从HGraph生成SSA

类SsaBuilder的作用：
将一个graph转变成SSA形式。该转化的liveness guarantees在下面列出。dex register被杀意味着代码中某个给定位置的值在其environment use中不再可用。下文中的merge被具化成HPhi。
(a)不需要merge的dex registers（即在一个join block，他们没有不同的值）在所有的environment use中都是可用的。注意：这不意味着在register allocation之后instruction将会有一个physical location。参见SsaLivenessAnalysis部分。
(b)需要merge且merge会带来不相容类型的dex registers将会被杀掉，为了该merge的environment use。
(c)当debuggable 标记被传到compiler后，需要merg且在merge后有一个proper type的dex registers，对所有的environment use都是可用的。如果debuggable 标记没有被设置，仅被environment使用的dex register的value会被杀掉。

class SsaLivenessAnalysis的作用：
计算instruction的liveness的分析。
(a)instruction的non-environment use会使得这个instruction live
(b)类型是object(即非primitive类型)的instruction的environment use会使得instruction live。这是由于必须使得有finalizers（对象被删除时要调用的）来删除native objects的objects保持alive（因为对象的删除是由虚拟机的垃圾收集器完成的,时间不确定,所以调用finalizer的时间也是不确定的）。
(c)当一个graph有debuggable这个属性时，有primitive类型的instruction的environment use似的instruction live。如果graph没有debuggable属性，environment use没有任何作用，而且可能在register allocation后得到一个’none’ value。
(b)(c)是通过SsaLivenessAnalysis::ShouldBeLiveForEnvironment来实现的。

class LiveRange的作用：
live range包含一个instruction或者一个temporary live的start和end。还含有LiveRange* next_;

class LiveInterval的作用：
interval是一个instruction live的不重合的live range的链表。每个有use的instruction都有一个interval。

class UsePosition的作用：
use position代表着在一个给定position的live interval use。

 160  private: 161   HInstruction* const user_; 162   HEnvironment* const environment_; 163   const size_t input_index_; 164   const size_t position_; 165   UsePosition* next_;

231 void SsaBuilder::BuildSsa() {232   // 1) Visit in reverse post order. We need to have all predecessors of a block visited233   // (with the exception of loops) in order to create the right environment for that234   // block. For loops, we create phis whose inputs will be set in 2).235   for (HReversePostOrderIterator it(*GetGraph()); !it.Done(); it.Advance()) {236     VisitBasicBlock(it.Current());237   }238 239   // 2) Set inputs of loop phis.240   for (size_t i = 0; i < loop_headers_.Size(); i++) {241     HBasicBlock* block = loop_headers_.Get(i);242     for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {243       HPhi* phi = it.Current()->AsPhi();244       for (size_t pred = 0; pred < block->GetPredecessors().Size(); pred++) {245         HInstruction* input = ValueOfLocal(block->GetPredecessors().Get(pred), phi->GetRegNumber());246         phi->AddInput(input);247       }248     }249   }250 251   // 3) Mark dead phis. This will mark phis that are only used by environments:252   // at the DEX level, the type of these phis does not need to be consistent, but253   // our code generator will complain if the inputs of a phi do not have the same254   // type. The marking allows the type propagation to know which phis it needs255   // to handle. We mark but do not eliminate: the elimination will be done in256   // step 9).257   SsaDeadPhiElimination dead_phis_for_type_propagation(GetGraph());258   dead_phis_for_type_propagation.MarkDeadPhis();259 260   // 4) Propagate types of phis. At this point, phis are typed void in the general261   // case, or float/double/reference when we created an equivalent phi. So we262   // need to propagate the types across phis to give them a correct type.263   PrimitiveTypePropagation type_propagation(GetGraph());264   type_propagation.Run();265 266   // 5) When creating equivalent phis we copy the inputs of the original phi which267   // may be improperly typed. This was fixed during the type propagation in 4) but268   // as a result we may end up with two equivalent phis with the same type for269   // the same dex register. This pass cleans them up.270   EquivalentPhisCleanup();271 272   // 6) Mark dead phis again. Step 4) may have introduced new phis.273   // Step 5) might enable the death of new phis.274   SsaDeadPhiElimination dead_phis(GetGraph());275   dead_phis.MarkDeadPhis();276 277   // 7) Now that the graph is correctly typed, we can get rid of redundant phis.278   // Note that we cannot do this phase before type propagation, otherwise279   // we could get rid of phi equivalents, whose presence is a requirement for the280   // type propagation phase. Note that this is to satisfy statement (a) of the281   // SsaBuilder (see ssa_builder.h).282   SsaRedundantPhiElimination redundant_phi(GetGraph());283   redundant_phi.Run();284 285   // 8) Fix the type for null constants which are part of an equality comparison.286   // We need to do this after redundant phi elimination, to ensure the only cases287   // that we can see are reference comparison against 0. The redundant phi288   // elimination ensures we do not see a phi taking two 0 constants in a HEqual289   // or HNotEqual.290   FixNullConstantType();291 292   // 9) Make sure environments use the right phi "equivalent": a phi marked dead293   // can have a phi equivalent that is not dead. We must therefore update294   // all environment uses of the dead phi to use its equivalent. Note that there295   // can be multiple phis for the same Dex register that are live (for example296   // when merging constants), in which case it is OK for the environments297   // to just reference one.298   for (HReversePostOrderIterator it(*GetGraph()); !it.Done(); it.Advance()) {299     HBasicBlock* block = it.Current();300     for (HInstructionIterator it_phis(block->GetPhis()); !it_phis.Done(); it_phis.Advance()) {301       HPhi* phi = it_phis.Current()->AsPhi();302       // If the phi is not dead, or has no environment uses, there is nothing to do.303       if (!phi->IsDead() || !phi->HasEnvironmentUses()) continue;304       HInstruction* next = phi->GetNext();305       if (!IsPhiEquivalentOf(next, phi)) continue;306       if (next->AsPhi()->IsDead()) {307         // If the phi equivalent is dead, check if there is another one.308         next = next->GetNext();309         if (!IsPhiEquivalentOf(next, phi)) continue;310         // There can be at most two phi equivalents.311         DCHECK(!IsPhiEquivalentOf(next->GetNext(), phi));312         if (next->AsPhi()->IsDead()) continue;313       }314       // We found a live phi equivalent. Update the environment uses of `phi` with it.315       phi->ReplaceWith(next);316     }317   }318 319   // 10) Deal with phis to guarantee liveness of phis in case of a debuggable320   // application. This is for satisfying statement (c) of the SsaBuilder321   // (see ssa_builder.h).322   if (GetGraph()->IsDebuggable()) {323     DeadPhiHandling dead_phi_handler(GetGraph());324     dead_phi_handler.Run();325   }326 327   // 11) Now that the right phis are used for the environments, and we328   // have potentially revive dead phis in case of a debuggable application,329   // we can eliminate phis we do not need. Regardless of the debuggable status,330   // this phase is necessary for statement (b) of the SsaBuilder (see ssa_builder.h),331   // as well as for the code generation, which does not deal with phis of conflicting332   // input types.333   dead_phis.EliminateDeadPhis();334 335   // 12) Clear locals.336   for (HInstructionIterator it(GetGraph()->GetEntryBlock()->GetInstructions());337        !it.Done();338        it.Advance()) {339     HInstruction* current = it.Current();340     if (current->IsLocal()) {341       current->GetBlock()->RemoveInstruction(current);342     }343   }344 }

待续