[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, 13 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e877fb1..f5e320e 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/AssumeBundleQueries.h"
 #include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/EHPersonalities.h"
 #include "llvm/Analysis/GuardUtils.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/Loads.h"
@@ -5273,6 +5274,18 @@ bool llvm::isGuaranteedToTransferExecutionToSuccessor(const Instruction *I) {
   if (isa<UnreachableInst>(I))
     return false;
 
+  if (isa<CatchPadInst>(I)) {
+    switch (classifyEHPersonality(I->getFunction()->getPersonalityFn())) {
+    default:
+      // A catchpad may invoke exception object constructors and such, which
+      // in some languages can be arbitrary code, so be conservative by default.
+      return false;
+    case EHPersonality::CoreCLR:
+      // For CoreCLR, it just involves a type test.
+      return true;
+    }
+  }
+
   // An instruction that returns without throwing must transfer control flow
   // to a successor.
   return !I->mayThrow() && I->willReturn();