[turbofan] Always inline small functions directly.

Introduce a flag --max_inlined_nodes_small (defaults to 10), which gives
the upper limit of AST nodes for a function to be considered "small" by
the inlining heuristic. These functions will always be inlined
immediately, independent of the budget.

R=jarin@chromium.org
BUG=v8:6395,v8:6278,v8:6344,v8:6394

Review-Url: https://codereview.chromium.org/2883853002
Cr-Commit-Position: refs/heads/master@{#45291}

src/compiler/js-inlining-heuristic.cc

@@ -65,6 +65,15 @@ bool CanInlineFunction(Handle<SharedFunctionInfo> shared) {
   return true;
 }
 
+bool IsSmallInlineFunction(Handle<SharedFunctionInfo> shared) {
+  // Don't forcibly inline functions that weren't compiled yet.
+  if (shared->ast_node_count() == 0) return false;
+
+  // Forcibly inline small functions.
+  if (shared->ast_node_count() <= FLAG_max_inlined_nodes_small) return true;
+  return false;
+}
+
 }  // namespace
 
 Reduction JSInliningHeuristic::Reduce(Node* node) {
@@ -91,7 +100,7 @@ Reduction JSInliningHeuristic::Reduce(Node* node) {
   }
 
   // Functions marked with %SetForceInlineFlag are immediately inlined.
-  bool can_inline = false, force_inline = true;
+  bool can_inline = false, force_inline = true, small_inline = true;
   for (int i = 0; i < candidate.num_functions; ++i) {
     Handle<SharedFunctionInfo> shared =
         candidate.functions[i].is_null()
@@ -103,6 +112,9 @@ Reduction JSInliningHeuristic::Reduce(Node* node) {
     if (CanInlineFunction(shared)) {
       can_inline = true;
     }
+    if (!IsSmallInlineFunction(shared)) {
+      small_inline = false;
+    }
   }
   if (force_inline) return InlineCandidate(candidate);
   if (!can_inline) return NoChange();
@@ -154,6 +166,13 @@ Reduction JSInliningHeuristic::Reduce(Node* node) {
     return NoChange();
   }
 
+  // Forcibly inline small functions here.
+  if (small_inline) {
+    TRACE("Inlining small function(s) at call site #%d:%s\n", node->id(),
+          node->op()->mnemonic());
+    return InlineCandidate(candidate);
+  }
+
   // In the general case we remember the candidate for later.
   candidates_.insert(candidate);
   return NoChange();

src/flag-definitions.h

@@ -361,6 +361,8 @@ DEFINE_INT(max_inlined_nodes, 200,
            "maximum number of AST nodes considered for a single inlining")
 DEFINE_INT(max_inlined_nodes_cumulative, 400,
            "maximum cumulative number of AST nodes considered for inlining")
+DEFINE_INT(max_inlined_nodes_small, 10,
+           "maximum number of AST nodes considered for small function inlining")
 DEFINE_FLOAT(min_inlining_frequency, 0.15, "minimum frequency for inlining")
 DEFINE_BOOL(loop_invariant_code_motion, true, "loop invariant code motion")
 DEFINE_BOOL(fast_math, true, "faster (but maybe less accurate) math functions")