-eph
changed the title
joker-eph
reviewed
Nov 28, 2025
joker-eph
approved these changes
Dec 1, 2025
Collaborator
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/203/builds/30502 Here is the relevant piece of the build log for the reference |
Collaborator
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/205/builds/29292 Here is the relevant piece of the build log for the reference |
Collaborator
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/204/builds/29313 Here is the relevant piece of the build log for the reference |
Contributor
|
I put up #170107 to add the missing MLIR library in linking. |
aahrun
pushed a commit
to aahrun/llvm-project
that referenced
this pull request
Dec 1, 2025
…n into after region (llvm#169892) When a `scf.if` directly precedes a `scf.condition` in the before region of a `scf.while` and both share the same condition, move the if into the after region of the loop. This helps simplify the control flow to enable uplifting `scf.while` to `scf.for`.
aahrun
pushed a commit
to aahrun/llvm-project
that referenced
this pull request
Dec 1, 2025
…er region in scf-uplift-while-to-for" (llvm#169888) Reverts llvm#165216 It is implemented in llvm#169892 .
augusto2112
pushed a commit
to augusto2112/llvm-project
that referenced
this pull request
Dec 3, 2025
…n into after region (llvm#169892) When a `scf.if` directly precedes a `scf.condition` in the before region of a `scf.while` and both share the same condition, move the if into the after region of the loop. This helps simplify the control flow to enable uplifting `scf.while` to `scf.for`.
augusto2112
pushed a commit
to augusto2112/llvm-project
that referenced
this pull request
Dec 3, 2025
…er region in scf-uplift-while-to-for" (llvm#169888) Reverts llvm#165216 It is implemented in llvm#169892 .
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
5 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
When a
scf.ifdirectly precedes ascf.conditionin the before region of ascf.whileand both share the same condition, move the if into the after region of the loop. This helps simplify the control flow to enable upliftingscf.whiletoscf.for.