[Bug ada/26797] [4.3 regression] ACATS cxh1001 fails

[kenner at vlsi1 dot ultra dot nyu dot edu]

------- Comment #34 from kenner at vlsi1 dot ultra dot nyu dot edu  2007-03-09 13:59 -------
Subject: Re:  [4.3 regression] ACATS cxh1001 fails

> You may well respond that you are only supposed to forget
> information you deduced from the range of the type, not information
> you worked out by some other means.  But these cannot really be
> distinguished, c.f. VRP.

Exactly.  That's indeed the problem.  It's similar to the whole
signed-overflow issue: what you're able to safely do with with
VRP-derived information depends on how you derived it and there's
no way of tracking that.

> Surely you only have to prove that it is in the range of Y, since if it
> is not in the range of X then you have a bounded error, and doing a NOP_EXPR
> should be perfectly legal.

Think of a range check being done in a subscript reference in the LHS of
an assignment.  A bounded error is not allowed to cause a memory store
outside the bounds of an array.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26797