Ensure the first predecessor of a loop is the pre header.
Note that the check in ssa_phi_elimination.cc was very defensive:
it does not affect the outcome of the algorithm whether the
loop phi takes itself as the first input.
It makes things consistent to always have the pre header as first
input.
Change-Id: Ic86248c1f38af67f7432782f6deefae1f4bf1ab6
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index 207c605..1d7e678 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -194,6 +194,11 @@
}
pre_header->AddSuccessor(header);
}
+
+ // Make sure the second predecessor of a loop header is the back edge.
+ if (header->GetPredecessors().Get(1) != info->GetBackEdges().Get(0)) {
+ header->SwapPredecessors();
+ }
}
void HGraph::SimplifyCFG() {
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index d6dfeae..142b517 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -331,6 +331,13 @@
block->successors_.Add(this);
}
+ void SwapPredecessors() {
+ DCHECK(predecessors_.Size() == 2);
+ HBasicBlock* temp = predecessors_.Get(0);
+ predecessors_.Put(0, predecessors_.Get(1));
+ predecessors_.Put(1, temp);
+ }
+
size_t GetPredecessorIndexOf(HBasicBlock* predecessor) {
for (size_t i = 0, e = predecessors_.Size(); i < e; ++i) {
if (predecessors_.Get(i) == predecessor) {
diff --git a/compiler/optimizing/ssa_phi_elimination.cc b/compiler/optimizing/ssa_phi_elimination.cc
index 65675dc..d541a62 100644
--- a/compiler/optimizing/ssa_phi_elimination.cc
+++ b/compiler/optimizing/ssa_phi_elimination.cc
@@ -83,6 +83,10 @@
}
}
+static bool LoopPreHeaderIsFirstPredecessor(HBasicBlock* block) {
+ return block->GetPredecessors().Get(0) == block->GetLoopInformation()->GetPreHeader();
+}
+
void SsaRedundantPhiElimination::Run() {
// Add all phis in the worklist.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
@@ -102,7 +106,10 @@
// Find if the inputs of the phi are the same instruction.
HInstruction* candidate = phi->InputAt(0);
- // A loop phi cannot have itself as the first phi.
+ // A loop phi cannot have itself as the first phi. Note that this
+ // check relies on our simplification pass ensuring the pre-header
+ // block is first in the list of predecessors of the loop header.
+ DCHECK(!phi->IsLoopHeaderPhi() || LoopPreHeaderIsFirstPredecessor(phi->GetBlock()));
DCHECK_NE(phi, candidate);
for (size_t i = 1; i < phi->InputCount(); ++i) {
