[SimpleLoopUnswitch] Drop uses of instructions before block deletion

[DaniilSuchkov]

Summary

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.

[asbirlea]

Thank you for the patch!
LGTM.

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

static void deleteDeadBlocksFromLoop(Loop &L,
 
   // Remove the loop mappings for the dead blocks and drop all the references
   // from these blocks to others to handle cyclic references as we start
   // deleting the blocks themselves.
   for (auto *BB : DeadBlockSet) {
     // 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();
   }
 
   // Actually delete the blocks now that they've been fully unhooked from the
   // IR.
   for (auto *BB : DeadBlockSet)
     BB->eraseFromParent();
 }

llvm/test/Transforms/SimpleLoopUnswitch/dead-blocks-uses-in-unreachablel-blocks.ll

-; XFAIL: *
 ; REQUIRES: asserts
 ; RUN: opt < %s -passes='unswitch<nontrivial>' -disable-output
 ; RUN: opt < %s -simple-loop-unswitch -enable-nontrivial-unswitch -disable-output
 
 
 ; Make sure we don't crash due to a dangling use of %tmp2 in bb7.
 define void @test.use_in_dead_block(i1 %arg1, i1 %arg2) {
 bb1: