Improve PRNG jumping when using threads

Improve PRNG jumping when using threads

Currently, when a new thread needs to use the RANDOM_NUMBER intrinsic,
the per-thread PRNG state is initialized by copying the master state
and then jumping forwards N*2**128 entries in the stream so that the
PRNG streams for different threads don't alias each other, where N is
the number of threads that have so far initialized the PRNG.

With this patch the master state itself is jumped forwards once each
time a new thread initializes the PRNG, thus obviating the need to
jump through all the N-1 previous streams. Effectively turning an O(N)
algorithm into an O(1) one.

Regtested on x86_64-pc-linux-gnu.

libgfortran/ChangeLog:

2019-09-05  Janne Blomqvist  <jb@gcc.gnu.org>

	* intrinsics/random.c (master_init): Replace with
	master_state.init.
	(njumps): Remove variable.
	(master_state): Make instance of struct prng_state.
	(init_rand_state): When jumping, update the master_state once
	instead of keeping track of how many jumps need to be done.
	(SZU64): Modify to handle new master_state.
	(SZ): Likewise.
	(random_seed_i4): Likewise.
	(random_seed_i8): Likewise.

From-SVN: r275397