This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Fold pointer range checks with equal spans


On Tue, 31 Jul 2018, Richard Biener wrote:

Also, when @2 == @0 + (@1+1) then the original condition is true but
((sizetype) @0 - (sizetype) @2 + @1) > (@1 * 2) is not?
   (sizetype) @0 - (sizetype) (@0 + @1 + 1) + @1 > @1 * 2
-> -1 > @1 * 2

which is false.  So I can't really see how you can apply this transform in
general (it would be fine for generating alias checks but a bit more
pessimistic).

But maybe I am missing something?

It relies on sizetype being unsigned: (sizetype)-1 > @1 * 2 is true.

Hmm, so mathematically this is

 (@0 - @2) % modreduce + @1 > @1 * 2

then, but I don't want to think too much about this since Marc didn't
object here ;)

We already transform abs(x)<=3 into (unsigned)x+3u<=6u, that's the usual
way we do range checking so I didn't pay much attention to that part.
(tempted to say: "I didn't want to think too much about this since
Richard was going to do it anyway" ;-)

Turning multiple comparisons of the form P + cst CMP Q + cst into a
range check on P - Q sounds good (we don't really have to restrict to
the case where the range is symmetric). Actually, just turning P + cst
CMP Q + cst into P - Q CMP cst should do it, we should already have code
to handle range checking on integers (modulo the issue of CSE P-Q and
Q-P). But I don't know if a couple :s is sufficient to make this
transformation a good canonicalization.

If we start from a comparison of pointer_plus, I think it would make
sense to use pointer_diff.

I believe currently we try to use pointer operations (pointer_plus,
pointer_diff, lt) only for related pointers and we cast to some integer
type for wilder cases (implementation of std::less in C++ for instance).
On the other hand, in an alias check, the 2 pointers are possibly
unrelated, so maybe the code emitted for an alias check should be
changed.

--
Marc Glisse


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]