Add a new pass AddReadAttrs which works out which functions

can get the readnone/readonly attributes, and gives them it.
The plan is to remove markmodref (which did the same thing
by querying GlobalsModRef) and delete the analogous
functionality from GlobalsModRef.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56341 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/AddReadAttrs.cpp

@@ -0,0 +1,135 @@
+//===- AddReadAttrs.cpp - Pass which marks functions readnone or readonly -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a simple interprocedural pass which walks the
+// call-graph, looking for functions which do not access or only read
+// non-local memory, and marking them readnone/readonly.  It implements
+// this as a bottom-up traversal of the call-graph.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "addreadattrs"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/CallGraphSCCPass.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/CallGraph.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/InstIterator.h"
+using namespace llvm;
+
+STATISTIC(NumReadNone, "Number of functions marked readnone");
+STATISTIC(NumReadOnly, "Number of functions marked readonly");
+
+namespace {
+  struct VISIBILITY_HIDDEN AddReadAttrs : public CallGraphSCCPass {
+    static char ID; // Pass identification, replacement for typeid
+    AddReadAttrs() : CallGraphSCCPass(&ID) {}
+
+    // runOnSCC - Analyze the SCC, performing the transformation if possible.
+    bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
+  };
+}
+
+char AddReadAttrs::ID = 0;
+static RegisterPass<AddReadAttrs>
+X("addreadattrs", "Mark functions readnone/readonly");
+
+Pass *llvm::createAddReadAttrsPass() { return new AddReadAttrs(); }
+
+
+bool AddReadAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
+  CallGraph &CG = getAnalysis<CallGraph>();
+
+  // Check if any of the functions in the SCC read or write memory.
+  // If they write memory then just give up.
+  bool ReadsMemory = false;
+  for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
+    Function *F = SCC[i]->getFunction();
+
+    if (F == 0)
+      // May write memory.
+      return false;
+
+    if (F->doesNotAccessMemory())
+      // Already perfect!
+      continue;
+
+    // Definitions with weak linkage may be overridden at linktime with
+    // something that writes memory, so treat them like declarations.
+    if (F->isDeclaration() || F->hasWeakLinkage()) {
+      if (!F->onlyReadsMemory())
+        // May write memory.
+        return false;
+
+      ReadsMemory = true;
+      continue;
+    }
+
+    // Scan the function body for explicit loads and stores, or calls to
+    // functions that may read or write memory.
+    for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
+      Instruction *I = &*II;
+      if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
+        if (LI->isVolatile())
+          // Volatile loads may have side-effects, so treat them as writing
+          // memory.
+          return false;
+        ReadsMemory = true;
+      } else if (isa<StoreInst>(I) || isa<MallocInst>(I) || isa<FreeInst>(I)) {
+        // Writes memory.
+        return false;
+      } else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
+        CallSite CS(I);
+
+        if (std::find(SCC.begin(), SCC.end(), CG[CS.getCalledFunction()]) !=
+            SCC.end())
+          // The callee is inside our current SCC - ignore it.
+          continue;
+
+        if (!CS.onlyReadsMemory())
+          // May write memory.
+          return false;
+
+        if (!CS.doesNotAccessMemory())
+          ReadsMemory = true;
+      }
+    }
+  }
+
+  // Success!  Functions in this SCC do not access memory, or only read memory.
+  // Give them the appropriate attribute.
+  bool MadeChange = false;
+  for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
+    Function *F = SCC[i]->getFunction();
+
+    if (F->doesNotAccessMemory())
+      // Already perfect!
+      continue;
+
+    if (F->onlyReadsMemory() && ReadsMemory)
+      // No change.
+      continue;
+
+    MadeChange = true;
+
+    // Clear out any existing attributes.
+    F->removeParamAttr(0, ParamAttr::ReadOnly | ParamAttr::ReadNone);
+
+    // Add in the new attribute.
+    F->addParamAttr(0, ReadsMemory ? ParamAttr::ReadOnly : ParamAttr::ReadNone);
+
+    if (ReadsMemory)
+      NumReadOnly++;
+    else
+      NumReadNone++;
+  }
+
+  return MadeChange;
+}