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Re: [RFC] Isolate & simplify paths with undefined behaviour


On Fri, 18 Oct 2013, Jeff Law wrote:

Back in 2011 I wrote code to detect cases when traversing a particular path could be proven to trigger undefined behaviour (such as a null pointer dereference). That original patch would find the control dependent edges leading to the dereference and eliminate those edges. The result being those paths with undefined behaviour simply vanished.

The problem with that implementation is there could have been observable side effects on those paths prior to triggering the undefined behaviour.

Maybe a new -fretroactive-undefined-behavior? (for later, obviously)

At that time I speculated we could isolate the path (via block copying) with undefined behaviour. On the duplicate path we'd replace the undefined behaviour with a trap and remove only the outgoing edges. That would still preserve any visible side effects on the undefined path up to the undefined behaviour and we still often get to simplify the main path, though not as much as the original version from 2011.

That's precisely what this patch does. When we have a PHI which feeds a memory dereference in the same block as the PHI and one (or more) of the PHI args is NULL we duplicate the block, redirect incoming edges with the NULL PHI args to the duplicate and change the statement with the memory dereference to a trap.

Cool. As a new pass doing something quite clear, I am trying to read it to learn, and have a couple minor comments:

* should cfg_altered be static (or a member of the pass class)?

* tree-vrp has a function infer_nonnull_range, do you think we could share it? We now store the VRP ranges for integers, but not for pointers. If we did (or maybe just a non-null bit), the pass could just test that bit on the variable found in the PHI.

--
Marc Glisse


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