Initialize hoisted object allocations (#49584)

(cherry picked from commit 5032a1a)

Author: pchintalapudi

src/llvm-julia-licm.cpp

@@ -290,6 +290,20 @@ struct JuliaLICM : public JuliaPassContext {
                         }
                         changed = true;
                     }
+                    IRBuilder<> builder(preheader->getTerminator());
+                    builder.SetCurrentDebugLocation(call->getDebugLoc());
+                    auto obj_i8 = builder.CreateBitCast(call, Type::getInt8PtrTy(call->getContext(), call->getType()->getPointerAddressSpace()));
+                    // Note that this alignment is assuming the GC allocates at least pointer-aligned memory
+                    auto align = Align(DL.getPointerSize(0));
+                    auto clear_obj = builder.CreateMemSet(obj_i8, ConstantInt::get(Type::getInt8Ty(call->getContext()), 0), call->getArgOperand(1), align);
+                    if (MSSAU.getMemorySSA()) {
+                        auto alloc_mdef = MSSAU.getMemorySSA()->getMemoryAccess(call);
+                        assert(isa<MemoryDef>(alloc_mdef) && "Expected alloc to be associated with a memory def!");
+                        auto clear_mdef = MSSAU.createMemoryAccessAfter(clear_obj, nullptr, alloc_mdef);
+                        assert(isa<MemoryDef>(clear_mdef) && "Expected memset to be associated with a memory def!");
+                        (void) clear_mdef;
+                    }
+                    changed = true;
                 }
             }
         }