[PATCH] D47139: [NaryReassociate] Detect deleted instr with WeakTrackingVH

[Karl-Johan Karlsson via Phabricator via llvm-commits]

Ka-Ka updated this revision to Diff 148166.
Ka-Ka marked 2 inline comments as done.
Ka-Ka added a comment.

The reason for me to use WeakTrackingVH instead of WeakVH from the beginning was only that it was used elsewhere in the file. However as you point out WeakVH should be fine as RAUW is not used in RecursivelyDeleteTriviallyDeadInstructions(). I adjust the code to use WeakVH.

I did considered using the lazy approach. I actually tried it out, but it seems like the algorithm used here actually require RecursivelyDeleteTriviallyDeadInstructions to be used in the loop. I end up with infinite loops when trying the lazy approach. As I'm not really familiar with the the algorithm used, I instead made this minimal change to fix the problem I saw in pr37539.

I updated the testcase according to what was suggested in the review.


Repository:
  rL LLVM

https://reviews.llvm.org/D47139

Files:
  lib/Transforms/Scalar/NaryReassociate.cpp
  test/Transforms/NaryReassociate/pr37539.ll


Index: test/Transforms/NaryReassociate/pr37539.ll
===================================================================
--- /dev/null
+++ test/Transforms/NaryReassociate/pr37539.ll
@@ -0,0 +1,29 @@
+; RUN: opt < %s -nary-reassociate -S -o - | FileCheck %s
+
+; The test check that compilation does not segv (see pr37539).
+
+define void @f1() {
+; CHECK-LABEL: @f1(
+; CHECK-NEXT:    br label %[[BB1:.*]]
+; CHECK:         [[BB1]]
+; CHECK-NEXT:    [[P1:%.*]] = phi i16 [ 0, [[TMP0:%.*]] ], [ [[A1:%.*]], %[[BB1]] ]
+; CHECK-NEXT:    [[SCEVGEP_OFFS:%.*]] = add i16 2, 0
+; CHECK-NEXT:    [[A1]] = add i16 [[P1]], [[SCEVGEP_OFFS]]
+; CHECK-NEXT:    br i1 false, label %[[BB1]], label %[[BB7:.*]]
+; CHECK:         [[BB7]]
+; CHECK-NEXT:    ret void
+;
+  br label %bb1
+
+bb1:
+  %p1 = phi i16 [ 0, %0 ], [ %a1, %bb1 ]
+  %p2 = phi i16 [ 0, %0 ], [ %a2, %bb1 ]
+  %scevgep.offs = add i16 2, 0
+  %a1 = add i16 %p1, %scevgep.offs
+  %scevgep.offs5 = add i16 2, 0
+  %a2 = add i16 %p2, %scevgep.offs5
+  br i1 false, label %bb1, label %bb7
+
+bb7:
+  ret void
+}
Index: lib/Transforms/Scalar/NaryReassociate.cpp
===================================================================
--- lib/Transforms/Scalar/NaryReassociate.cpp
+++ lib/Transforms/Scalar/NaryReassociate.cpp
@@ -240,10 +240,17 @@
           Changed = true;
           SE->forgetValue(&*I);
           I->replaceAllUsesWith(NewI);
-          // If SeenExprs constains I's WeakTrackingVH, that entry will be
-          // replaced with
-          // nullptr.
+          WeakVH NewIExist = NewI;
+          // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
+          // entry will be replaced with nullptr if deleted.
           RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
+          if (!NewIExist) {
+            // Rare occation where the new instruction (NewI) have been removed,
+            // probably due to parts of the input code was dead from the
+            // beginning, reset the iterator and start over from the beginning
+            I = BB->begin();
+            continue;
+          }
           I = NewI->getIterator();
         }
         // Add the rewritten instruction to SeenExprs; the original instruction


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