我有带闪存的 MCU(像往常一样)。链接器将 .struct_init、.struct_init_const、.struct_not_init 段放置到属于闪存段 20 的地址。它在链接描述文件中被硬编码。
考虑以下测试代码:test.h
typedef struct
{
int val1;
int val2;
} mystruct_t;
测试.cpp
#include "test.h"
// each variable is placed in dedicated section
// sections are placed in flash section20
// linker exports symbols with address of eaach section
__attribute__((section(".struct_init")))
mystruct_t struct_init = {
.val1 = 1,.val2 = 2};
__attribute__((section(".struct_init_const")))
extern const mystruct_t struct_init_const = {
.val1 = 1, .val2 = 2};
__attribute__((section(".struct_not_init")))
mystruct_t struct_not_init;
主文件
#include <stdint.h>
// This symbols exported by linker
// contains addresses of corresponding sections
extern uintptr_t LNK_STRUCT_INIT_ADDR;
extern uintptr_t LNK_STRUCT_INIT_CONST_ADDR;
extern uintptr_t LNK_STRUCT_NOT_INIT_ADDR;
// Pointers for indirect access to data
mystruct_t* struct_init_ptr = (mystruct_t*)LNK_STRUCT_INIT_ADDR;
const mystruct_t* struct_init_const_ptr = (const mystruct_t*)LNK_STRUCT_INIT_CONST_ADDR;
mystruct_t* struct_not_init_ptr = (mystruct_t*)LNK_STRUCT_NOT_INIT_ADDR;
// Extern variables declarations for DIRECT access data
extern mystruct_t struct_init;
extern const mystruct_t struct_init_const;
extern mystruct_t struct_not_init;
// This is some variables representing config values
// They can be more complex objects(classes) with internal state and logic..
int param1_direct;
int param1_init_const_direct;
int param1_not_init_direct;
int param1_indirect;
int param2_init_const_indirect;
int param1_not_init_indirect;
int main(void)
{
// local variables init with direct access
int param1_direct_local = struct_init.val1;
int param1_init_const_direct_local = struct_init_const.val1;
int param1_not_init_direct_local = struct_not_init.val1;
// local variables init with indirect access
int param1_indirect_local = struct_init_ptr->val1;
int param2_init_const_indirect_local = struct_init_const_ptr->val1;
int param1_not_init_indirect_local = struct_not_init_ptr->val1;
//global variables init direct
param1_direct = struct_init.val1;
param1_init_const_direct = struct_init_const.val1;
param1_not_init_direct = struct_not_init.val1;
//global variables init indirect
param1_indirect = struct_init_ptr->val1;
param2_init_const_indirect = struct_init_const_ptr->val1;
param1_not_init_indirect = struct_not_init_ptr->val1;
while(1){
// use all variables we init above
// usage of variables may also occure in some functions or methods
// directly or indirectly called from this loop
}
}
我想确保 param1_ 变量的初始化将导致从闪存中获取数据。因为可以使用引导加载程序更改闪存部分 20 中的数据(在主固件未运行时)。
问题是:LTO(和其他优化)是否可以丢弃从闪存中获取的数据并仅替换已知值,因为它们在链接时由于初始化而已知。什么方法更好?如果 LTO 可以替换值 - 那么应该避免初始化吗?我知道 volatile 可以提供帮助,但在这种情况下真的需要它吗?
代码示例显示了访问和初始化数据的不同方法。not_init 版本似乎是最好的,因为编译器不能替代任何东西。但是有一些默认参数是个好主意,所以如果可以使用,我更喜欢初始化版本。
应该选择什么方法?
目前我正在使用 GCC 4.9.3,但这是关于任何 C/C++ 编译器的一般问题。