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

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

Ka-Ka created this revision.
Ka-Ka added reviewers: jingyue, tra, meheff, grosser.

If NaryReassociate succeed it will, when replacing the old instruction
with the new instruction, also recursively delete trivially
dead instructions from the old instruction. However, if the input to the
NaryReassociate pass contain dead code it is not save to recursively
delete trivially deadinstructions as it might lead to deleting the newly
created instruction.

This patch will fix the problem by using WeakTrackingVH to detect this
rare case, when the newly created instruction is dead, and it will then
restart the basic block iteration from the beginning.

This fixes pr37539


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 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; 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.
+          WeakTrackingVH NewIExist = NewI;
+          // If SeenExprs and NewIExist contains I's WeakTrackingVH, 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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