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, 5 insertions, 0 deletions

diff --git a/compiler/optimizing/induction_var_range.cc b/compiler/optimizing/induction_var_range.cc
index 8568062933..72719f89bf 100644
--- a/compiler/optimizing/induction_var_range.cc
+++ b/compiler/optimizing/induction_var_range.cc
@@ -384,6 +384,11 @@ bool InductionVarRange::HasKnownTripCount(const HLoopInformation* loop,
                                           /*out*/ int64_t* trip_count) const {
   bool is_constant = false;
   CheckForFiniteAndConstantProps(loop, &is_constant, trip_count);
+  // Set negative trip counts as 0, since it means that no trips would happen. Note that if the
+  // `is_constant` value is false, `trip_count` would be disregareded.
+  if (*trip_count < 0) {
+    *trip_count = 0;
+  }
   return is_constant;
 }