Don't print unreachable vasm blocks below a certain trace level

Summary: Certain optimizations can cause us to end up with more unreachable
than reachable blocks, and printing them makes the output very noisy. They do
matter for passes that iterate over unit.blocks, though, so it's important to
be aware that they exist. When the vasm trace level is below 6, we now print
something like "43 unreachable blocks not shown. Set TRACE=vasm:6 or greater."
in place of the unreachable blocks.

Reviewed By: @ottoni

Differential Revision: D1921821

hphp/runtime/vm/jit/vasm-print.cpp

@@ -305,16 +305,20 @@ std::string show(const Vunit& unit) {
   }
 
   // Print unreachable blocks last.
-  bool printedMsg{false};
-  for (size_t b = 0; b < unit.blocks.size(); b++) {
-    if (!reachableSet.test(b)) {
-      if (!printedMsg) {
-        out << "\nUnreachable blocks:\n";
-        printedMsg = true;
-      }
-      printBlock(out, unit, preds, Vlabel{b});
+  auto const numUnreachable = reachableSet.size() - reachableSet.count();
+  if (numUnreachable == 0) return out.str();
+
+  if (Trace::moduleEnabledRelease(Trace::vasm, kVasmUnreachableLevel)) {
+    out << "\nUnreachable blocks:\n";
+    for (size_t b = 0; b < unit.blocks.size(); b++) {
+      if (!reachableSet.test(b)) printBlock(out, unit, preds, Vlabel{b});
     }
+  } else {
+    out << folly::format("\n{} unreachable blocks not shown. "
+                         "Set TRACE=vasm:{} or greater to print them.\n",
+                         numUnreachable, kVasmUnreachableLevel);
   }
+
   return out.str();
 }
 

hphp/runtime/vm/jit/vasm-print.h

@@ -54,6 +54,7 @@ constexpr int kVasmJumpsLevel = 4;
 constexpr int kVasmExitsLevel = 4;
 constexpr int kVasmDCELevel = 4;
 constexpr int kVasmLowerLevel = 4;
+constexpr int kVasmUnreachableLevel = 6;
 
 // Print the cfg digraph followed by a vasm code listing, if the trace level is
 // above `level'.