[llvm] 025e431 - [LoopFusion] Forget loop and block dispositions after latch merge (#166233)

[via llvm-commits]

Author: Alireza Torabian
Date: 2025-11-04T16:48:39-05:00
New Revision: 025e431e7450cada2724b19eb59354a6c020fa4f

URL: https://github.com/llvm/llvm-project/commit/025e431e7450cada2724b19eb59354a6c020fa4f
DIFF: https://github.com/llvm/llvm-project/commit/025e431e7450cada2724b19eb59354a6c020fa4f.diff

LOG: [LoopFusion] Forget loop and block dispositions after latch merge (#166233)

Merging the latches of loops may affect the dispositions, so they should
be forgotten after the merge. This patch fixed the crash in loop fusion
[#164082](https://github.com/llvm/llvm-project/issues/164082).

Added: 
    llvm/test/Transforms/LoopFusion/pr164082.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 19eccb9e17020..9ffa602416b05 100644
--- a/llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ b/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) {

diff  --git a/llvm/test/Transforms/LoopFusion/pr164082.ll b/llvm/test/Transforms/LoopFusion/pr164082.ll
new file mode 100644
index 0000000000000..652557cef48f8
--- /dev/null
+++ b/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
+}