Re: [PATCH v2 4/6] Do not use HP_TIMING_NOW for random bits

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Adhemerval,

> +# include <random-bits.h>
> +# define RANDOM_BITS(Var) ((Var) = random_bits ())
> 
> This define is not used (removed above).

> I think we need to still define it if we eventually decide to sync it back
> to gnulib.

Well this is the question - do we really need all this clutter just for gnulib?
It looks to me we should keep the code as clean as possible (so we don't
need any the !_LIBC code given these files are always in LIBC).

> I fact the new line should not be added, since random_time_bits should already
> get the random_bits() value. In any case I think we can remove random_time_bits
> altogether and just call RANDOM_BITS on value instead.

Agreed.

> And it seems 'value' is static by design, but I do agree there is no impeding
> reason to continue to do so.

Indeed.

---
diff --git a/sysdeps/posix/tempname.c b/sysdeps/posix/tempname.c
index 5217cb38e1..d062e4b82f 100644
--- a/sysdeps/posix/tempname.c
+++ b/sysdeps/posix/tempname.c
@@ -73,6 +73,13 @@
 #ifdef _LIBC
 # include <random-bits.h>
 # define RANDOM_BITS(Var) ((Var) = random_bits ())
+# else
+# define RANDOM_BITS(Var) \
+    {                                                                         \
+      struct timeval tv;                                                      \
+      __gettimeofday (&tv, NULL);                                             \
+      (Var) = ((uint64_t) tv.tv_usec << 16) ^ tv.tv_sec;                      \
+    }
 #endif
 
I don't see the point of this, especially using gettimeofday. If we want to export
these changes to gnulib, we could just add random_bits to gnulib.


-  value += random_time_bits ^ __getpid ();
-  value += random_bits () ^ __getpid ();
+  RANDOM_BITS (value);
+  value ^= __getpid ();
+  /* Shuffle the lower bits to minimize the pid bias due low maximum value.  */
+  value = (value << 24) | (value >> 8);

random_bits already does that shuffle, so doing it again doesn't help. It's better
to avoid the aliasing of getpid with the random bits, eg. value ^= __get_pid << 32
so we end up with more than 32 random bits.

Wilco