Replace constant conditional branches by unconditional branches
BUG=
R=jpp@chromium.org, stichnot@chromium.org
Review URL: https://codereview.chromium.org/1847423003 .
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 9fc6634..9c59dac 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -84,11 +84,15 @@
OutEdges = Insts.rbegin()->getTerminatorEdges();
}
-// Validate each Phi instruction in the node with respect to control flow. For
-// every phi argument, its label must appear in the predecessor list. For each
-// predecessor, there must be a phi argument with that label. We don't check
-// that phi arguments with the same label have the same value.
-void CfgNode::validatePhis() {
+// Ensure each Phi instruction in the node is consistent with respect to control
+// flow. For each predecessor, there must be a phi argument with that label.
+// If a phi argument's label doesn't appear in the predecessor list (which can
+// happen as a result of e.g. unreachable node elimination), its value is
+// modified to be zero, to maintain consistency in liveness analysis. This
+// allows us to remove some dead control flow without a major rework of the phi
+// instructions. We don't check that phi arguments with the same label have the
+// same value.
+void CfgNode::enforcePhiConsistency() {
for (Inst &Instr : Phis) {
auto *Phi = llvm::cast<InstPhi>(&Instr);
// We do a simple O(N^2) algorithm to check for consistency. Even so, it
@@ -106,8 +110,11 @@
break;
}
}
- if (!Found)
- llvm::report_fatal_error("Phi error: label for bad incoming edge");
+ if (!Found) {
+ // Predecessor was unreachable, so if (impossibly) the control flow
+ // enters from that predecessor, the value should be zero.
+ Phi->clearOperandForTarget(Label);
+ }
}
for (CfgNode *InNode : getInEdges()) {
bool Found = false;
