Cache lookups at megamorphic call sites in optimized code.

runtime/vm/stub_code_ia32.cc

Index: runtime/vm/stub_code_ia32.cc
diff --git a/runtime/vm/stub_code_ia32.cc b/runtime/vm/stub_code_ia32.cc
index 140eb70bb1eab70ac52fe2ed55645e6e3f312b9a..3f6fbc1e2cb053617962e47a64c32aeab2eff6e8 100644
--- a/runtime/vm/stub_code_ia32.cc
+++ b/runtime/vm/stub_code_ia32.cc
@@ -564,6 +564,38 @@ void StubCode::GenerateDeoptimizeStub(Assembler* assembler) {
 }
 
 
+void StubCode::GenerateMegamorphicMissStub(Assembler* assembler) {
+  const Immediate raw_null =
+      Immediate(reinterpret_cast<intptr_t>(Instructions::null()));
+  AssemblerMacros::EnterStubFrame(assembler);
+  __ pushl(raw_null);  // Space for the result of the runtime call.
+  // Load the receiver into EAX.  The argument count in the arguments
+  // descriptor is a smi.

[srdjan, 2012/12/03 19:10:46]

Please document that EDX contains argument descriptor.

[Kevin Millikin (Google), 2012/12/06 14:03:11]

OK.

+  __ movl(EAX, FieldAddress(EDX, Array::data_offset()));
+  // Three words in the stack above the last argument.
+  __ movl(EAX, Address(ESP, EAX, TIMES_2, 3 * kWordSize));
+  __ pushl(EAX);  // Receiver.
+  __ pushl(ECX);  // IC data.
+  __ pushl(EDX);  // Arguments descriptor.
+  __ CallRuntime(kMegamorphicCacheMissHandlerRuntimeEntry);
+  __ popl(EDX);  // Arguments descriptor.

[srdjan, 2012/12/03 19:10:46]

You need to preserve/restore argument descriptor and ic data explicitly and must
assume that any of outgoing arguments will be destroyed/changed in C code.

[Kevin Millikin (Google), 2012/12/06 14:03:11]

Thank you, good catch.

+  __ popl(ECX);  // IC data.
+  __ popl(EAX);  // Discard.
+  __ popl(EAX);  // Return value from the runtime call (instructions).
+  __ LeaveFrame();

[srdjan, 2012/12/03 19:10:46]

null as result should not be possible, should it? Or is it used for noSuchMethod
handling? 

[Kevin Millikin (Google), 2012/12/06 14:03:11]

No such method, implicit closures.

+
+  Label lookup;
+  __ cmpl(EAX, raw_null);
+  __ j(EQUAL, &lookup, Assembler::kNearJump);
+  __ addl(EAX, Immediate(Instructions::HeaderSize() - kHeapObjectTag));
+  __ jmp(EAX);
+
+  __ Bind(&lookup);
+  __ int3();

[Vyacheslav Egorov (Google), 2012/12/03 14:55:22]

What's up with this int3?

If it can not be reached then kill it (or make a purely debug code).

If it can be reached then it means our tests do not cover this case. Please
ensure test coverage for this.

Ditto x64.

[Kevin Millikin (Google), 2012/12/06 14:03:11]

Thanks for catching it.  It's a reminder to myself to add a test that covers
this case since we don't have any.  I forgot about it when sending this out for
review.

It turns out that the code below didn't quite work because indirect calls break
our pattern matching on instance calls used by the InstanceFunctionLookup stub.

I have already fixed that separately.

+  __ jmp(&StubCode::InstanceFunctionLookupLabel());
+}
+
+
 // Called for inline allocation of arrays.
 // Input parameters:
 //   EDX : Array length as Smi.