Implement irreducible loop support in optimizing.

So we don't fallback to the interpreter in the presence of
irreducible loops.

Implications:
- A loop pre-header does not necessarily dominate a loop header.
- Non-constant redundant phis will be kept in loop headers, to
  satisfy our linear scan register allocation algorithm.
- while-graph optimizations, such as gvn, licm, lse, and dce
  need to know when they are dealing with irreducible loops.

Change-Id: I2cea8934ce0b40162d215353497c7f77d6c9137e

1 files changed, 9 insertions, 5 deletions

diff --git a/compiler/optimizing/register_allocator.cc b/compiler/optimizing/register_allocator.cc
index d399bc2d7a..5ab4547e22 100644
--- a/compiler/optimizing/register_allocator.cc
+++ b/compiler/optimizing/register_allocator.cc
@@ -179,9 +179,11 @@ void RegisterAllocator::AllocateRegistersInternal() {
       ProcessInstruction(inst_it.Current());
     }
 
-    if (block->IsCatchBlock()) {
-      // By blocking all registers at the top of each catch block, we force
-      // intervals used after catch to spill.
+    if (block->IsCatchBlock() ||
+        (block->GetLoopInformation() != nullptr && block->GetLoopInformation()->IsIrreducible())) {
+      // By blocking all registers at the top of each catch block or irreducible loop, we force
+      // intervals belonging to the live-in set of the catch/header block to be spilled.
+      // TODO(ngeoffray): Phis in this block could be allocated in register.
       size_t position = block->GetLifetimeStart();
       BlockRegisters(position, position + 1);
     }
@@ -1864,8 +1866,10 @@ void RegisterAllocator::Resolve() {
   // Resolve non-linear control flow across branches. Order does not matter.
   for (HLinearOrderIterator it(*codegen_->GetGraph()); !it.Done(); it.Advance()) {
     HBasicBlock* block = it.Current();
-    if (block->IsCatchBlock()) {
-      // Instructions live at the top of catch blocks were forced to spill.
+    if (block->IsCatchBlock() ||
+        (block->GetLoopInformation() != nullptr && block->GetLoopInformation()->IsIrreducible())) {
+      // Instructions live at the top of catch blocks or irreducible loop header
+      // were forced to spill.
       if (kIsDebugBuild) {
         BitVector* live = liveness_.GetLiveInSet(*block);
         for (uint32_t idx : live->Indexes()) {