ARM: Emit RSB (reverse-subtract) for subtraction with constant lhs

src/arm/lithium-arm.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1310 matching lines @@
   } else {
     return DoArithmeticT(Token::MUL, instr);
   }
 }
 
 
 LInstruction* LChunkBuilder::DoSub(HSub* instr) {
   if (instr->representation().IsInteger32()) {
     ASSERT(instr->left()->representation().IsInteger32());
     ASSERT(instr->right()->representation().IsInteger32());
+
+    if (instr->left()->IsConstant()) {
+      // If lhs is constant, do reverse subtraction instead.
+      return DoRSub(instr);
+    }
+
     LOperand* left = UseRegisterAtStart(instr->left());
     LOperand* right = UseOrConstantAtStart(instr->right());
     LSubI* sub = new(zone()) LSubI(left, right);
     LInstruction* result = DefineAsRegister(sub);
     if (instr->CheckFlag(HValue::kCanOverflow)) {
       result = AssignEnvironment(result);
     }
     return result;
   } else if (instr->representation().IsDouble()) {
     return DoArithmeticD(Token::SUB, instr);
   } else {
     return DoArithmeticT(Token::SUB, instr);
   }
 }
 
+
+LInstruction* LChunkBuilder::DoRSub(HSub* instr) {
+  ASSERT(instr->representation().IsInteger32());
+  ASSERT(instr->left()->representation().IsInteger32());
+  ASSERT(instr->right()->representation().IsInteger32());
+
+  // Note: The lhs of the subtraction becomes the rhs of the
+  // reverse-subtraction.
+  LOperand* left = UseRegisterAtStart(instr->right());
+  LOperand* right = UseOrConstantAtStart(instr->left());
+  LRSubI* rsb = new(zone()) LRSubI(left, right);
+  LInstruction* result = DefineAsRegister(rsb);
+  if (instr->CheckFlag(HValue::kCanOverflow)) {
+    result = AssignEnvironment(result);
+  }
+  return result;
+}
+
+
 LInstruction* LChunkBuilder::DoMultiplyAdd(HMul* mul, HValue* addend) {
   LOperand* multiplier_op = UseRegisterAtStart(mul->left());
   LOperand* multiplicand_op = UseRegisterAtStart(mul->right());
   LOperand* addend_op = UseRegisterAtStart(addend);
   return DefineSameAsFirst(new(zone()) LMultiplyAddD(addend_op, multiplier_op,
                                                      multiplicand_op));
 }
 
 LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
   if (instr->representation().IsInteger32()) {
@@ skipping 944 matching lines @@
 
 
 LInstruction* LChunkBuilder::DoLoadFieldByIndex(HLoadFieldByIndex* instr) {
   LOperand* object = UseRegister(instr->object());
   LOperand* index = UseRegister(instr->index());
   return DefineAsRegister(new(zone()) LLoadFieldByIndex(object, index));
 }
 
 
 } }  // namespace v8::internal