[SimplifyCFG] simplifyUnreachable(): erase instructions iff they are guaranteed to transfer execution to unreachable

This replaces the current ad-hoc implementation,
by syncing the code from InstCombine's implementation in `InstCombinerImpl::visitUnreachableInst()`,
with one exception that here in SimplifyCFG we are allowed to remove EH instructions.

Effectively, this now allows SimplifyCFG to remove calls (iff they won't throw and will return),
arithmetic/logic operations, etc.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D105374

1 files changed, 18 insertions, 32 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index b8f6bf9..f08ab18 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4655,6 +4655,7 @@ bool SimplifyCFGOpt::simplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
   return false;
 }
 
+// WARNING: keep in sync with InstCombinerImpl::visitUnreachableInst()!
 bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
   BasicBlock *BB = UI->getParent();
 
@@ -4665,39 +4666,24 @@ bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
   while (UI->getIterator() != BB->begin()) {
     BasicBlock::iterator BBI = UI->getIterator();
     --BBI;
-    // Do not delete instructions that can have side effects which might cause
-    // the unreachable to not be reachable; specifically, calls and volatile
-    // operations may have this effect.
-    if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI))
-      break;
 
-    if (BBI->mayHaveSideEffects()) {
-      if (auto *SI = dyn_cast<StoreInst>(BBI)) {
-        // Temporarily disable removal of volatile stores preceding unreachable,
-        // pending a potential LangRef change permitting volatile stores to
-        // trap.
-        // TODO: Either remove this code, or properly integrate the check into
-        // isGuaranteedToTransferExecutionToSuccessor().
-        if (SI->isVolatile())
-          break;
-      } else if (isa<CatchPadInst>(BBI)) {
-        // A catchpad may invoke exception object constructors and such, which
-        // in some languages can be arbitrary code, so be conservative by
-        // default.
-        // For CoreCLR, it just involves a type test, so can be removed.
-        if (classifyEHPersonality(BB->getParent()->getPersonalityFn()) !=
-            EHPersonality::CoreCLR)
-          break;
-      } else if (!isa<LoadInst>(BBI) && !isa<AtomicRMWInst>(BBI) &&
-                 !isa<AtomicCmpXchgInst>(BBI) && !isa<FenceInst>(BBI) &&
-                 !isa<VAArgInst>(BBI) && !isa<LandingPadInst>(BBI)) {
-        break;
-      }
-      // Note that deleting LandingPad's here is in fact okay, although it
-      // involves a bit of subtle reasoning. If this inst is a LandingPad,
-      // all the predecessors of this block will be the unwind edges of Invokes,
-      // and we can therefore guarantee this block will be erased.
-    }
+    if (!isGuaranteedToTransferExecutionToSuccessor(&*BBI))
+      break; // Can not drop any more instructions. We're done here.
+    // Otherwise, this instruction can be freely erased,
+    // even if it is not side-effect free.
+
+    // Temporarily disable removal of volatile stores preceding unreachable,
+    // pending a potential LangRef change permitting volatile stores to trap.
+    // TODO: Either remove this code, or properly integrate the check into
+    // isGuaranteedToTransferExecutionToSuccessor().
+    if (auto *SI = dyn_cast<StoreInst>(&*BBI))
+      if (SI->isVolatile())
+        break; // Can not drop this instruction. We're done here.
+
+    // Note that deleting EH's here is in fact okay, although it involves a bit
+    // of subtle reasoning. If this inst is an EH, all the predecessors of this
+    // block will be the unwind edges of Invoke/CatchSwitch/CleanupReturn,
+    // and we can therefore guarantee this block will be erased.
 
     // Delete this instruction (any uses are guaranteed to be dead)
     BBI->replaceAllUsesWith(PoisonValue::get(BBI->getType()));