On 05/24/2018 03:40 PM, Marc Glisse wrote:
On Wed, 23 May 2018, Martin Sebor wrote:
On 05/23/2018 08:57 AM, Jeff Law wrote:
On 05/10/2018 04:05 PM, Marc Glisse wrote:
On Thu, 10 May 2018, Martin Sebor wrote:
Can you please comment/respond to Jeff's question below and
confirm whether my understanding of the restriction (below)
is correct?
I don't remember it at all, I really should have expanded that
comment...
The documentation of nonzero_chars seems to indicate that, unless
full_string_p, it is only a lower bound on the length of the string, so
not suitable for this kind of alias check. I don't know if we also have
easy access to some upper bound.
(I noticed while looking at this pass that it could probably use
POINTER_DIFF_EXPR more)
So ISTM that we'd need to guard the code that uses si->nonzero_chars in
maybe_invalidate to also check FULL_STRING_P since it appears we're
using si->nonzero_chars as a string length.
I'm not sure I see why. Can you explain?
Here's my explanation of the current approach. si->nonzero_chars
is the lower bound on si's length. maybe_invalidate() invalidates
the string length which is only necessary when something overwrites
one of the first si->nonzero_chars of the array. It doesn't matter
whether si is nul-terminated at that point.
When you say "invalidates the string length", does that mean it only
invalidates nonzero_chars (the lower bound), or does that also determine
if you can CSE a call to strlen before and after this instruction? Your
argument only applies in the first case.
"invalidates the string length" means that maybe_invalidate(STMT)
removes the length information record for a string whose length
might be modified by STMT.
If the exact string length is known (full_string_p is true) then
its length can only be modified by writing a nul into one of
the leading nonzero_chars. If the exact string length is not
known (i.e., full_string_p is false), then nonzero_chars is not
constant and writes into the string are assumed to change its
length, and thus invalidate it.
But I'm not sure I understand from your brief description what
use case you have in mind. If you have an example I'll test it.
Martin
