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
Review URL: https://codereview.chromium.org/1027933002
diff --git a/src/IceCfg.h b/src/IceCfg.h
index b4c3748..8f74d07 100644
--- a/src/IceCfg.h
+++ b/src/IceCfg.h
@@ -135,9 +135,9 @@
// Passes over the CFG.
void translate();
// After the CFG is fully constructed, iterate over the nodes and
- // compute the predecessor edges, in the form of
- // CfgNode::InEdges[].
- void computePredecessors();
+ // compute the predecessor and successor edges, in the form of
+ // CfgNode::InEdges[] and CfgNode::OutEdges[].
+ void computeInOutEdges();
void renumberInstructions();
void placePhiLoads();
void placePhiStores();
