Fix tree-ssa-strlen handling of partial clobbers (PR85814)

Fix tree-ssa-strlen handling of partial clobbers (PR85814)

In this PR we have:

  c_5 = c_4(D) + 4;
  c_12 = c_5 + 1;
  *c_5 = 2;
  a = 2;		// A
  c_21 = c_12 + 1;
  *c_12 = 2;
  a = 2;		// B
  c_28 = c_21 + 1;
  *c_21 = 2;
  a = 2;
  c_7 = c_28 + 1;
  *c_28 = 2;

where a is a global int.  We decide that A can't clobber *c_5 == c_4[4]
because the latter implies that c_4 is an object of 5 bytes or more,
whereas a has exactly 4 bytes.

The assumption for B and *c_5 is the same, but when considering B and
*c_12, we only follow the definition of c_12 to c_5 + 1 (for good
reason) and so have *c_12 == c_5[1].  We then don't have the same
size guarantee and so assume that B could clobber *c_12.  This leads
to a situation in which the strinfo for c_5 is still valid but the
next strinfo (c_12) isn't.  We then segfaulted while trying to get
the strinfo for c_21 + 1 == c_5 + 3 because get_stridx_plus_constant
assumed that c_5's next strinfo (c_12) would be valid too.

And of course it should be valid really.  It doesn't make sense for the
string based at c_5 to be valid but a substring of it to be invalid.
I don't think we can guarantee that such weird corner cases never
happen though, even if we tried to avoid this one.

One possibility would be to mark c_12 as valid on the basis that c_5
is valid, but I'm not sure the complication is worth it given that it
seems to trigger very rarely.  A better optimisation would be to get
the unroller to clean up after itself a bit more...

Although this particular instance of the bug relies on r249880, I think
we could have similar problems in GCC 7.  It would be much harder to
trigger though, especially since it relies on unfolded IR like the above.

2018-05-21  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/85814
	* tree-ssa-strlen.c (get_stridx_plus_constant): Cope with
	a null return from get_strinfo when unsharing the next
	strinfo in the chain.

gcc/testsuite/
	PR tree-optimization/85814
	* gcc.dg/torture/pr85814.c: New test.

From-SVN: r260488