[Turbofan] Reduce register allocation work when we can.

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index 174ee300fee63348fbf0791801528d628da2b2a2..142da9bf76dd65fc1edf337cc30d15bd8f142aae 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -813,6 +813,7 @@ InstructionSequence::InstructionSequence(Isolate* isolate,
       next_virtual_register_(0),
       reference_maps_(zone()),
       representations_(zone()),
+      representation_mask_(0),
       deoptimization_entries_(zone()),
       current_block_(nullptr) {}
 
@@ -918,6 +919,7 @@ void InstructionSequence::MarkAsRepresentation(MachineRepresentation rep,
   DCHECK_IMPLIES(representations_[virtual_register] != rep,
                  representations_[virtual_register] == DefaultRepresentation());
   representations_[virtual_register] = rep;
+  representation_mask_ |= 1 << static_cast<int>(rep);
 }

[Mircea Trofin, 2016/11/04 22:57:25]

dcheck for rep's value?

also, this would have problems if we have more than 32 different reps and the
instruction takes more than 32 vregs - probably very unlikely case, but how
about using a BitVector instead, dimensioned to the number of possible
representations, and then it's a bit more readable to see what's happening here
and elsewhere we use representation_mask_?

[bbudge, 2016/11/04 23:30:33]

Since there are only 11 representations currently, we have plenty of room in an
int. This collapses the representations for all vregs in an InstructionSequence
into a single int so the number of vregs isn't an issue.

I'll add a static_assert in machine-type.h as a defensive measure. Note, we also
use these bit masks of representations in MoveOptimizer and GapResolver to
quickly check if we need aliasing tests.

 
 int InstructionSequence::AddDeoptimizationEntry(