[LoopFusion] Forget loop and block dispositions after latch merge

Merging the latches of loops may affect the dispositions, so they
should be forgotten after the merge. This patch will fix the crash
due to using loop dispositions after forgetting them.

Author: 1997alireza

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -1796,14 +1796,16 @@ struct LoopFuser {
     // mergeLatch may remove the only block in FC1.
     SE.forgetLoop(FC1.L);
     SE.forgetLoop(FC0.L);
-    // Forget block dispositions as well, so that there are no dangling
-    // pointers to erased/free'ed blocks.
-    SE.forgetBlockAndLoopDispositions();
 
     // Move instructions from FC0.Latch to FC1.Latch.
     // Note: mergeLatch requires an updated DT.
     mergeLatch(FC0, FC1);
 
+    // Forget block dispositions as well, so that there are no dangling
+    // pointers to erased/free'ed blocks. It should be done after mergeLatch()
+    // since merging the latches may affect the dispositions.
+    SE.forgetBlockAndLoopDispositions();
+
     // Merge the loops.
     SmallVector<BasicBlock *, 8> Blocks(FC1.L->blocks());
     for (BasicBlock *BB : Blocks) {
@@ -2092,14 +2094,16 @@ struct LoopFuser {
     // mergeLatch may remove the only block in FC1.
     SE.forgetLoop(FC1.L);
     SE.forgetLoop(FC0.L);
-    // Forget block dispositions as well, so that there are no dangling
-    // pointers to erased/free'ed blocks.
-    SE.forgetBlockAndLoopDispositions();
 
     // Move instructions from FC0.Latch to FC1.Latch.
     // Note: mergeLatch requires an updated DT.
     mergeLatch(FC0, FC1);
 
+    // Forget block dispositions as well, so that there are no dangling
+    // pointers to erased/free'ed blocks. It should be done after mergeLatch()
+    // since merging the latches may affect the dispositions.
+    SE.forgetBlockAndLoopDispositions();
+
     // Merge the loops.
     SmallVector<BasicBlock *, 8> Blocks(FC1.L->blocks());
     for (BasicBlock *BB : Blocks) {

llvm/test/Transforms/LoopFusion/pr164082.ll

@@ -0,0 +1,65 @@
+; REQUIRES: asserts
+; RUN: opt -passes=loop-fusion -disable-output -stats < %s 2>&1 | FileCheck -check-prefix=STAT %s
+; STAT: 1 loop-fusion - Loops fused
+
+; C Code
+;
+;;  for (int i = 0; i < 100; ++i)
+;;      Array[i][i] = -i;
+;;  for (int row = 0; row < 100; ++row)
+;;      for (int col = 0; col < 100; ++col)
+;;          if (col != row)
+;;              Array[row][col] = row + col;
+;
+; Loop fusion should not crash anymore as now forgetBlockAndLoopDispositions()
+; is trigerred after mergeLatch() during the fusion.
+
+define i32 @forget_dispositions() nounwind {
+entry:
+  %Array = alloca [100 x [100 x i32]], align 4
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv33 = phi i64 [ 0, %entry ], [ %indvars.iv.next34, %for.body ]
+  %0 = trunc i64 %indvars.iv33 to i32
+  %sub = sub i32 0, %0
+  %arrayidx2 = getelementptr inbounds [100 x [100 x i32]], ptr %Array, i64 0, i64 %indvars.iv33, i64 %indvars.iv33
+  store i32 %sub, ptr %arrayidx2, align 4
+  %indvars.iv.next34 = add i64 %indvars.iv33, 1
+  %lftr.wideiv35 = trunc i64 %indvars.iv.next34 to i32
+  %exitcond36 = icmp eq i32 %lftr.wideiv35, 100
+  br i1 %exitcond36, label %for.cond6.preheader, label %for.body
+
+for.cond6.preheader:                              ; preds = %for.body, %for.inc17
+  %indvars.iv29 = phi i64 [ %indvars.iv.next30, %for.inc17 ], [ 0, %for.body ]
+  br label %for.body8
+
+for.body8:                                        ; preds = %for.inc14, %for.cond6.preheader
+  %indvars.iv = phi i64 [ 0, %for.cond6.preheader ], [ %indvars.iv.next, %for.inc14 ]
+  %1 = trunc i64 %indvars.iv to i32
+  %2 = trunc i64 %indvars.iv29 to i32
+  %cmp9 = icmp eq i32 %1, %2
+  br i1 %cmp9, label %for.inc14, label %if.then
+
+if.then:                                          ; preds = %for.body8
+  %3 = add i64 %indvars.iv, %indvars.iv29
+  %arrayidx13 = getelementptr inbounds [100 x [100 x i32]], ptr %Array, i64 0, i64 %indvars.iv29, i64 %indvars.iv
+  %4 = trunc i64 %3 to i32
+  store i32 %4, ptr %arrayidx13, align 4
+  br label %for.inc14
+
+for.inc14:                                        ; preds = %for.body8, %if.then
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv27 = trunc i64 %indvars.iv.next to i32
+  %exitcond28 = icmp eq i32 %lftr.wideiv27, 100
+  br i1 %exitcond28, label %for.inc17, label %for.body8
+
+for.inc17:                                        ; preds = %for.inc14
+  %indvars.iv.next30 = add i64 %indvars.iv29, 1
+  %lftr.wideiv31 = trunc i64 %indvars.iv.next30 to i32
+  %exitcond32 = icmp eq i32 %lftr.wideiv31, 100
+  br i1 %exitcond32, label %for.exit, label %for.cond6.preheader
+
+for.exit:                                    ; preds = %for.inc17
+  ret i32 0
+}