[InstSimplify] Fold call null/undef to poison

Calling null or undef results in immediate undefined behavior.
Return poison instead of undef in this case, similar to what
we do for immediate UB due to division by zero.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -5690,11 +5690,11 @@ Value *llvm::SimplifyCall(CallBase *Call, const SimplifyQuery &Q) {
   if (Call->isMustTailCall())
     return nullptr;
 
-  // call undef -> undef
-  // call null -> undef
+  // call undef -> poison
+  // call null -> poison
   Value *Callee = Call->getCalledOperand();
   if (isa<UndefValue>(Callee) || isa<ConstantPointerNull>(Callee))
-    return UndefValue::get(Call->getType());
+    return PoisonValue::get(Call->getType());
 
   if (Value *V = tryConstantFoldCall(Call, Q))
     return V;

llvm/test/Transforms/GVN/PRE/volatile.ll

@@ -204,7 +204,7 @@ define i32 @test9(i32* %V) {
 ; CHECK-LABEL: @test9(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[LOAD:%.*]] = call i32 undef()
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
 entry:
   %load = call i32 undef()

llvm/test/Transforms/InstSimplify/call.ll

@@ -454,7 +454,7 @@ define i32 @call_null() {
 ; CHECK-LABEL: @call_null(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CALL:%.*]] = call i32 null()
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
 entry:
   %call = call i32 null()
@@ -465,7 +465,7 @@ define i32 @call_undef() {
 ; CHECK-LABEL: @call_undef(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CALL:%.*]] = call i32 undef()
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
 entry:
   %call = call i32 undef()

llvm/test/Transforms/InstSimplify/undef.ll

@@ -172,7 +172,7 @@ define i64 @test17(i64 %a) {
 define i64 @test18(i64 %a) {
 ; CHECK-LABEL: @test18(
 ; CHECK-NEXT:    [[R:%.*]] = call i64 undef(i64 [[A:%.*]])
-; CHECK-NEXT:    ret i64 undef
+; CHECK-NEXT:    ret i64 poison
 ;
   %r = call i64 (i64) undef(i64 %a)
   ret i64 %r