Improvements in lithium code generation. Recognizing if some operands are constants, we can often s…

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index c6ba51770331417db3b30847214692805a4708ea..9d04f0e2bc8cc4ded55e42a6153d4c8f5d9cbfcd 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -3287,14 +3287,22 @@ void LCodeGen::DoLoadExternalArrayPointer(
 
 void LCodeGen::DoAccessArgumentsAt(LAccessArgumentsAt* instr) {
   Register arguments = ToRegister(instr->arguments());
-  Register length = ToRegister(instr->length());
-  Register index = ToRegister(instr->index());
   Register result = ToRegister(instr->result());
-  // There are two words between the frame pointer and the last argument.
-  // Subtracting from length accounts for one of them add one more.
-  __ sub(length, length, index);
-  __ add(length, length, Operand(1));
-  __ ldr(result, MemOperand(arguments, length, LSL, kPointerSizeLog2));
+  if (instr->length()->IsConstantOperand() &&
+      instr->index()->IsConstantOperand()) {
+    int const_index = ToInteger32(LConstantOperand::cast(instr->index()));
+    int const_length = ToInteger32(LConstantOperand::cast(instr->length()));
+    int index = (const_length - const_index) + 1;
+    __ ldr(result, MemOperand(arguments, index * kPointerSize));
+  } else {
+    Register length = ToRegister(instr->length());
+    Register index = ToRegister(instr->index());
+    // There are two words between the frame pointer and the last argument.
+    // Subtracting from length accounts for one of them add one more.
+    __ sub(length, length, index);
+    __ add(length, length, Operand(1));
+    __ ldr(result, MemOperand(arguments, length, LSL, kPointerSizeLog2));
+  }
 }