Optimising bitwise operations in C

Question

I had a problem in hand as this : "Exercise 2-6. Write a function setbits(x,p,n,y) that returns x with the n bits that begin at position p set to the rightmost n bits of y, leaving the other bits unchanged."

I've written a function for this as below. This is working as expected.

int func_setx(int x,int p,int n,int y)
{
    int a_t= ~0 << (p+n);
    int b_t= ~a_t >> n;
    int x_t= x& (a_t | b_t);    // a temporary x which has the bits to be changed as 0 , rest of the bits unchanged.

    int mask= (y << p) & (~(~0 << (p+n)));     // a mask which has required bits from y in positions to be set , rest all bits 0.

    printf("\nCheckpoint : mask= %x  x_t= %x\n",mask,x_t);

    int result= mask|x_t;
    return result;
}

But I somehow feel the logic is long and can be optimized, but can't think of any other way yet. Can anyone suggest any optimization to this please?

You'll probably find an answer at Bit Twiddling Hacks — and you should bookmark the page even if you don't find the answer there. — Jonathan Leffler
– Jonathan Leffler, Commented Jun 27, 2013 at 5:14
Numerous duplicates, e.g. k&r exercise confusion with bit-operations and returns x with the n bits that begin at position p set to the rightmost n bits of y, leaving other bits unchanged and Setting Bits in C — Paul R
– Paul R, Commented Jun 27, 2013 at 8:32

Carl Norum · Accepted Answer · 2013-06-27 04:51:14Z

10

To make an n bit mask:

mask_y = (1U << n) - 1;

To start it at bit p:

mask_x = mask_y << p;

Clear the appropriate bits in x:

x &= ~mask_x;

Extract the bits from y:

y &= mask_y;

Upshift them to position p:

y <<= p;

Put it all together:

result = x | y;

Or in a more compact form:

mask = (1U << n) - 1;
result = x & ~(mask << p);
result |= (y & mask) << p;

edited Jun 27, 2013 at 4:51

answered Jun 27, 2013 at 4:37

Carl Norum

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4 Comments

Carl Norum Over a year ago

Assuming 32-bit integers, if n == 32, this solution causes undefined behaviour. That case is just return y, though, so it's easy to special-case. Alternately, use 1ULL (or however you want to get a 64-bit type) instead of 1U.

legends2k Over a year ago

+1 for not just giving the code but also explaining what it does step-by-step

Jim Balter Over a year ago

Carl, your comment should properly be part of your answer.

Carl Norum Over a year ago

It's more of an aside than part of the answer, really, which is why I left it a comment. NBD.

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