add details. test=develop

paddle/fluid/framework/details/memory_optimize_pass.cc

@@ -69,11 +69,13 @@ std::unique_ptr<ir::Graph> MemoryOptimizePass::ApplyImpl(
     }
 
     for (auto& var : op->outputs) {
-      if (!NodeCanReused(var) || cfg_->Use(op).count(var->Name()) == 0 ||
-          skip_set_.count(var->Name()))
+      if (skip_set_.count(var->Name())) {
+        VLOG(3) << "Skip set contains variable of " << var->Name()
+                << "disable reuse on it. skipped";
         continue;
+      }
+      if (NodeCanReused(var) && cfg_->Use(op).count(var->Name()) == 0) {
         ir::Node* cache = pool_.FindBestFitNode(var);
-
         if (var->Name() == FLAGS_memory_optimize_debug) {
           VLOG(3) << "start match var " << DebugString(var) << " of op "
                   << op->Name();
@@ -82,42 +84,43 @@ std::unique_ptr<ir::Graph> MemoryOptimizePass::ApplyImpl(
                   << ((cache == nullptr) ? "False" : "True");
         }
 
-      if (cache == nullptr) continue;
+        if (cache != nullptr) {
           if (var->Name() == cache->Name()) {
-        VLOG(3) << "The same cache variable is cascade reused." << var->Name()
-                << " is re-filled to the pool after"
+            VLOG(3) << "The same cache variable is cascade reused."
+                    << var->Name() << " is re-filled to the pool after"
                     << "the reused op is finished. Current op can not "
                     << "replace it again. Skip this candidate.";
             continue;
+          }
 
           int node_idx_in_pool = pool_.GetNodeIndexInPool(cache);
           VLOG(3) << string::Sprintf(
               "!!! %s,  %s => %s, cache idx %d, pool size %d",
               std::to_string(reuse_id++), DebugString(var), DebugString(cache),
               node_idx_in_pool, static_cast<int>(pool_.size()));
-
-        // update CFG Graph on the fly.
-        // reused var maybe re-fill into the pool
+          // NOTE(dzhwinter): update the ProgramDesc/IR Graph
+          // and the CFG Graph on the fly.
+          //
+          // IR Graph define the dependence relationship between nodes.
+          //
+          // ProgramDesc defines the input/output vars. Its used in
+          // CreateOp, CreateVar when running happens.
+          //
+          // CFG Graph store the liveness information, when reuse happens
+          // we also need to update the variable liveness.
           cfg_->RenameVarInCFGGraph(var->Name(), cache->Name(), idx);
-        // NOTE(dzhwinter): we need to both update the ProgramDesc
-        // and IR Graph. because op_desc/var_desc is used in CreateOp,
-        // CreateVar when running happens. But IR Graph
-        // define the dependence relationship between nodes.
           RenameVarInGraphDesc(var->Name(), cache->Name(), idx);
           RenameVarInGraphNode(var->Name(), cache->Name(), idx, graph.get());
 
           pool_.Erase(cache);
         }
-
+      }
+    }
     // fill the pool
-      std::unordered_set<std::string> unlived_vars;
     for (auto var : cfg_->LiveIn(op)) {
       if (cfg_->LiveOut(op).count(var) == 0) {
-          unlived_vars.emplace(var);
-        }
-      }
-      for (auto var : unlived_vars) {
         ir::Node* var_node = cfg_->GetNodeByName(var, op);
+        if (var_node == nullptr) continue;
         if (NodeCanReused(var_node) && !pool_.Has(var_node)) {
           pool_.Insert(var_node);
         }