Fixed register allocation corner case.

src/lithium-allocator.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
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 //     * Redistributions in binary form must reproduce the above
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@@ skipping 188 matching lines @@
 UsePosition* LiveRange::NextRegisterPosition(LifetimePosition start) {
   UsePosition* pos = NextUsePosition(start);
   while (pos != NULL && !pos->RequiresRegister()) {
     pos = pos->next();
   }
   return pos;
 }
 
 
 bool LiveRange::CanBeSpilled(LifetimePosition pos) {
-  // TODO(kmillikin): Comment. Now.
-  if (pos.Value() <= Start().Value() && HasRegisterAssigned()) return false;
-
   // We cannot spill a live range that has a use requiring a register
   // at the current or the immediate next position.
   UsePosition* use_pos = NextRegisterPosition(pos);
   if (use_pos == NULL) return true;
   return
       use_pos->pos().Value() > pos.NextInstruction().InstructionEnd().Value();
 }
 
 
 UsePosition* LiveRange::FirstPosWithHint() const {
@@ skipping 1688 matching lines @@
     if (use_pos[i].Value() > use_pos[reg].Value()) {
       reg = i;
     }
   }
 
   LifetimePosition pos = use_pos[reg];
 
   if (pos.Value() < register_use->pos().Value()) {
     // All registers are blocked before the first use that requires a register.
     // Spill starting part of live range up to that use.
-    //
-    // Corner case: the first use position is equal to the start of the range.
-    // In this case we have nothing to spill and SpillBetween will just return
-    // this range to the list of unhandled ones. This will lead to the infinite
-    // loop.
-    ASSERT(current->Start().Value() < register_use->pos().Value());

[Vyacheslav Egorov (Chromium), 2013/03/11 08:43:15]

I wonder if we should turn ASSERT in the callee into the CHECK to prevent hangs
and get crashes instead.

[Sven Panne, 2013/03/11 09:48:03]

Sounds like a good idea, the check is extremely cheap. I'll change SpillBetween
accordingly.

     SpillBetween(current, current->Start(), register_use->pos());
     return;
   }
 
   if (block_pos[reg].Value() < current->End().Value()) {
     // Register becomes blocked before the current range end. Split before that
     // position.
     LiveRange* tail = SplitBetween(current,
                                    current->Start(),
                                    block_pos[reg].InstructionStart());
@@ skipping 188 matching lines @@
     LiveRange* current = live_ranges()->at(i);
     if (current != NULL) current->Verify();
   }
 }
 
 
 #endif
 
 
 } }  // namespace v8::internal