[llvm-commits] Debug-check in SSAUpdater detecting PHI/CFG missmatches

[Ludwig Meier]

On Jul 22, 2011, 09:03 -0700, Bob Wilson wrote:
> On Jul 16, 2011, at 2:56 AM, Ludwig Meier wrote:
> 
> > This patch adds a debug-check to SSAUpdater, that verifies that the
> > predecessor list (as derived from the CFG) matches the predecessor list
> > in the PHI-nodes.
> > 
> > For performance reasons the SSAUpdater uses the incoming-edges of a
> > PHI-Node (if one exists) to determine the predecessors of a basic-block.
> > When the PHI-nodes incoming edges differ from the CFG, subsequent calls
> > to SSAUpdater::GetValue...() may crash.
> > This occurs only when the IR is currently invalid, of course.
> 
> I think this should go in as a verifier check.  We don't want to have to debugging checks for well-formed IR scattered throughout LLVM.

The verifier probably already catches this.
But you wont get to the verifyer in this case, because using the
SSAUpdater in invalid IR would cause a segfault earlier. To find the
reason for the crash, you have to analyze the internals of SSAUpdater
(to figure out that SSAUpdater uses the PHI-nodes to enumerate a blocks
predecessors).
With this check added you would hopefully not have to poke around in
SSAUpaters internals to find whats wrong.

As a sidenote: In my case I was well aware, that I had invalid
PHI-nodes. I am using SSAUpdater in the process of fixing them. Now I'm
syncing the PHI-nodes to the CFG before using the SSAUpdater.
-- 
Ludwig Meier