MIPS: Fix zlib critical performance regression.

src/mips/lithium-codegen-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.7
 // 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 4782 matching lines @@
   // Heap number map check.
   __ lw(scratch1, FieldMemOperand(input_reg, HeapObject::kMapOffset));
   __ LoadRoot(at, Heap::kHeapNumberMapRootIndex);
   // This 'at' value and scratch1 map value are used for tests in both clauses
   // of the if.
 
   if (instr->truncating()) {
     // Performs a truncating conversion of a floating point number as used by
     // the JS bitwise operations.
     Label no_heap_number, check_bools, check_false;
-    __ Branch(&no_heap_number, ne, scratch1, Operand(at));  // HeapNumber map?
+    // Check HeapNumber map.
+    __ Branch(USE_DELAY_SLOT, &no_heap_number, ne, scratch1, Operand(at));
     __ mov(scratch2, input_reg);

[Paul Lind, 2014/01/31 18:54:40]

nit: we should add '// In delay slot.' here, to stay consistent with other DS
branches here. I'll add this when I land it.

     __ TruncateHeapNumberToI(input_reg, scratch2);
     __ Branch(&done);
 
     // Check for Oddballs. Undefined/False is converted to zero and True to one
     // for truncating conversions.
     __ bind(&no_heap_number);
     __ LoadRoot(at, Heap::kUndefinedValueRootIndex);
     __ Branch(&check_bools, ne, input_reg, Operand(at));
     ASSERT(ToRegister(instr->result()).is(input_reg));
     __ Branch(USE_DELAY_SLOT, &done);
@@ skipping 921 matching lines @@
   __ Subu(scratch, result, scratch);
   __ lw(result, FieldMemOperand(scratch,
                                 FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal