Convert assignments from C++ to C

Question

I would like help to convert these made assignments into C++ to C:

string binario = bitset<256>(address).to_string();
int InicioDado = 256-log2(cacheconfig.lineSize);
string dado = binario.substr(InicioDado, log2(cacheconfig.lineSize));
int InicioIndex = InicioDado-log2(cacheconfig.numLines/cacheconfig.associativity);
string index = binario.substr(InicioIndex ,log2(cacheconfig.numLines/cacheconfig.associativity));
int InicioTag = 256-cacheconfig.lineSize;
string tag = binario.substr(InicioTag, 256-InicioTag-log2(cacheconfig.lineSize));
long unsigned int Tag = bitset<256>(tag).to_ulong();
long unsigned int Index = bitset<256>(index).to_ulong();
long unsigned int Dado = bitset<256>(dado).to_ulong();

Whereas adress is a variable unsigned long long int and the rest are like int.

Answer 1

I’m guessing that cacheconfig.numLines, cacheconfig.associativity and cacheconfig.lineSize are of the type unsigned int. If they’re not, maybe you’ll need a cast somewhere, or you can change them to be unsigned.

The first step is to arrange these common subexpressions in one place, and thereby define the variables a and b:

unsigned int a = log2(cacheconfig.numLines / cacheconfig.associativity);
unsigned int b = log2(cacheconfig.lineSize);
string binario = bitset<256>(address).to_string();
int InicioDado = 256 - b;
string dado = binario.substr(InicioDado, b);
int InicioIndex = InicioDado - a;
string index = binario.substr(InicioIndex, a);
int InicioTag = 256 - cacheconfig.lineSize;
string tag = binario.substr(InicioTag, 256 - InicioTag - b);
long unsigned int Tag = bitset<256>(tag).to_ulong();
long unsigned int Index = bitset<256>(index).to_ulong();
long unsigned int Dado = bitset<256>(dado).to_ulong();

Let’s eliminate the variables of the type int intermediaries:

unsigned int a = log2(cacheconfig.numLines / cacheconfig.associativity);
unsigned int b = log2(cacheconfig.lineSize);
string binario = bitset<256>(address).to_string();
string dado = binario.substr(256 - b, b);
string index = binario.substr(256 - b - a, a);
string tag = binario.substr(256 - cacheconfig.lineSize, cacheconfig.lineSize - b);
long unsigned int Tag = bitset<256>(tag).to_ulong();
long unsigned int Index = bitset<256>(index).to_ulong();
long unsigned int Dado = bitset<256>(dado).to_ulong();

Now, let’s see what this does. You take the address, converts to a binary string using bitset, Insert this string into some parts and use the bitset to convert these binary substrings to numbers later. Since this is a numerical calculation, it does not seem ideal to use strings in it. This seems to be a task for bitwise operators <<, >> and &.

First you take the last b bits, which are the Dado. The a previous bits are the Index. The Tag It seems a little strange, but it’s the last cacheconfig.lineSize bits with the exception of the last b bits. This is strange because there is an intersection between the Tag and the Index.

Well, to get the last k bits of a number x, being k >= 1, we can do this:

x & ((1 << k) - 1)

To discard the last k digits, we do that:

x >> k

Let’s create macros to make it easier (adding the parentheses around each parameter and macro as a whole to be hygienic):

#define OBTER_ULTIMOS_BITS(x, k) ((x) & ((1 << (k)) - 1))
#define DESCARTAR_ULTIMOS_BITS(x, k) ((x) >> (k))

So your code stays like this:

unsigned int a = log2(cacheconfig.numLines / cacheconfig.associativity);
unsigned int b = log2(cacheconfig.lineSize);
long unsigned int Tag = DESCARTAR_ULTIMOS_BITS(OBTER_ULTIMOS_BITS(address, cacheconfig.lineSize), b);
long unsigned int Index = DESCARTAR_ULTIMOS_BITS(OBTER_ULTIMOS_BITS(address, b + a), b);
long unsigned int Dado = OBTER_ULTIMOS_BITS(address, b);

And this code is the same in both C and C++.

Maybe you’ll still need a function that (re)defines your log2. Here follows an implementation that I copied from that reply. Only include it if you cannot for some reason use the default implementation:

unsigned int log2(unsigned int x) {
    unsigned int resultado = 0;
    while (x >>= 1) resultado++;
    return resultado;
}