tree-ssa-strlen: Fix pdata->maxlen computation [PR110603]

tree-ssa-strlen: Fix pdata->maxlen computation [PR110603]

On the following testcase we emit an invalid range of [2, 1] due to
UB in the source.  Older VRP code silently swapped the boundaries and
made [1, 2] range out of it, but newer code just ICEs on it.

The reason for pdata->minlen 2 is that we see a memcpy in this case
setting both elements of the array to non-zero value, so strlen (a)
can't be smaller than 2.  The reason for pdata->maxlen 1 is that in
char a[2] array without UB there can be at most 1 non-zero character
because there needs to be '\0' termination in the buffer too.

IMHO we shouldn't create invalid ranges like that and even creating
for that case a range [1, 2] looks wrong to me, so the following patch
just doesn't set maxlen in that case to the array size - 1, matching
what will really happen at runtime when triggering such UB (strlen will
be at least 2, perhaps more or will crash).
This is what the second hunk of the patch does.

The first hunk fixes a fortunately harmless thinko.
If the strlen pass knows the string length (i.e. get_string_length
function returns non-NULL), we take a different path, we get to this
only if all we know is that there are certain number of non-zero
characters but we don't know what it is followed with, whether further
non-zero characters or zero termination or either of that.
If we know exactly how many non-zero characters it is, such as
char a[42];
...
  memcpy (a, "01234567890123456789", 20);
then we take an earlier if for the INTEGER_CST case and set correctly
just pdata->minlen to 20 in that case, but if we have something like
  int len;
  ...
  if (len < 15 || len > 32) return;
  memcpy (a, "0123456789012345678901234567890123456789", len);
then we have [15, 32] range for the nonzero_chars and we set pdata->minlen
correctly to 15, but incorrectly set also pdata->maxlen to 32.  That is
not what the above implies, it just means that in some cases we know that
there are at least 32 non-zero characters, followed by something we don't
know.  There is no guarantee that there is '\0' right after it, so it
means nothing.
The reason this is harmless, just confusing, is that the code a few lines
later fortunately overwrites this incorrect pdata->maxlen value with
something different (either array length - 1 or all ones etc.).

2024-01-29  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/110603
	* tree-ssa-strlen.cc (get_range_strlen_dynamic): Remove incorrect
	setting of pdata->maxlen to vr.upper_bound (which is unconditionally
	overwritten anyway).  Avoid creating invalid range with minlen
	larger than maxlen.  Formatting fix.

	* gcc.c-torture/compile/pr110603.c: New test.