[Arm]: Enable use of extended out-of-line constant pool for Arm.

src/arm/full-codegen-arm.cc

Index: src/arm/full-codegen-arm.cc
diff --git a/src/arm/full-codegen-arm.cc b/src/arm/full-codegen-arm.cc
index f1e6380a5cf55eaf2668632eabfecb54496d6999..302055173b700f724020f972ae14a81dd2bee90d 100644
--- a/src/arm/full-codegen-arm.cc
+++ b/src/arm/full-codegen-arm.cc
@@ -346,13 +346,27 @@ void FullCodeGenerator::EmitProfilingCounterDecrement(int delta) {
 }
 
 
+static const int kProfileCounterResetSequenceLength = 5 * Assembler::kInstrSize;
+
+
 void FullCodeGenerator::EmitProfilingCounterReset() {
+  Assembler::BlockConstPoolScope block_const_pool(masm_);
+  PredictableCodeSizeScope predictable_code_size_scope(
+      masm_, kProfileCounterResetSequenceLength);
+  Label start;
+  __ bind(&start);
   int reset_value = FLAG_interrupt_budget;
   if (info_->is_debug()) {
     // Detect debug break requests as soon as possible.
     reset_value = FLAG_interrupt_budget >> 4;
   }
   __ mov(r2, Operand(profiling_counter_));
+  // The mov instruction above can be either 1, 2 or 3 instructions depending
+  // upon whether it is an extended constant pool - insert nop to compensate.
+  ASSERT(masm_->InstructionsGeneratedSince(&start) <= 3);
+  while (masm_->InstructionsGeneratedSince(&start) != 3) {
+    __ nop();
+  }
   __ mov(r3, Operand(Smi::FromInt(reset_value)));
   __ str(r3, FieldMemOperand(r2, Cell::kValueOffset));
 }
@@ -4753,10 +4767,18 @@ static Address GetInterruptImmediateLoadAddress(Address pc) {
   Address load_address = pc - 2 * Assembler::kInstrSize;
   if (!FLAG_enable_ool_constant_pool) {
     ASSERT(Assembler::IsLdrPcImmediateOffset(Memory::int32_at(load_address)));
+  } else if (Assembler::IsLdrPpRegOffset(Memory::int32_at(load_address))) {
+    // This is an extended constant pool lookup.
+    load_address -= 2 * Assembler::kInstrSize;
+    ASSERT(Assembler::IsMovW(Memory::int32_at(load_address)));
+    ASSERT(Assembler::IsMovT(
+        Memory::int32_at(load_address + Assembler::kInstrSize)));
   } else if (Assembler::IsMovT(Memory::int32_at(load_address))) {
+    // This is a movw_movt immediate load.
     load_address -= Assembler::kInstrSize;
     ASSERT(Assembler::IsMovW(Memory::int32_at(load_address)));
   } else {
+    // This is a small constant pool lookup.
     ASSERT(Assembler::IsLdrPpImmediateOffset(Memory::int32_at(load_address)));
   }
   return load_address;
@@ -4778,12 +4800,16 @@ void BackEdgeTable::PatchAt(Code* unoptimized_code,
       //   ; load interrupt stub address into ip - either of:
       //   ldr ip, [pc/pp, <constant pool offset>]  |   movw ip, <immed low>

[ulan, 2014/07/01 13:29:47]

This comments suggests only two cases, but there are three (the third being
movw_movt immediate)

[rmcilroy, 2014/07/02 16:35:54]

Done.

       //                                            |   movt ip, <immed high>
+      //                                            |   ldr  ip, [pp, ip]
       //   blx ip
+      //  <reset profiling counter>
       //  ok-label
 
-      // Calculate branch offet to the ok-label - this is the difference between
-      // the branch address and |pc| (which points at <blx ip>) plus one instr.
-      int branch_offset = pc + Assembler::kInstrSize - branch_address;
+      // Calculate branch offset to the ok-label - this is the difference
+      // between the branch address and |pc| (which points at <blx ip>) plus
+      // kProfileCounterResetSequence instructions
+      int branch_offset = pc - Instruction::kPCReadOffset - branch_address +

[ulan, 2014/07/01 13:29:48]

Where does Instruction::kPCReadOffset come from?

[rmcilroy, 2014/07/02 16:35:54]

If you look at Assembler::branch_offset() it applies kPCReadOffset to a pc
offset.  This is used by Assembler::b(Label..) before calling through to the raw
Assembler::b(int..) function.

I think the code here was wrong originally, but "got away with it" due to the
fact that it didn't take the code generated by EmitProfilingCounterReset into
account in the calculation (I think it jumped to the __str(r3,
FieldMemOffset(r2, Cell::kValueOffset) at L371 rather than the instruction after
as intended, but this worked because r3 still held the old counter value.

[ulan, 2014/07/03 09:19:55]

Thanks, now I got it. Nice catch!

+          kProfileCounterResetSequenceLength;
       patcher.masm()->b(branch_offset, pl);
       break;
     }
@@ -4795,6 +4821,7 @@ void BackEdgeTable::PatchAt(Code* unoptimized_code,
       //   ldr ip, [pc/pp, <constant pool offset>]  |   movw ip, <immed low>
       //                                            |   movt ip, <immed high>

[ulan, 2014/07/01 13:29:47]

Update the comment?

[rmcilroy, 2014/07/02 16:35:54]

Done.

       //   blx ip
+      //  <reset profiling counter>
       //  ok-label
       patcher.masm()->nop();
       break;