[libgfortran,patch] Scramble bytes in the user-provided random seeds (PR 32812)

[FX Coudert]

Hi all,

Current library implementation of RANDOM_NUMBER() is such that the  
highest bits of the user-provided seed determine the mots significant  
bits of floating-points values return, and the lowest bits of the  
seed control the least significant bits of fp values. Thus, a user  
providing us with a seed with good lower bits but poor higher bits  
will find that the fp values are not very random at all... one such  
example being the use of DATE_AND_TIME to provide for random seeds.

This patch adds a little to the quality of implementation by adding a  
(bijective) scrambling operation between the seed the user gives us  
and the seed we actually use. Two functions, scramble_seed and  
unscramble_seed, are used when  RANDOM_NUMBER() is called with a PUT=  
or GET= argument, which shuffles bytes of the seed.

Regtested on x86_64-linux, with both -m32 and -m64. Tested manually  
with different seeds of different qualities.


FX

PS: Of course, we're not adding quality to poor seeds (I'm not  
changing entropy with a bijective function!). We're just providing a  
little invisible help to one particular case of half-bad, half-good  
seeds.


-- 
François-Xavier Coudert
http://www.homepages.ucl.ac.uk/~uccafco/
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