编译器优化不能编译常量吗？

Question

I have the following string declared as a constant in my code. The purpose is to provide a crude and simple way of storing simple metadata in the compiled output.

const char myString1[] ="abc123\0";
const char myString2[] = {'a','b','c','1','2','3','\0'};

When I inspect the output with a hex editor, I see other string constants but "abc123" does not appear. This leads me to believe that the optimizations that are enabled are causing the lines not to be compiled, as they are never referenced in the program.

Is there a way in code to force this to compile, or another way (in code) of getting this metadata into the binary? I don't want to do any manipulation of the binary post-compile, the goal is to keep it as simple as possible.

compiler flags

-O2 -g -Wall -c -fmessage-length=0 -fno-builtin -ffunction-sections -mcpu=cortex-m3 -mthumb

Answer 1

I think you are looking for the used attribute:

`used'

This attribute, attached to a variable, means that the variable must be emitted even if it appears that the variable is not referenced.

When applied to a static data member of a C++ class template, the attribute also means that the member will be instantiated if the class itself is instantiated.

Apply it like

__attribute__((used))
const char myString1[] ="abc123\0";
__attribute__((used))
const char myString2[] = {'a','b','c','1','2','3','\0'};

Given the compiler flags you posted, it is almost certainly the linker. The -ffunction-sections flag puts each definition into its own section in the object files. This allows the linker to easily determine that a data item or function is not referenced and omit it from the final binary.

Answer 2

Use the binutils strings command to see if these strings are present in your binary.

If they have been optimized out, you can try to use the volatile qualifier when you declare them. Note that if they are not used even with the volatile qualifier some compilers can still optimized them out.

Answer 3

I've come up with a solution that uses attributes and involves modifying the link script.

First I define a custom section called ".metadata".

__attribute__ ((section(".metadata")))

Then, in the SECTIONS block of the .ld script I added a KEEP(*(.metadata)) which will force the linker to include .metadata even if it's not used

.text :
{
    KEEP(*(.isr_vector))
    KEEP(*(.metadata))
    *(.text*)
    *(.rodata*)

} > MFlash32

NOTE

I found that the __attribute__ keyword had to be on the same line as the variable or else it didn't actually show up in the binary, though the .metadata section did show up in the memory map.

Answer 4

If you have these variables in file scope, the compiler must provide the strings, since he can't know if they will be used from a different compilation unit. So any of your ".o" files where you place these variables, must contain the string.

Now a clever linker could decide for the final binary that these constants are not needed. (I have never observed that, though.) If this is the case for your platform, you should use the variable on a "hypothetical" path, that in reality will never be taken by the program. Something like

int main(int argc, char*argv[]){
  switch (argv[0][0]) {
    case 1: return myString1[argv[0][1]];
    case 2: return myString2[argv[0][1]];
  }
  ...
}