copy elision: test case: scalar variable declaration

```zig
export fn entry() void {
    var x: i32 = 1234;
}
```

```llvm
define void @entry() #2 !dbg !41 {
Entry:
  %x = alloca i32, align 4
  store i32 1234, i32* %x, align 4, !dbg !48
  call void @llvm.dbg.declare(metadata i32* %x, metadata !45, metadata !DIExpression()), !dbg !48
  ret void, !dbg !49
}
```

Author: andrewrk

src/all_types.hpp

@@ -2215,6 +2215,7 @@ struct IrInstructionDeclVarGen {
     IrInstruction base;
 
     ZigVar *var;
+    IrInstruction *var_ptr;
 };
 
 struct IrInstructionCondBr {
@@ -3324,7 +3325,8 @@ struct IrInstructionResultPtrCast {
 struct IrInstructionAlloca {
     IrInstruction base;
 
-    IrInstruction *ty;
+    IrInstruction *child_type;
+    const char *name_hint;
 };
 
 static const size_t slice_ptr_index = 0;

src/codegen.cpp

@@ -1844,12 +1844,12 @@ static LLVMValueRef gen_assign_raw(CodeGen *g, LLVMValueRef ptr, ZigType *ptr_ty
     return nullptr;
 }
 
-static void gen_var_debug_decl(CodeGen *g, ZigVar *var) {
+static void gen_var_debug_decl(CodeGen *g, ZigVar *var, LLVMValueRef value_ref) {
     assert(var->di_loc_var != nullptr);
     AstNode *source_node = var->decl_node;
     ZigLLVMDILocation *debug_loc = ZigLLVMGetDebugLoc((unsigned)source_node->line + 1,
             (unsigned)source_node->column + 1, get_di_scope(g, var->parent_scope));
-    ZigLLVMInsertDeclareAtEnd(g->dbuilder, var->value_ref, var->di_loc_var, debug_loc,
+    ZigLLVMInsertDeclareAtEnd(g->dbuilder, value_ref, var->di_loc_var, debug_loc,
             LLVMGetInsertBlock(g->builder));
 }
 
@@ -2000,7 +2000,7 @@ static bool iter_function_params_c_abi(CodeGen *g, ZigType *fn_type, FnWalk *fn_
                 clear_debug_source_node(g);
                 gen_store_untyped(g, LLVMGetParam(llvm_fn, fn_walk->data.inits.gen_i), var->value_ref, var->align_bytes, false);
                 if (var->decl_node) {
-                    gen_var_debug_decl(g, var);
+                    gen_var_debug_decl(g, var, var->value_ref);
                 }
                 fn_walk->data.inits.gen_i += 1;
                 break;
@@ -2035,7 +2035,7 @@ static bool iter_function_params_c_abi(CodeGen *g, ZigType *fn_type, FnWalk *fn_
             }
             case FnWalkIdInits:
                 if (var->decl_node) {
-                    gen_var_debug_decl(g, var);
+                    gen_var_debug_decl(g, var, var->value_ref);
                 }
                 fn_walk->data.inits.gen_i += 1;
                 break;
@@ -2073,7 +2073,7 @@ static bool iter_function_params_c_abi(CodeGen *g, ZigType *fn_type, FnWalk *fn_
                 }
                 case FnWalkIdInits:
                     if (var->decl_node) {
-                        gen_var_debug_decl(g, var);
+                        gen_var_debug_decl(g, var, var->value_ref);
                     }
                     fn_walk->data.inits.gen_i += 1;
                     break;
@@ -2111,7 +2111,7 @@ static bool iter_function_params_c_abi(CodeGen *g, ZigType *fn_type, FnWalk *fn_
                     LLVMValueRef bitcasted = LLVMBuildBitCast(g->builder, var->value_ref, ptr_to_int_type_ref, "");
                     gen_store_untyped(g, arg, bitcasted, var->align_bytes, false);
                     if (var->decl_node) {
-                        gen_var_debug_decl(g, var);
+                        gen_var_debug_decl(g, var, var->value_ref);
                     }
                     fn_walk->data.inits.gen_i += 1;
                     break;
@@ -2206,7 +2206,7 @@ void walk_function_params(CodeGen *g, ZigType *fn_type, FnWalk *fn_walk) {
                 }
 
                 if (variable->decl_node) {
-                    gen_var_debug_decl(g, variable);
+                    gen_var_debug_decl(g, variable, variable->value_ref);
                 }
                 break;
             }
@@ -3175,18 +3175,16 @@ static LLVMValueRef ir_render_bool_not(CodeGen *g, IrExecutable *executable, IrI
     return LLVMBuildICmp(g->builder, LLVMIntEQ, value, zero, "");
 }
 
-static LLVMValueRef ir_render_decl_var(CodeGen *g, IrExecutable *executable,
-        IrInstructionDeclVarGen *decl_var_instruction)
-{
-    ZigVar *var = decl_var_instruction->var;
+static LLVMValueRef ir_render_decl_var(CodeGen *g, IrExecutable *executable, IrInstructionDeclVarGen *instruction) {
+    ZigVar *var = instruction->var;
 
     if (!type_has_bits(var->value->type))
         return nullptr;
 
     if (var->ref_count == 0 && g->build_mode != BuildModeDebug)
         return nullptr;
 
-    gen_var_debug_decl(g, var);
+    gen_var_debug_decl(g, var, ir_llvm_value(g, instruction->var_ptr));
     return nullptr;
 }
 
@@ -5304,7 +5302,7 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdStoreResult:
             zig_panic("TODO");
         case IrInstructionIdAlloca:
-            zig_panic("TODO");
+            return instruction->llvm_value; // handled before function code generation
     }
     zig_unreachable();
 }
@@ -6194,8 +6192,11 @@ static void do_code_gen(CodeGen *g) {
         // allocate temporary stack data
         for (size_t alloca_i = 0; alloca_i < fn_table_entry->alloca_list.length; alloca_i += 1) {
             IrInstructionAlloca *instruction = fn_table_entry->alloca_list.at(alloca_i);
-            ZigType *slot_type = instruction->base.value.type;
-            instruction->base.llvm_value = build_alloca(g, slot_type, "", get_abi_alignment(g, slot_type));
+            ZigType *ptr_type = instruction->base.value.type;
+            assert(ptr_type->id == ZigTypeIdPointer);
+            ZigType *child_type = ptr_type->data.pointer.child_type;
+            instruction->base.llvm_value = build_alloca(g, child_type, instruction->name_hint,
+                    get_ptr_align(g, ptr_type));
         }
 
         ImportTableEntry *import = get_scope_import(&fn_table_entry->fndef_scope->base);
@@ -6221,8 +6222,6 @@ static void do_code_gen(CodeGen *g) {
                 continue;
 
             if (var->src_arg_index == SIZE_MAX) {
-                var->value_ref = build_alloca(g, var->value->type, buf_ptr(&var->name), var->align_bytes);
-
                 var->di_loc_var = ZigLLVMCreateAutoVariable(g->dbuilder, get_di_scope(g, var->parent_scope),
                         buf_ptr(&var->name), import->di_file, (unsigned)(var->decl_node->line + 1),
                         var->value->type->di_type, !g->strip_debug_symbols, 0);