Frees up LR on arm for use by the register allocator.

runtime/vm/stub_code_arm.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_ARM)
 
 #include "vm/assembler.h"
 #include "vm/code_generator.h"
 #include "vm/cpu.h"
@@ skipping 458 matching lines @@
 //   +------------------+
 //   | PC marker        |
 //   +------------------+
 //   | ...              | <- SP of optimized frame
 //
 // Parts of the code cannot GC, part of the code can GC.
 static void GenerateDeoptimizationSequence(Assembler* assembler,
                                            bool preserve_result) {
   // DeoptimizeCopyFrame expects a Dart frame, i.e. EnterDartFrame(0), but there
   // is no need to set the correct PC marker or load PP, since they get patched.
-  __ mov(IP, Operand(LR));
-  __ mov(LR, Operand(0));
-  __ EnterFrame((1 << PP) | (1 << FP) | (1 << IP) | (1 << LR), 0);
+
+  // IP has the potentially live LR value. LR was clobbered by the call with
+  // the return address, so move it into IP to set up the Dart frame.
+  __ eor(IP, IP, Operand(LR));
+  __ eor(LR, IP, Operand(LR));
+  __ eor(IP, IP, Operand(LR));
+
+  // Set up the frame manually. We can't use EnterFrame because we can't
+  // clobber LR (or any other register) with 0, yet.
+  __ sub(SP, SP, Operand(kWordSize));  // Make room for PC marker of 0.
+  __ Push(IP);  // Push return address.
+  __ Push(FP);
+  __ mov(FP, Operand(SP));
+  __ Push(PP);
+
+  // Now that IP holding the return address has been written to the stack,
+  // we can clobber it with 0 to write the null PC marker.
+  __ mov(IP, Operand(0));
+  __ str(IP, Address(SP, +3 * kWordSize));
+
   // The code in this frame may not cause GC. kDeoptimizeCopyFrameRuntimeEntry
   // and kDeoptimizeFillFrameRuntimeEntry are leaf runtime calls.
   const intptr_t saved_result_slot_from_fp =
       kFirstLocalSlotFromFp + 1 - (kNumberOfCpuRegisters - R0);
   // Result in R0 is preserved as part of pushing all registers below.
 
   // Push registers in their enumeration order: lowest register number at
   // lowest address.
   __ PushList(kAllCpuRegistersList);
 
@@ skipping 1482 matching lines @@
   const Register right = R0;
   __ ldr(left, Address(SP, 1 * kWordSize));
   __ ldr(right, Address(SP, 0 * kWordSize));
   GenerateIdenticalWithNumberCheckStub(assembler, left, right, temp);
   __ Ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM