Re: [csmith-bugs] 2.0.0: read from and write to the same location within two sequence points

["Xuejun Yang" <jxyang@cs.utah.edu>]

In essence, line 61 is doing something like:

*p = func_10(g_2, g_2)

where p points to g_2 and func_10 modifies g_2.

I think the accesses to g_2 constitute no ambiguity: firstly function
parameters are evaluated before the function; secondly a function are
evaluated before its return value is used. In other words, the accesses to
g_2 follow the sequence:

read g_2
read g_2
sequence point
write g_2 (in func_10)
sequence point
write g_2 (through *p)

The first two reads can be re-arranged. But that doesn't introduce
ambiguity.

-Xuejun

> 
> Hello,
> 
> Csmith 2.0.0 generated the attached program.
> 
> If instrumented with a printf() statement before and after line 61, a
compilation
> on a 64-bit little-endian platform shows that line 61 is reached, and the
> functions called there all terminate.
> It only takes a cursory glance at func_10() to see that if it terminates,
it
> terminates having modified g_2.
> Therefore it seems to me that line 61 contains undefined behavior, as
> g_2 is accessed there multiple times for reading and for writing in
addition to
> the modification by func_10().