[llvm] 90de4a4 - [LoopFusion] Fix sink instructions (#147501)

[via llvm-commits]

Author: Madhur Amilkanthwar
Date: 2025-07-28T12:08:43+05:30
New Revision: 90de4a4ac96ef314e3af9c43c516d5aaf105777a

URL: https://github.com/llvm/llvm-project/commit/90de4a4ac96ef314e3af9c43c516d5aaf105777a
DIFF: https://github.com/llvm/llvm-project/commit/90de4a4ac96ef314e3af9c43c516d5aaf105777a.diff

LOG: [LoopFusion] Fix sink instructions (#147501)

If we have instructions in second loop's preheader which can be sunk, we
should also be adjusting PHI nodes to receive values from the fused loop's latch block.

Fixes #128600

Added: 
    llvm/test/Transforms/LoopFusion/sunk-phi-nodes.ll

Modified: 
    llvm/lib/Transforms/Scalar/LoopFuse.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LoopFuse.cpp b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
index d6bd92d520e28..b5eb647a042b9 100644
--- a/llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopFuse.cpp
@@ -1176,6 +1176,28 @@ struct LoopFuser {
     return true;
   }
 
+  /// This function fixes PHI nodes after fusion in \p SafeToSink.
+  /// \p SafeToSink instructions are the instructions that are to be moved past
+  /// the fused loop. Thus, the PHI nodes in \p SafeToSink should be updated to
+  /// receive values from the fused loop if they are currently taking values
+  /// from the first loop (i.e. FC0)'s latch.
+  void fixPHINodes(ArrayRef<Instruction *> SafeToSink,
+                   const FusionCandidate &FC0,
+                   const FusionCandidate &FC1) const {
+    for (Instruction *Inst : SafeToSink) {
+      // No update needed for non-PHI nodes.
+      PHINode *Phi = dyn_cast<PHINode>(Inst);
+      if (!Phi)
+        continue;
+      for (unsigned I = 0; I < Phi->getNumIncomingValues(); I++) {
+        if (Phi->getIncomingBlock(I) != FC0.Latch)
+          continue;
+        assert(FC1.Latch && "FC1 latch is not set");
+        Phi->setIncomingBlock(I, FC1.Latch);
+      }
+    }
+  }
+
   /// Collect instructions in the \p FC1 Preheader that can be hoisted
   /// to the \p FC0 Preheader or sunk into the \p FC1 Body
   bool collectMovablePreheaderInsts(
@@ -1481,6 +1503,9 @@ struct LoopFuser {
       assert(I->getParent() == FC1.Preheader);
       I->moveBefore(*FC1.ExitBlock, FC1.ExitBlock->getFirstInsertionPt());
     }
+    // PHI nodes in SinkInsts need to be updated to receive values from the
+    // fused loop.
+    fixPHINodes(SinkInsts, FC0, FC1);
   }
 
   /// Determine if two fusion candidates have identical guards

diff  --git a/llvm/test/Transforms/LoopFusion/sunk-phi-nodes.ll b/llvm/test/Transforms/LoopFusion/sunk-phi-nodes.ll
new file mode 100644
index 0000000000000..36c6bdde781c7
--- /dev/null
+++ b/llvm/test/Transforms/LoopFusion/sunk-phi-nodes.ll
@@ -0,0 +1,65 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=loop-fusion -S < %s 2>&1 | FileCheck %s
+define dso_local i32 @check_sunk_phi_nodes() {
+; CHECK-LABEL: define dso_local i32 @check_sunk_phi_nodes() {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    [[SUM1_02:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[ADD:%.*]], %[[FOR_INC6:.*]] ]
+; CHECK-NEXT:    [[I_01:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[INC:%.*]], %[[FOR_INC6]] ]
+; CHECK-NEXT:    [[I1_04:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[INC7:%.*]], %[[FOR_INC6]] ]
+; CHECK-NEXT:    [[SUM2_03:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[ADD5:%.*]], %[[FOR_INC6]] ]
+; CHECK-NEXT:    [[ADD]] = add nsw i32 [[SUM1_02]], [[I_01]]
+; CHECK-NEXT:    br label %[[FOR_INC:.*]]
+; CHECK:       [[FOR_INC]]:
+; CHECK-NEXT:    [[MUL:%.*]] = mul nsw i32 [[I1_04]], [[I1_04]]
+; CHECK-NEXT:    [[ADD5]] = add nsw i32 [[SUM2_03]], [[MUL]]
+; CHECK-NEXT:    br label %[[FOR_INC6]]
+; CHECK:       [[FOR_INC6]]:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_01]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[INC]], 10
+; CHECK-NEXT:    [[INC7]] = add nsw i32 [[I1_04]], 1
+; CHECK-NEXT:    [[CMP3:%.*]] = icmp slt i32 [[INC7]], 10
+; CHECK-NEXT:    br i1 [[CMP3]], label %[[FOR_BODY]], label %[[FOR_END8:.*]]
+; CHECK:       [[FOR_END8]]:
+; CHECK-NEXT:    [[SUM2_0_LCSSA:%.*]] = phi i32 [ [[ADD5]], %[[FOR_INC6]] ]
+; CHECK-NEXT:    [[SUM1_0_LCSSA:%.*]] = phi i32 [ [[ADD]], %[[FOR_INC6]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[SUM1_0_LCSSA]], [[SUM2_0_LCSSA]]
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.inc
+  %sum1.02 = phi i32 [ 0, %entry ], [ %add, %for.inc ]
+  %i.01 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %add = add nsw i32 %sum1.02, %i.01
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.01, 1
+  %cmp = icmp slt i32 %inc, 10
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.inc
+  %sum1.0.lcssa = phi i32 [ %add, %for.inc ]
+  br label %for.body4
+
+for.body4:                                        ; preds = %for.end, %for.inc6
+  %i1.04 = phi i32 [ 0, %for.end ], [ %inc7, %for.inc6 ]
+  %sum2.03 = phi i32 [ 0, %for.end ], [ %add5, %for.inc6 ]
+  %mul = mul nsw i32 %i1.04, %i1.04
+  %add5 = add nsw i32 %sum2.03, %mul
+  br label %for.inc6
+
+for.inc6:                                         ; preds = %for.body4
+  %inc7 = add nsw i32 %i1.04, 1
+  %cmp3 = icmp slt i32 %inc7, 10
+  br i1 %cmp3, label %for.body4, label %for.end8
+
+for.end8:                                         ; preds = %for.inc6
+  %sum2.0.lcssa = phi i32 [ %add5, %for.inc6 ]
+  %0 = add i32 %sum1.0.lcssa, %sum2.0.lcssa
+  ret i32 %0
+}
+