How can I do these asm ROR and BT/RCR operations in C? Rotate, or shift in a bit from another number

Question

I'm converting some assembly code to C to be able to use it with the current compiler environment I have to work with.

I've reached 2 operations I don't know how to translate to C. Anyone know how to do it?

In both, offset is an unsigned 32-bit integer and shift is a signed integer value. C_FLAG is a bool.

OP1:

__asm {
    __asm mov ecx, shift
    __asm ror offset, cl
}

OP2:

__asm {
    __asm bt dword ptr C_FLAG, 0
    __asm rcr offset, 1
}

Thank you very much for your expertise.

P.S.: I'm not the original developer, nor I have seen many x86 assembly code...

Answer 1

Assuming that the flags are not used to hold persistent state (which is a reasonable assumption), OP1 is equivalent to:

/* Precondition: `*offset` is in range 0..0xFFFFFFFF */
void inline op1(unsigned long *offset, int shift)
{
    shift = ((unsigned)shift) % 32;

    if (shift)
        *offset = ((*offset >> shift) | (*offset << (32 - shift))) & 0xFFFFFFFFUL;
}

and OP2 is equivalent to:

/* Precondition: `*offset` is in range 0..0xFFFFFFFF */
void inline op2(unsigned long *offset, unsigned long C_FLAG)
{
    *offset = (*offset >> 1) | ((C_FLAG & 1) << 31);
}

(On systems with 32 bit long, the precondition is automatically satisfied).