Refactor a CallGraphPropertyAnalysis helper [NFC] (#2441)

This moves code out of Asyncify into a general helper class. The class
automates scanning the functions for a property, then propagating it to
functions that call them. In Asyncify, the property is "may call something
that leads to sleep", and we propagate backwards to callers, to find
all those that may sleep.

This will be useful in a future exceptions-optimizing pass I want to write,
where the property will be "may throw". We will then be able to remove
exceptions overhead in cases that definitely do not throw.

1 files changed, 70 insertions, 20 deletions

diff --git a/src/ir/module-utils.h b/src/ir/module-utils.h
index 16ecd54b0..52765035c 100644
--- a/src/ir/module-utils.h
+++ b/src/ir/module-utils.h
@@ -20,6 +20,7 @@
 #include "ir/find_all.h"
 #include "ir/manipulation.h"
 #include "pass.h"
+#include "support/unique_deferring_queue.h"
 #include "wasm.h"
 
 namespace wasm {
@@ -288,22 +289,33 @@ template<typename T> inline void iterDefinedEvents(Module& wasm, T visitor) {
   }
 }
 
-// Performs a parallel map on function in the module, emitting a map object
-// of function => result.
-// TODO: use in inlining and elsewhere
-template<typename T> struct ParallelFunctionMap {
+// Helper class for analyzing the call graph.
+//
+// Provides hooks for running some initial calculation on each function (which
+// is done in parallel), writing to a FunctionInfo structure for each function.
+// Then you can call propagateBack() to propagate a property of interest to the
+// calling functions, transitively.
+//
+// For example, if some functions are known to call an import "foo", then you
+// can use this to find which functions call something that might eventually
+// reach foo, by initially marking the direct callers as "calling foo" and
+// propagating that backwards.
+template<typename T> struct CallGraphPropertyAnalysis {
+  Module& wasm;
+
+  // The basic information for each function about whom it calls and who is
+  // called by it.
+  struct FunctionInfo {
+    std::set<Function*> callsTo;
+    std::set<Function*> calledBy;
+  };
 
   typedef std::map<Function*, T> Map;
   Map map;
 
   typedef std::function<void(Function*, T&)> Func;
 
-  struct Info {
-    Map* map;
-    Func work;
-  };
-
-  ParallelFunctionMap(Module& wasm, Func work) {
+  CallGraphPropertyAnalysis(Module& wasm, Func work) : wasm(wasm) {
     // Fill in map, as we operate on it in parallel (each function to its own
     // entry).
     for (auto& func : wasm.functions) {
@@ -317,27 +329,65 @@ template<typename T> struct ParallelFunctionMap {
       }
     }
 
-    // Run on the implemented functions.
     struct Mapper : public WalkerPass<PostWalker<Mapper>> {
       bool isFunctionParallel() override { return true; }
 
-      Mapper(Info* info) : info(info) {}
+      Mapper(Module* module, Map* map, Func work)
+        : module(module), map(map), work(work) {}
+
+      Mapper* create() override { return new Mapper(module, map, work); }
 
-      Mapper* create() override { return new Mapper(info); }
+      void visitCall(Call* curr) {
+        (*map)[this->getFunction()].callsTo.insert(
+          module->getFunction(curr->target));
+      }
 
-      void doWalkFunction(Function* curr) {
-        assert((*info->map).count(curr));
-        info->work(curr, (*info->map)[curr]);
+      void visitFunction(Function* curr) {
+        assert((*map).count(curr));
+        work(curr, (*map)[curr]);
       }
 
     private:
-      Info* info;
+      Module* module;
+      Map* map;
+      Func work;
     };
 
-    Info info = {&map, work};
-
     PassRunner runner(&wasm);
-    Mapper(&info).run(&runner, &wasm);
+    Mapper(&wasm, &map, work).run(&runner, &wasm);
+
+    // Find what is called by what.
+    for (auto& pair : map) {
+      auto* func = pair.first;
+      auto& info = pair.second;
+      for (auto* target : info.callsTo) {
+        map[target].calledBy.insert(func);
+      }
+    }
+  }
+
+  // Propagate a property from a function to those that call it.
+  void propagateBack(std::function<bool(const T&)> hasProperty,
+                     std::function<bool(const T&)> canHaveProperty,
+                     std::function<void(T&)> addProperty) {
+    // The work queue contains items we just learned can change the state.
+    UniqueDeferredQueue<Function*> work;
+    for (auto& func : wasm.functions) {
+      if (hasProperty(map[func.get()])) {
+        work.push(func.get());
+      }
+    }
+    while (!work.empty()) {
+      auto* func = work.pop();
+      for (auto* caller : map[func].calledBy) {
+        // If we don't already have the property, and we are not forbidden
+        // from getting it, then it propagates back to us now.
+        if (!hasProperty(map[caller]) && canHaveProperty(map[caller])) {
+          addProperty(map[caller]);
+          work.push(caller);
+        }
+      }
+    }
   }
 };