Add vasm dominator utility functions

Summary:
Add vasm dominator utility functions. These are mainly copies of the
equivalent HHIR functions, but works on vasm.

Reviewed By: markw65

Differential Revision: D13285649

fbshipit-source-id: a41fef0e6e985b4055124a689d61f6209371aede

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

@@ -18,6 +18,9 @@
 
 #include "hphp/runtime/vm/jit/vasm-instr.h"
 #include "hphp/runtime/vm/jit/vasm-unit.h"
+#include "hphp/runtime/vm/jit/vasm-visit.h"
+
+#include "hphp/util/dataflow-worklist.h"
 
 namespace HPHP { namespace jit {
 
@@ -161,4 +164,66 @@ Vloc make_const(Vunit& unit, Type type) {
 
 //////////////////////////////////////////////////////////////////////
 
+bool dominates(Vlabel b1, Vlabel b2, const VIdomVector& idoms) {
+  assertx(b1.isValid() && b2.isValid());
+  for (auto b = b2; b.isValid(); b = idoms[b]) {
+    if (b == b1) return true;
+  }
+  return false;
+}
+
+VIdomVector findDominators(const Vunit& unit,
+                           const jit::vector<Vlabel>& rpo) {
+  assertx(!rpo.empty() && rpo[0] == unit.entry);
+
+  auto const preds = computePreds(unit);
+  VIdomVector idom(unit.blocks.size());
+
+  jit::vector<size_t> rpoOrder(unit.blocks.size());
+  for (size_t i = 0; i < rpo.size(); ++i) rpoOrder[rpo[i]] = i;
+
+  dataflow_worklist<size_t> worklist(rpo.size());
+
+  idom[unit.entry] = unit.entry;
+  for (auto const succ : succs(unit.blocks[0])) {
+    worklist.push(rpoOrder[succ]);
+  }
+
+  while (!worklist.empty()) {
+    auto const block = rpo[worklist.pop()];
+    auto const& blockPreds = preds[block];
+
+    // Find the first already processed predecessor (there must be at least one
+    // because we shouldn't be on the worklist otherwise).
+    auto predIt = std::find_if(
+      blockPreds.begin(),
+      blockPreds.end(),
+      [&] (Vlabel p) { return idom[p].isValid(); }
+    );
+    assertx(predIt != blockPreds.end());
+    auto p1 = *predIt;
+
+    // For all other already processed predecessors
+    for (++predIt; predIt != blockPreds.end(); ++predIt) {
+      auto p2 = *predIt;
+      if (p2 == p1 || !idom[p2].isValid()) continue;
+      do {
+        // Find earliest common predecessor of p1 and p2
+        while (rpoOrder[p1] < rpoOrder[p2]) p2 = idom[p2];
+        while (rpoOrder[p2] < rpoOrder[p1]) p1 = idom[p1];
+      } while (p1 != p2);
+    }
+
+    if (!idom[block].isValid() || idom[block] != p1) {
+      idom[block] = p1;
+      for (auto const succ : succs(unit.blocks[block])) {
+        worklist.push(rpoOrder[succ]);
+      }
+    }
+  }
+
+  idom[unit.entry] = Vlabel{}; // entry has no dominator
+  return idom;
+}
+
 }}

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

@@ -39,6 +39,22 @@ bool is_trivial_nop(const Vinstr&);
  */
 bool splitCriticalEdges(Vunit& unit);
 
+///////////////////////////////////////////////////////////////////////////////
+
+/*
+ * Compute a mapping of each (reachable) block in a Vunit to its immediate
+ * dominator, using the provided reverse post-order sort of the blocks.
+ */
+using VIdomVector = jit::vector<Vlabel>;
+VIdomVector findDominators(const Vunit&, const jit::vector<Vlabel>& rpo);
+
+/*
+ * Test if b1 dominates b2
+ */
+bool dominates(Vlabel b1, Vlabel b2, const VIdomVector&);
+
+///////////////////////////////////////////////////////////////////////////////
+
 /*
  * Return a Vloc holding the constant value represented by the given Type.
  */