Re: [PATCH] adjust object size computation for union accesses and PHIs (PR 92765)

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Jan 15, 2020 at 09:52:57PM +0100, Jakub Jelinek wrote:
> This looks wrong.  For one, this function is used for two purposes now and
> you tweak it for one, but more importantly, whether he initial stmt
> you see is a PHI or not can't make a difference, how is that case e.g.
> different from _1 = PHI <_3, _4>; _2 = _1 + 1; and asking about _2?
> For _1, you'd use (correctly) the maximum, but if called on _2, you'd ask
> (wrongly) for minimum instead of maximum.

And now with testcases.  strlenopt-95.c shows the above.

> This also looks like a hack to shut up the particular testcases instead of
> really playing with what the IL provides.  Instead of the unions, consider
> e.g. C++ placement new, have a pointer to a buffer into which you placement
> new one structure, take address of some member in it, pass it to something,
> if it doesn't have a destructor do a C++ placement new into the same buffer
> but with different structure, take address of a different member with the
> same address as the first member, do the str*cmp on it that invokes this
> stuff.  SCCVN will (likely) find out that the values of those two pointers
> are the same and just use the former pointer in the latter case.

And strlenopt-93.C shows the latter.  strlenopt-94.C is similar, just to
show that it breaks equally badly with non-PODs that will be constructed by
placement new and destructed later.

	Jakub

Attachment: strlenopt-95.c
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Attachment: strlenopt-93.C
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Attachment: strlenopt-94.C
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