Refine and filter bespoke sink layouts by the known layout

Summary:
We might profile that a given array sink is likely to see a particular array
layout, but we might also know something about the actual layout we are
getting and that might contradict or refine the prediction.

So, take advantage of that knowledge and avoid emitting dead checks that always
side exit.

Differential Revision: D57518393

fbshipit-source-id: ddfb68597472d8575d1e29b4fcdfbdd94421a153

hphp/runtime/base/bespoke/layout-selection.cpp

@@ -1017,7 +1017,8 @@ ArrayLayout layoutForSource(SrcKey sk) {
 // Computes the layouts for a given sink by looking at the layouts for each
 // TransID. If there are <= 2 total layouts then they are returned in frequency
 // order. If there are more than 2 layouts, they will be unioned and returned.
-SinkLayouts layoutsForSink(const jit::TransIDSet& ids, SrcKey sk) {
+SinkLayouts layoutsForSink(
+    const jit::TransIDSet& ids, SrcKey sk, ArrayLayout knownLayout) {
   auto result = SinkLayouts{};
 
   // Track layout frequencies across the multiple layouts
@@ -1031,8 +1032,9 @@ SinkLayouts layoutsForSink(const jit::TransIDSet& ids, SrcKey sk) {
 
       // Record the frequencies for each layout
       for (auto const& layout : sls.layouts) {
-        layoutFrequencies[layout.layout.toUint16()] +=
-          layout.coverage * transCounter;
+        auto const l = layout.layout & knownLayout;
+        if (l == ArrayLayout::Bottom()) continue;
+        layoutFrequencies[l.toUint16()] += layout.coverage * transCounter;
       }
     }
   }
@@ -1060,14 +1062,14 @@ SinkLayouts layoutsForSink(const jit::TransIDSet& ids, SrcKey sk) {
     }
 
     if (sl.layout == ArrayLayout::Bottom()) {
-      return {{{ArrayLayout::Top(), 1.0}}, false};
+      return {{{knownLayout, 1.0}}, false};
     }
 
     result.layouts = {std::move(sl)};
   }
 
   if (result.layouts.empty()) {
-    return {{{ArrayLayout::Top(), 1.0}}, false};
+    return {{{knownLayout, 1.0}}, false};
   }
 
   return result;

hphp/runtime/base/bespoke/layout-selection.h

@@ -31,7 +31,11 @@ using LayoutWeightVector =
 // Layout selection. We'll call selectBespokeLayouts in RTA, and then we'll
 // be able to use layoutForSource / layoutForSink for optimized code.
 jit::ArrayLayout layoutForSource(SrcKey sk);
-SinkLayouts layoutsForSink(const jit::TransIDSet& ids, SrcKey sk);
+SinkLayouts layoutsForSink(
+  const jit::TransIDSet& ids,
+  SrcKey sk,
+  jit::ArrayLayout knownLayout
+);
 void selectBespokeLayouts();
 
 }

hphp/runtime/vm/jit/irgen-bespoke.cpp

@@ -1841,7 +1841,8 @@ void handleSink(IRGS& env, const NormalizedInstruction& ni,
 
   emitLogArrayReach(env, loc, sk);
 
-  auto const sinkLayouts = bespoke::layoutsForSink(env.profTransIDs, ni.source);
+  auto const sinkLayouts = bespoke::layoutsForSink(
+    env.profTransIDs, ni.source, type.arrSpec().layout());
 
   if (isIteratorOp(sk.op())) {
     emitVanilla(env);

hphp/runtime/vm/jit/irgen-iter-spec.cpp

@@ -697,9 +697,11 @@ bool specializeIterInit(IRGS& env, Offset doneOffset,
   auto const layout = [&]{
     if (!allowBespokeArrayLikes()) return ArrayLayout::Vanilla();
     if (!arrayTypeCouldBeBespoke(baseDT)) return ArrayLayout::Vanilla();
-    auto const sl = bespoke::layoutsForSink(env.profTransIDs, curSrcKey(env));
+    auto const knownLayout = base->type().arrSpec().layout();
+    auto const sl = bespoke::layoutsForSink(
+      env.profTransIDs, curSrcKey(env), knownLayout);
     assertx(sl.layouts.size() == 1);
-    return sl.sideExit ? sl.layouts[0].layout : ArrayLayout::Top();
+    return sl.layouts[0].layout;
   }();
   auto const accessor = getAccessor(baseDT, keyTypes, layout, data);