How to put constants in code memory - c++

Question

I need to keep some information about each function in my program in the form of a constant number. I was wondering if it is possible to put the constant for a function just before it in the code memory, so if a function is called through a function pointer, that information could be read by subtracting the value of the function pointer.
To illustrate further, my code memory should look as follows.
ConstantForFunc1
Func1:
....
ConstantForFunc2
Func2:
....
And following is an example code of how I would read that information
FuncPointer f = &Func2;
int constantForFunc2 = *((int*)(f - sizeof(int)));
And note that using Hash tables is too slow for what I'm trying to achieve, so I need a very fast method. And all this modification, which is inserting constants and code to read from them is done by a compiler pass, which I'm writing and which modifies the LLVM IR. Using structures would be too cumbersome for the compiler pass, as it would have to modify a lot of code.

Answer 1

What you are doing doesn't make sense, yet:
You could use structs maybe?
struct example
{
int constantForFunc;
void (*ptrToFunc)();
};
//After declaring, maybe 3, functions
struct example funcList[3] = {{5, &func1}, {10, &func2}, {15, &func3}};
int currentFuncConstant=funcList[1].constantForFunc;
(*funcList[1].ptrToFunc)();
I haven't used function pointers to be honest, probaby has mistakes.

Answer 2

Is this not acceptable at all?:
#include <iostream>
using namespace std;
const int Const__Fxn1 = 1;
void Fxn1()
{
cout << "Fxn1" << endl;
}
const int Const__Fxn2 = 2;
void Fxn2()
{
cout << "Fxn2" << endl;
}
#define GetFxnConst(FxnName) Const__ ## FxnName
int main()
{
cout << GetFxnConst(Fxn1) << endl;
cout << GetFxnConst(Fxn2) << endl;
return 0;
}
Option 2:
#include <iostream>
#include <cstring>
using namespace std;
const volatile int v1 = 0;
volatile unsigned v2 = 0;
void Fxn1()
{
if (v1) { v2 = 0x12345601; }
cout << "Fxn1" << endl;
}
void Fxn2()
{
if (v1) { v2 = 0x12345602; }
cout << "Fxn2" << endl;
}
int FindFxnConst(void(*f)())
{
const unsigned char* p = (const unsigned char*)f;
while (memcmp(p, "\x56\x34\x12", 3))
p++;
return p[-1];
}
int main()
{
Fxn1();
cout << FindFxnConst(Fxn1) << endl;
Fxn2();
cout << FindFxnConst(Fxn2) << endl;
return 0;
}
Output (Ideone):
Fxn1
1
Fxn2
2
You can embed more than 8 bits of data per function by using other magic prefixes, e.g.:
if (v1)
{
v2 = 0x12345611; // byte 1
v2 = 0x789ABC22; // byte 2
v2 = 0xDEF01233; // byte 3
v2 = 0xFEDCBA44; // byte 4
}
This is not necessarily a reliable solution, let alone portable.

Answer 3

Since the addresses of the functions are known from the executable binary (unless they are loaded from a shared library ofcourse), if you have the address space layout randomization (ASLR) off, you could use gperf to generate a highly efficient hash function for you and use that hash function to get the constants for each function.
However, for this, you will have to compile your program twice, first to get the addresses of the functions from the generated binary, so that you could give those addresses as an input to gperf and recompile using the hash function generated by gperf. But you have to be careful that the addresses of the functions from the first compilation do not become different in the second compilation. I am not sure, how to achieve that.
An alternative, would be to do something like gperf just after your program is loaded, so you don't have to compile twice. But I don't know how to do that.