Why test the bit and then set it?

The "tstbit(i)" is being done on vector A, and the "setbit(i)" is being done on vector B - but only when the "tstbit(i)" returns true.
This is in accordance with the op's description of finding "every set bit in vector A, then set the corresponding bit(s) in vector B".
Of course, this is a rather naive and inefficient approach. A smarter approach would be to do B = A|B, or in gmp parlance mpz_ior(B, A, B);

Why would calling a function to set a bit be quicker than setting the bit directly?

As you suggest, they might not be any quicker, and could even be slower. For single bit operations, you're probably right. (The assembler code on which gmp is generally built could count as something in its favour - but it's not always going to come into play.)
Personally however, at least where *big* vectors are involved, I'd want to see benchmarks done before ruling out gmp.

Cheers,
Rob

In reply to Re^9: Bit vector fiddling with Inline C by syphilis
in thread Bit vector fiddling with Inline C by oxone

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