Insert dummy conversion instruction for impossible conversion requests.

runtime/vm/flow_graph_optimizer.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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 379 matching lines @@
     converted = new UnboxFloat32x4Instr(use->CopyWithType(), deopt_id);
   } else if ((from == kUnboxedFloat32x4) && (to == kTagged)) {
     converted = new BoxFloat32x4Instr(use->CopyWithType());
   } else if ((from == kTagged) && (to == kUnboxedUint32x4)) {
     ASSERT((deopt_target != NULL) || (use->Type()->ToCid() == kUint32x4Cid));
     const intptr_t deopt_id = (deopt_target != NULL) ?
         deopt_target->DeoptimizationTarget() : Isolate::kNoDeoptId;
     converted = new UnboxUint32x4Instr(use->CopyWithType(), deopt_id);
   } else if ((from == kUnboxedUint32x4) && (to == kTagged)) {
     converted = new BoxUint32x4Instr(use->CopyWithType());
+  } else {
+    const intptr_t deopt_id = (deopt_target != NULL) ?
+        deopt_target->DeoptimizationTarget() : Isolate::kNoDeoptId;
+    // We have failed to find a suitable conversion instruction.
+    // Insert a "dummy" conversion instruction with the correct
+    // "to" representation. The inserted instruction will trigger a
+    // a deoptimization if executed. See issue #12417 for a discussion.
+    if (to == kUnboxedDouble) {
+      converted = new UnboxDoubleInstr(use->CopyWithType(), deopt_id);

[Florian Schneider, 2013/08/15 09:11:35]

How about inserting a Box- and an Unbox-instruction to avoid deoptimization here
and in the other cases?

[srdjan, 2013/08/15 14:34:39]

What John has observed is that this code is actually never executed. It occurs
when inlining a function. The argument types are known and match one control
flow, but other (never possible) control flows are not eliminated and legal code
must still be generated for them as well (control flows are formed by is-a test
on arguments). John may provide more information.

[Cutch, 2013/08/15 14:35:02]

We are being asked to convert an UnboxedFloat32x4 to an UnboxedDouble. Even if
we inserted a BoxFloat32x4Instr followed by an UnboxDoubleInstr a deoptimization
will be triggered.

[Florian Schneider, 2013/08/16 10:31:18]

Still, I don't understand how this should work: Even if never executed, the
representations still should match: Unbox-instructions need a boxed input
(tagged) which comes in a regular register. An unboxed float32x4 value comes in
a XMM register.

[Cutch, 2013/08/16 15:57:05]

So what ends up happening is that unboxed -> tagged conversion routine is
inserted anyways. I'm sending you a CL which makes this explicit and fixes the
issue you found below.

+    } else if (to == kUnboxedUint32x4) {
+      converted = new UnboxDoubleInstr(use->CopyWithType(), deopt_id);

[Florian Schneider, 2013/08/16 10:31:18]

Ok, but shouldn't this be UnboxUint32x4Instr here?

[Cutch, 2013/08/16 15:57:05]

Yes.

+    } else if (to == kUnboxedFloat32x4) {
+      converted = new UnboxDoubleInstr(use->CopyWithType(), deopt_id);

[Florian Schneider, 2013/08/16 10:31:18]

Likewise, UnboxFloat32x4Instr here?

+    } else if (to == kUnboxedMint) {
+      converted = new UnboxIntegerInstr(use->CopyWithType(), deopt_id);
+    }
   }
   ASSERT(converted != NULL);
   use->BindTo(converted);
   InsertBefore(insert_before, converted, use->instruction()->env(),
                Definition::kValue);
 }
 
 
 void FlowGraphOptimizer::ConvertUse(Value* use, Representation from_rep) {
   const Representation to_rep =
@@ skipping 7060 matching lines @@
   }
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits.length(); i++) {
     CreateMaterializationAt(exits[i], alloc, alloc->cls(), *fields);
   }
 }
 
 
 }  // namespace dart