MIPS: Optimize AccessArgumentsAt.

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index e40021dfeab574f4e883c6b7b1eae41c2e0ec79c..5a8249efc3bcd27a23b3b8d80265b48897cca3fc 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -3037,22 +3037,44 @@ void LCodeGen::DoLoadExternalArrayPointer(
 void LCodeGen::DoAccessArgumentsAt(LAccessArgumentsAt* instr) {
   Register arguments = ToRegister(instr->arguments());
   Register result = ToRegister(instr->result());
-  if (instr->length()->IsConstantOperand() &&
-      instr->index()->IsConstantOperand()) {
-    int const_index = ToInteger32(LConstantOperand::cast(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.
+  if (instr->length()->IsConstantOperand()) {
     int const_length = ToInteger32(LConstantOperand::cast(instr->length()));
-    int index = (const_length - const_index) + 1;
-    __ lw(result, MemOperand(arguments, index * kPointerSize));
+    if (instr->index()->IsConstantOperand()) {
+      int const_index = ToInteger32(LConstantOperand::cast(instr->index()));
+      int index = (const_length - const_index) + 1;
+      __ lw(result, MemOperand(arguments, index * kPointerSize));
+    } else {
+      Register index = ToRegister(instr->index());
+      __ li(at, Operand(const_length + 1));
+      __ Subu(result, at, index);
+      __ sll(at, result, kPointerSizeLog2);
+      __ Addu(at, arguments, at);
+      __ lw(result, MemOperand(at));
+    }
+  } else if (instr->index()->IsConstantOperand()) {
+    Register length = ToRegister(instr->length());
+    int const_index = ToInteger32(LConstantOperand::cast(instr->index()));
+    int loc = const_index - 1;
+    if (loc != 0) {
+      __ Subu(result, length, Operand(loc));
+      __ sll(at, result, kPointerSizeLog2);
+      __ Addu(at, arguments, at);
+      __ lw(result, MemOperand(at));
+    } else {
+      __ sll(at, length, kPointerSizeLog2);
+      __ Addu(at, arguments, at);
+      __ lw(result, MemOperand(at));
+    }
   } 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.
-    __ subu(length, length, index);
-    __ Addu(length, length, Operand(1));
-    __ sll(length, length, kPointerSizeLog2);
-    __ Addu(at, arguments, Operand(length));
-    __ lw(result, MemOperand(at, 0));
+    __ Subu(result, length, index);
+    __ Addu(result, result, 1);
+    __ sll(at, result, kPointerSizeLog2);
+    __ Addu(at, arguments, at);
+    __ lw(result, MemOperand(at));
   }
 }