Revert "Revert "ART: Reduce the instructions generated by packed switch.""

This reverts commit b4c137630fd2226ad07dfd178ab15725374220f1.

The underlying issue was fixed by https://android-review.googlesource.com/188271 .

Bug: 26121945
Change-Id: I58b08eb1a9f0a5c861f8cda93522af64bcf63920

1 files changed, 22 insertions, 10 deletions

diff --git a/compiler/optimizing/code_generator_mips.cc b/compiler/optimizing/code_generator_mips.cc
index 9dc9167824..f872bfe6c8 100644
--- a/compiler/optimizing/code_generator_mips.cc
+++ b/compiler/optimizing/code_generator_mips.cc
@@ -4248,19 +4248,31 @@ void InstructionCodeGeneratorMIPS::VisitPackedSwitch(HPackedSwitch* switch_instr
   HBasicBlock* default_block = switch_instr->GetDefaultBlock();
 
   // Create a set of compare/jumps.
+  Register temp_reg = TMP;
+  __ Addiu32(temp_reg, value_reg, -lower_bound);
+  // Jump to default if index is negative
+  // Note: We don't check the case that index is positive while value < lower_bound, because in
+  // this case, index >= num_entries must be true. So that we can save one branch instruction.
+  __ Bltz(temp_reg, codegen_->GetLabelOf(default_block));
+
   const ArenaVector<HBasicBlock*>& successors = switch_instr->GetBlock()->GetSuccessors();
-  for (int32_t i = 0; i < num_entries; ++i) {
-    int32_t case_value = lower_bound + i;
-    MipsLabel* successor_label = codegen_->GetLabelOf(successors[i]);
-    if (case_value == 0) {
-      __ Beqz(value_reg, successor_label);
-    } else {
-      __ LoadConst32(TMP, case_value);
-      __ Beq(value_reg, TMP, successor_label);
-    }
+  // Jump to successors[0] if value == lower_bound.
+  __ Beqz(temp_reg, codegen_->GetLabelOf(successors[0]));
+  int32_t last_index = 0;
+  for (; num_entries - last_index > 2; last_index += 2) {
+    __ Addiu(temp_reg, temp_reg, -2);
+    // Jump to successors[last_index + 1] if value < case_value[last_index + 2].
+    __ Bltz(temp_reg, codegen_->GetLabelOf(successors[last_index + 1]));
+    // Jump to successors[last_index + 2] if value == case_value[last_index + 2].
+    __ Beqz(temp_reg, codegen_->GetLabelOf(successors[last_index + 2]));
+  }
+  if (num_entries - last_index == 2) {
+    // The last missing case_value.
+    __ Addiu(temp_reg, temp_reg, -1);
+    __ Beqz(temp_reg, codegen_->GetLabelOf(successors[last_index + 1]));
   }
 
-  // Insert the default branch for every other value.
+  // And the default for any other value.
   if (!codegen_->GoesToNextBlock(switch_instr->GetBlock(), default_block)) {
     __ B(codegen_->GetLabelOf(default_block));
   }