[PATCH] D80551: [SimpleLoopUnswitch] Drop uses of instructions before block deletion

[Daniil Suchkov via Phabricator via llvm-commits]

DaniilSuchkov created this revision.
DaniilSuchkov added reviewers: asbirlea, aqjune, fhahn.
Herald added subscribers: llvm-commits, george.burgess.iv, hiraditya.
Herald added a project: LLVM.

Currently if instructions defined in a block are used in unreachable blocks and SimpleLoopUnswitch attempts deleting the block, it triggers following assertion:

  While deleting: i32 %tmp2
  Use still stuck around after Def is destroyed:  %tmp4 = add i32 %tmp2, 1
  opt: llvm-project/llvm/lib/IR/Value.cpp:96: llvm::Value::~Value(): Assertion `materialized_use_empty() && "Uses remain when a value is destroyed!"' failed.
  Stack dump:
  0.  Program arguments: bin/opt -passes=unswitch<nontrivial> -disable-output dead-blocks-uses-in-unreachablel-blocks.ll 
   <...>
   #6 0x00007f584b9590e6 __assert_fail_base (/lib64/libc.so.6+0x2f0e6)
   #7 0x00007f584b959192 (/lib64/libc.so.6+0x2f192)
   #8 0x000000000216b58d (bin/opt+0x216b58d)
   #9 0x000000000216b5e0 llvm::Value::deleteValue() (bin/opt+0x216b5e0)
  #10 0x00000000020212ca llvm::BasicBlock::~BasicBlock() (bin/opt+0x20212ca)
  #11 0x0000000002021535 llvm::BasicBlock::eraseFromParent() (bin/opt+0x2021535)
  #12 0x00000000006c5e2e deleteDeadBlocksFromLoop(llvm::Loop&, llvm::SmallVectorImpl<llvm::BasicBlock*>&, llvm::DominatorTree&, llvm::LoopInfo&, llvm::MemorySSAUpdater*) (bin/opt+0x6c5e2e)
  #13 0x00000000006c8489 unswitchNontrivialInvariants(llvm::Loop&, llvm::Instruction&, llvm::ArrayRef<llvm::Value*>, llvm::SmallVectorImpl<llvm::BasicBlock*>&, llvm::DominatorTree&, llvm::LoopInfo&, llvm::AssumptionCache&, llvm::function_ref<void (bool, llvm::ArrayRef<llvm::Loop*>)>, llvm::ScalarEvolution*, llvm::MemorySSAUpdater*) (bin/opt+0x6c8489)
  #14 0x0000000002737d07 unswitchBestCondition(llvm::Loop&, llvm::DominatorTree&, llvm::LoopInfo&, llvm::AssumptionCache&, llvm::TargetTransformInfo&, llvm::function_ref<void (bool, llvm::ArrayRef<llvm::Loop*>)>, llvm::ScalarEvolution*, llvm::MemorySSAUpdater*) (bin/opt+0x2737d07)
  #15 0x0000000002738d91 llvm::SimpleLoopUnswitchPass::run(llvm::Loop&, llvm::AnalysisManager<llvm::Loop, llvm::LoopStandardAnalysisResults&>&, llvm::LoopStandardAnalysisResults&, llvm::LPMUpdater&) (bin/opt+0x2738d91)
   <...>

This patch fixes it by replacing all uses of instructions from BB with undefs before BB deletion.


https://reviews.llvm.org/D80551

Files:
  llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
  llvm/test/Transforms/SimpleLoopUnswitch/dead-blocks-uses-in-unreachablel-blocks.ll


Index: llvm/test/Transforms/SimpleLoopUnswitch/dead-blocks-uses-in-unreachablel-blocks.ll
===================================================================
--- llvm/test/Transforms/SimpleLoopUnswitch/dead-blocks-uses-in-unreachablel-blocks.ll
+++ llvm/test/Transforms/SimpleLoopUnswitch/dead-blocks-uses-in-unreachablel-blocks.ll
@@ -1,4 +1,3 @@
-; XFAIL: *
 ; REQUIRES: asserts
 ; RUN: opt < %s -passes='unswitch<nontrivial>' -disable-output
 ; RUN: opt < %s -simple-loop-unswitch -enable-nontrivial-unswitch -disable-output
Index: llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -1574,6 +1574,11 @@
     // Check that the dominator tree has already been updated.
     assert(!DT.getNode(BB) && "Should already have cleared domtree!");
     LI.changeLoopFor(BB, nullptr);
+    // Drop all uses of the instructions to make sure we won't have dangling
+    // uses in other blocks.
+    for (auto &I : *BB)
+      if (!I.use_empty())
+        I.replaceAllUsesWith(UndefValue::get(I.getType()));
     BB->dropAllReferences();
   }
 


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