[SimplifyCFG] Replace calls to null/undef with unreachable

Calling null is undefined behavior, a call to undef can be trivially
treated as a call to null.

llvm-svn: 273776

2 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 4d76aae2971..edacfd20f99 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1414,6 +1414,12 @@ static bool markAliveBlocks(Function &F,
       }
 
       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
+        Value *Callee = CI->getCalledValue();
+        if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
+          changeToUnreachable(CI, /*UseLLVMTrap=*/false);
+          Changed = true;
+          break;
+        }
         if (CI->doesNotReturn()) {
           // If we found a call to a no-return function, insert an unreachable
           // instruction after it.  Make sure there isn't *already* one there
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index d22f5c63dee..7924c33038a 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5388,7 +5388,7 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
   if (I->use_empty())
     return false;
 
-  if (C->isNullValue()) {
+  if (C->isNullValue() || isa<UndefValue>(C)) {
     // Only look at the first use, avoid hurting compile time with long uselists
     User *Use = *I->user_begin();
 
@@ -5417,6 +5417,10 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
       if (!SI->isVolatile())
         return SI->getPointerAddressSpace() == 0 &&
                SI->getPointerOperand() == I;
+
+    // A call to null is undefined.
+    if (auto CS = CallSite(Use))
+      return CS.getCalledValue() == I;
   }
   return false;
 }