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fix incorrect sequence generation in *rand48 prng functions

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patch by Jens Gustedt. this fixes a bug reported by Nadav Har'El. the
underlying issue was that a left-shift by 16 bits after promotion of
unsigned short to int caused integer overflow. while some compilers
define this overflow case as "shifting into the sign bit", doing so
doesn't help; the sign bit then gets extended through the upper bits
in subsequent arithmetic as unsigned long long. this patch imposes a
promotion to unsigned prior to the shift, so that the result is
well-defined and matches the specified behavior.

(cherry picked from commit 05cef96d9e)
This commit is contained in:
Rich Felker 2014-09-22 17:55:08 -04:00
parent 0a7aca63c0
commit 9f93f6d119
1 changed files with 2 additions and 2 deletions
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4
src/prng/__rand48_step.c
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@ -3,8 +3,8 @@
uint64_t __rand48_step(unsigned short *xi, unsigned short *lc)
{
uint64_t a, x;
x = xi[0] | xi[1]<<16 | xi[2]+0ULL<<32;
a = lc[0] | lc[1]<<16 | lc[2]+0ULL<<32;
x = xi[0] | xi[1]+0U<<16 | xi[2]+0ULL<<32;
a = lc[0] | lc[1]+0U<<16 | lc[2]+0ULL<<32;
x = a*x + lc[3];
xi[0] = x;
xi[1] = x>>16;