Subzero: Prune unreachable nodes after constructing the Cfg.

Subzero: Prune unreachable nodes after constructing the Cfg.

The gcc torture test suite has examples where there is a function call (to a routine that throws an exception or aborts or something), followed by an "unreachable" instruction, followed by more code that may e.g. return a value to the caller.  In these examples, the code following the unreachable is itself unreachable.

Problems arise when the unreachable code references a variable defined in the reachable code.  This triggers a liveness consistency error because the use of the variable has no reaching definition.

It's a bit surprising that LLVM actually allows this, but it does so we need to deal with it.

The solution is, after initial CFG construction, do a traversal starting from the entry node and then delete any undiscovered nodes.

There is code in Subzero that assumes Cfg::Nodes[i]->Number == i, so the nodes need to be renumbered after pruning.  The alternative was to set Nodes[i]=nullptr and not change the node number, but that would mean peppering the code base with CfgNode null checks.

BUG= none
R=jvoung@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=69d3f9c6698150a88fcc89a20bb93823895dfaf8

[Jim Stichnoth, 2015-03-20 23:24:27]

stichnot@chromium.org changed reviewers:
+ jvoung@chromium.org, kschimpf@google.com, mtrofin@chromium.org

[jvoung (off chromium), 2015-03-23 17:13:22]

lgtm

[Jim Stichnoth, 2015-03-23 17:33:42]

Committed patchset #1 (id:1) manually as
69d3f9c6698150a88fcc89a20bb93823895dfaf8 (presubmit successful).