Eliminate never taken loops

If a loop is never taken we can safely eliminate it.

Known limitation: Due to how the loop trip calculation works,
if the calculation would overflow we would get "unknown" number
of trips. This could happen even for loops with 0 real iterations
like `(int i = Integer.MAX_VALUE; i < Integer.MIN_VALUE; i++)`.

Bug: 336236538
Fixes: 336236538
Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b
Test: m test-art-host-gtest-art_compiler_tests64
Change-Id: I26cae89a593e3375f9f1990a47f3c8973a76364c

1 files changed, 8 insertions, 1 deletions

diff --git a/compiler/optimizing/loop_optimization.cc b/compiler/optimizing/loop_optimization.cc
index 8d698d6c82..215986910b 100644
--- a/compiler/optimizing/loop_optimization.cc
+++ b/compiler/optimizing/loop_optimization.cc
@@ -1094,9 +1094,16 @@ bool HLoopOptimization::TryFullUnrolling(LoopAnalysisInfo* analysis_info, bool g
 bool HLoopOptimization::TryLoopScalarOpts(LoopNode* node) {
   HLoopInformation* loop_info = node->loop_info;
   int64_t trip_count = LoopAnalysis::GetLoopTripCount(loop_info, &induction_range_);
+  if (trip_count == 0) {
+    // Mark the loop as dead.
+    HIf* loop_hif = loop_info->GetHeader()->GetLastInstruction()->AsIf();
+    int32_t constant = loop_info->Contains(*loop_hif->IfTrueSuccessor()) ? 0 : 1;
+    loop_hif->ReplaceInput(graph_->GetIntConstant(constant), 0u);
+    return true;
+  }
+
   LoopAnalysisInfo analysis_info(loop_info);
   LoopAnalysis::CalculateLoopBasicProperties(loop_info, &analysis_info, trip_count);
-
   if (analysis_info.HasInstructionsPreventingScalarOpts() ||
       arch_loop_helper_->IsLoopNonBeneficialForScalarOpts(&analysis_info)) {
     return false;