dart2js: Inline across deferred output units if that leads to reduced size.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 3c55c53fbf232ae09de1ba053af0b1e2921f0501..c0980beca5a0c5ed1b101bba5a0167bdc77cae7d 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -1293,12 +1293,6 @@ class SsaBuilder extends NewResolvedVisitor {
     if (cachedCanBeInlined == false) return false;
 
     bool meetsHardConstraints() {
-      // Don't inline from one output unit to another. If something is deferred
-      // it is to save space in the loading code.

[floitsch, 2015/05/04 23:32:58]

this comment is incomplete: we must not inline code from deferred libraries,
because they might contain confidential code (like experiments).
However, we are allowed to inline over output-unit boundaries (as long as they
don't cross a deferred library-prefix).

I would reduce this test to: "!inSameOutputUnit &&
element_is_accessed_through_a_deferred_prefix".
I'm not sure this is something we can know here, though.

[herhut, 2015/05/05 14:19:12]

I changed it to not inline across deferred imports independent of output unit,
which I think is what you were asking for.

-      if (!compiler.deferredLoadTask
-          .inSameOutputUnit(element,compiler.currentElement)) {
-        return false;
-      }
       if (compiler.disableInlining) return false;
 
       assert(selector != null
@@ -1348,12 +1342,13 @@ class SsaBuilder extends NewResolvedVisitor {
         return InlineWeeder.canBeInlined(function, -1, false);
       }
       // TODO(sra): Measure if inlining would 'reduce' the size.  One desirable
-      // case we miss my doing nothing is inlining very simple constructors
+      // case we miss by doing nothing is inlining very simple constructors
       // where all fields are initialized with values from the arguments at this
       // call site.  The code is slightly larger (`new Foo(1)` vs `Foo$(1)`) but
       // that usually means the factory constructor is left unused and not
       // emitted.
-      return false;
+      // We at least inline bodies that are empty (and thus have a size of 1).
+      return InlineWeeder.canBeInlined(function, 1, true);
     }
 
     bool heuristicSayGoodToGo() {
@@ -1364,7 +1359,14 @@ class SsaBuilder extends NewResolvedVisitor {
 
       if (element.isSynthesized) return true;
 
-      if (inExpressionOfThrow || inLazyInitializerExpression) {
+      // Don't inline from one output unit to another, unless it reduces size.
+      // Similarly do not inline code that is rarely executed.
+      bool fromDifferentOutputUnit = !compiler.deferredLoadTask
+          .inSameOutputUnit(element,compiler.currentElement);
+
+      if (inExpressionOfThrow ||
+          inLazyInitializerExpression ||
+          fromDifferentOutputUnit) {
         return reductiveHeuristic();
       }