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Full Discussion: Which is more expensive ?
Top Forums Programming Which is more expensive ? Post 302131762 by Perderabo on Monday 13th of August 2007 03:39:02 PM
Old 08-13-2007
I would code your first snippet but compile with an optimizer. These days optimizers will unroll loops if unrolling is advantageous. That particular loop is not a real great candidate for unrolling anyway. A better candidate would be:

for(i=0; i<100; i++) A[i]=0;

Most superscalar cpus can execute:
A[i]=0;
A[i+1]=0;
A[i+2]=0;
simultaneously. How deep it can go depends on the cpu and that's why leaving unrolling to an optimizer is a good idea. The optimizer should know the target cpu. But your case involved a system call which is different. You're only saving some loop overhead.

Apparently, if you explicitly unroll a loop when it is not advantageous, most optimizers will not reroll the loop. At least this was the case circa 1998 when my copy of "High Performance Computing" was published. If you have that book, see chapter 8, "Loop Optimizations" and chapter 9, "Understanding Parallelism". This is still a great book and it's not just for Fortran programmers.

Anyway, if you are not in control of which fd's might be open, you need to to loop up to OPEN_MAX closing them. High fd's might have been opened and then setrlimit() called lower to the max fd.
 

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DSA_generate_parameters(3SSL)					      OpenSSL					     DSA_generate_parameters(3SSL)

NAME
DSA_generate_parameters - generate DSA parameters SYNOPSIS
#include <openssl/dsa.h> DSA *DSA_generate_parameters(int bits, unsigned char *seed, int seed_len, int *counter_ret, unsigned long *h_ret, void (*callback)(int, int, void *), void *cb_arg); DESCRIPTION
DSA_generate_parameters() generates primes p and q and a generator g for use in the DSA. bits is the length of the prime to be generated; the DSS allows a maximum of 1024 bits. If seed is NULL or seed_len < 20, the primes will be generated at random. Otherwise, the seed is used to generate them. If the given seed does not yield a prime q, a new random seed is chosen and placed at seed. DSA_generate_parameters() places the iteration count in *counter_ret and a counter used for finding a generator in *h_ret, unless these are NULL. A callback function may be used to provide feedback about the progress of the key generation. If callback is not NULL, it will be called as follows: o When a candidate for q is generated, callback(0, m++, cb_arg) is called (m is 0 for the first candidate). o When a candidate for q has passed a test by trial division, callback(1, -1, cb_arg) is called. While a candidate for q is tested by Miller-Rabin primality tests, callback(1, i, cb_arg) is called in the outer loop (once for each witness that confirms that the candidate may be prime); i is the loop counter (starting at 0). o When a prime q has been found, callback(2, 0, cb_arg) and callback(3, 0, cb_arg) are called. o Before a candidate for p (other than the first) is generated and tested, callback(0, counter, cb_arg) is called. o When a candidate for p has passed the test by trial division, callback(1, -1, cb_arg) is called. While it is tested by the Miller- Rabin primality test, callback(1, i, cb_arg) is called in the outer loop (once for each witness that confirms that the candidate may be prime). i is the loop counter (starting at 0). o When p has been found, callback(2, 1, cb_arg) is called. o When the generator has been found, callback(3, 1, cb_arg) is called. RETURN VALUE
DSA_generate_parameters() returns a pointer to the DSA structure, or NULL if the parameter generation fails. The error codes can be obtained by ERR_get_error(3). BUGS
Seed lengths > 20 are not supported. SEE ALSO
dsa(3), ERR_get_error(3), rand(3), DSA_free(3) HISTORY
DSA_generate_parameters() appeared in SSLeay 0.8. The cb_arg argument was added in SSLeay 0.9.0. In versions up to OpenSSL 0.9.4, callback(1, ...) was called in the inner loop of the Miller-Rabin test whenever it reached the squaring step (the parameters to callback did not reveal how many witnesses had been tested); since OpenSSL 0.9.5, callback(1, ...) is called as in BN_is_prime(3), i.e. once for each witness. 1.0.0e 2002-09-25 DSA_generate_parameters(3SSL)
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