On Fri, 12 Jan 2018, Jakub Jelinek wrote:
Apparently Linux kernel contains various UB code that has been worked around
through -fno-strict-overflow in 7.x and before, but when
POINTER_TYPE_OVERFLOW_UNDEFINED has been removed it now fails to boot.
The following patch follows the comments in the PR, essentially reverts
Bin's removal of that, except that it is now controlled by a separate option
and is included in TYPE_OVERFLOW_{WRAPS,UNDEFINED} macros.
I am pretty sure there are other patterns in match.pd that need protection
now, with pointer_diff.
(for op (simple_comparison)
(simplify
(op (pointer_diff@3 @0 @2) (pointer_diff @1 @2))
(if (!TYPE_OVERFLOW_SANITIZED (TREE_TYPE (@2)))
(op @0 @1))))
This is ready for sanitizers but not for wrapping pointers. And there are
a few more like it.
There were discussions at some point of implementing -fwrapp in the
front-end, generating (unsigned)q-(unsigned)p or (unsigned)p<(unsigned)q
for pointer operations. It has the advantage that the middle-end doesn't
need to know about those variants, but it might have some fallout (and I
am not sure what to do when the middle-end creates new pointer
operations), and I can understand that in stage 3 you are more interested
in an approach that looks like a reversal to a former known-ok state.
--
Marc Glisse
