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, 12 insertions, 0 deletions

diff --git a/compiler/optimizing/ssa_liveness_analysis.cc b/compiler/optimizing/ssa_liveness_analysis.cc
index b9d8731cc2..a5609fc466 100644
--- a/compiler/optimizing/ssa_liveness_analysis.cc
+++ b/compiler/optimizing/ssa_liveness_analysis.cc
@@ -273,6 +273,18 @@ void SsaLivenessAnalysis::ComputeLiveRanges() {
     }
 
     if (block->IsLoopHeader()) {
+      if (kIsDebugBuild && block->GetLoopInformation()->IsIrreducible()) {
+        // To satisfy our liveness algorithm, we need to ensure loop headers of
+        // irreducible loops do not have any live-in instructions, except constants
+        // and the current method, which can be trivially re-materialized.
+        for (uint32_t idx : live_in->Indexes()) {
+          HInstruction* instruction = GetInstructionFromSsaIndex(idx);
+          DCHECK(instruction->GetBlock()->IsEntryBlock()) << instruction->DebugName();
+          DCHECK(!instruction->IsParameterValue()) << instruction->DebugName();
+          DCHECK(instruction->IsCurrentMethod() || instruction->IsConstant())
+              << instruction->DebugName();
+        }
+      }
       size_t last_position = block->GetLoopInformation()->GetLifetimeEnd();
       // For all live_in instructions at the loop header, we need to create a range
       // that covers the full loop.