[llvm-reduce] Do not replace alloca with null pointers (#164075)

The lifetime intrinsics only accept allocas or poison. This patch uses
poison as the replacement value of alloca.

Author: dtcxzyw

llvm/test/tools/llvm-reduce/reduce-instructions-alloca.ll

@@ -0,0 +1,16 @@
+; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=instructions --test FileCheck --test-arg --check-prefixes=CHECK,INTERESTING --test-arg %s --test-arg --input-file %s -o %t
+; RUN: FileCheck -check-prefixes=CHECK,RESULT %s < %t
+
+; CHECK-LABEL: define void @alloca(
+; INTERESTING: call void @llvm.lifetime.start.p0(
+; INTERESTING: call void @llvm.lifetime.end.p0(
+
+; RESULT: call void @llvm.lifetime.start.p0(ptr poison)
+; RESULT-NEXT: call void @llvm.lifetime.end.p0(ptr poison)
+; RESULT-NEXT: ret void
+define void @alloca(ptr %ptr) {
+  %alloca = alloca i32, align 4
+  call void @llvm.lifetime.start.p0(ptr %alloca)
+  call void @llvm.lifetime.end.p0(ptr %alloca)
+  ret void
+}

llvm/tools/llvm-reduce/deltas/ReduceInstructions.cpp

@@ -13,6 +13,8 @@
 
 #include "ReduceInstructions.h"
 #include "Utils.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instructions.h"
 
 using namespace llvm;
 
@@ -37,7 +39,9 @@ void llvm::reduceInstructionsDeltaPass(Oracle &O, ReducerWorkItem &WorkItem) {
       for (auto &Inst :
            make_early_inc_range(make_range(BB.begin(), std::prev(BB.end())))) {
         if (!shouldAlwaysKeep(Inst) && !O.shouldKeep()) {
-          Inst.replaceAllUsesWith(getDefaultValue(Inst.getType()));
+          Inst.replaceAllUsesWith(isa<AllocaInst>(Inst)
+                                      ? PoisonValue::get(Inst.getType())
+                                      : getDefaultValue(Inst.getType()));
           Inst.eraseFromParent();
         }
       }