[RDF] Add option to keep dead phi nodes in DFG

Dead phi nodes are needed for code motion (such as copy propagation),
where a new use would be placed in a location that would be dominated
by a dead phi. Such a transformation is not legal for copy propagation,
and the existence of the phi would prevent it, but if the phi is not
there, it may appear to be valid.

llvm-svn: 267932

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp b/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
index d735acf457b..642a8785def 100644
--- a/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonRDFOpt.cpp
@@ -289,7 +289,10 @@ bool HexagonRDFOpt::runOnMachineFunction(MachineFunction &MF) {
   HexagonRegisterAliasInfo HAI(HRI);
   TargetOperandInfo TOI(HII);
   DataFlowGraph G(MF, HII, HRI, *MDT, MDF, HAI, TOI);
-  G.build();
+  // Dead phi nodes are necessary for copy propagation: we can add a use
+  // of a register in a block where it would need a phi node, but which
+  // was dead (and removed) during the graph build time.
+  G.build(BuildOptions::KeepDeadPhis);
 
   if (RDFDump)
     dbgs() << "Starting copy propagation on: " << MF.getName() << '\n'