Optimise the deoptimisation check to improve performance on modern ARM cores....

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
===================================================================
--- src/arm/lithium-codegen-arm.cc	(revision 7916)
+++ src/arm/lithium-codegen-arm.cc	(working copy)
@@ -85,6 +85,7 @@
   return GeneratePrologue() &&
       GenerateBody() &&
       GenerateDeferredCode() &&
+      GenerateDeoptJumpTable() &&
       GenerateSafepointTable();
 }
 
@@ -249,13 +250,52 @@
     __ jmp(code->exit());
   }
 
-  // Force constant pool emission at the end of deferred code to make
+  return !is_aborted();
+}
+
+
+bool LCodeGen::GenerateDeoptJumpTable() {
+  // Emit previous constant pool entries so that we can safely emit the
+  // deoptimization jump table.
+  masm()->CheckConstPool(true, false);
+  // Check that the jump table is accessible from everywhere in the function
+  // code, ie that offsets to the table can be encoded in the 24bit signed
+  // immediate of a branch instruction.
+  // To simplify we consider the code size from the first instruction to the
+  // end of the jump table. We also don't consider the pc load delta.
+  // Each entry in the jump table generates only one instruction.
+  if (!is_int24((masm()->pc_offset() / Assembler::kInstrSize) +
+      deopt_jump_table_.length())) {
+    Abort("Generated code is too large");
+  }
+  // We also need to check that the jump table will require fewer constant

[Søren Thygesen Gjesse, 2011/05/18 06:47:13]

I think we can avoid this restriction - see below.

[Alexandre, 2011/05/18 09:13:07]

Removed.

On 2011/05/18 06:47:13, Søren Gjesse wrote:

+  // pool entries than the maximum authorized.
+  // Each deopt entry will require exactly one constant pool entry.
+  if (deopt_jump_table_.length() > Assembler::MaxNumPRInfo()) {
+    Abort("Too many deoptimization entries");
+  }
+
+  // Block the constant pool emission during the jump table emission.
+  __ BlockConstPoolFor(deopt_jump_table_.length());
+  __ RecordComment("[ Deoptimisation jump table");
+  Label table_start;
+  __ bind(&table_start);
+  for (int i = 0; i < deopt_jump_table_.length(); i++) {
+    __ bind(&deopt_jump_table_[i].label);

[Søren Thygesen Gjesse, 2011/05/18 06:47:13]

How about avoiding using the const pool all together by emitting a -4 pc
relative load of pc for each table entry.

ldr pc, [pc, #-4]
<address of deopt table entry 0>
ldr pc, [pc, #-4]
<address of deopt table entry 1>
...

[Alexandre, 2011/05/18 09:13:07]

Done.
I also moved back the forced emission of the constant pool in the deferred code
generation.

It puts a heavier burden on the previous check, but that should never be a
problem.
It is also worth noting that - if we ever need it - we could go around the first
constraint by generating jump tables regularly instead of only at the end of the
code.

On 2011/05/18 06:47:13, Søren Gjesse wrote:

+    __ mov(pc, Operand(reinterpret_cast<int32_t>(deopt_jump_table_[i].address),
+                       RelocInfo::RUNTIME_ENTRY));
+  }
+  ASSERT(masm()->InstructionsGeneratedSince(&table_start) ==
+      deopt_jump_table_.length());
+  __ RecordComment("]");
+
+  // Force constant pool emission at the end of the jump table to make
   // sure that no constant pools are emitted after the official end of
   // the instruction sequence.
   masm()->CheckConstPool(true, false);
 
-  // Deferred code is the last part of the instruction sequence. Mark
-  // the generated code as done unless we bailed out.
+  // The deoptimization jump table is the last part of the instruction
+  // sequence. Mark the generated code as done unless we bailed out.
   if (!is_aborted()) status_ = DONE;
   return !is_aborted();
 }
@@ -595,19 +635,18 @@
     return;
   }
 
+  if (FLAG_trap_on_deopt) __ stop("trap_on_deopt", cc);
+
   if (cc == al) {
-    if (FLAG_trap_on_deopt) __ stop("trap_on_deopt");
     __ Jump(entry, RelocInfo::RUNTIME_ENTRY);
   } else {
-    if (FLAG_trap_on_deopt) {
-      Label done;
-      __ b(&done, NegateCondition(cc));
-      __ stop("trap_on_deopt");
-      __ Jump(entry, RelocInfo::RUNTIME_ENTRY);
-      __ bind(&done);
-    } else {
-      __ Jump(entry, RelocInfo::RUNTIME_ENTRY, cc);
+    // We often have several deopts to the same entry, reuse the last
+    // jump entry if this is the case.
+    if (deopt_jump_table_.is_empty() ||
+        (deopt_jump_table_.last().address != entry)) {
+      deopt_jump_table_.Add(JumpTableEntry(entry));
     }
+    __ b(cc, &deopt_jump_table_.last().label);
   }
 }