Implement GlobalConstifier/trivialstore.llx, and also do some

simplifications of the resultant program to avoid making later passes
do it all.

This allows us to constify globals that just have the same constant that
they are initialized stored into them.

Suprisingly this comes up ALL of the freaking time, dozens of times in
SPEC, 30 times in vortex alone.

For example, on 256.bzip2, it allows us to constify these two globals:

%smallMode = internal global ubyte 0             ; <ubyte*> [#uses=8]
%verbosity = internal global int 0               ; <int*> [#uses=49]

Which (with later optimizations) results in the bytecode file shrinking
from 82286 to 69686 bytes!  Lets hear it for IPO :)

For the record, it's nuking lots of "if (verbosity > 2) { do lots of stuff }"
code.

llvm-svn: 16793

llvm/lib/Transforms/IPO/GlobalConstifier.cpp

@@ -26,10 +26,12 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/ADT/Statistic.h"
 #include <set>
+#include <algorithm>
 using namespace llvm;
 
 namespace {
   Statistic<> NumMarked ("constify", "Number of globals marked constant");
+  Statistic<> NumDeleted("constify", "Number of globals deleted");
 
   struct Constifier : public ModulePass {
     bool runOnModule(Module &M);
@@ -50,6 +52,7 @@ static bool ContainingFunctionIsTriviallyDead(Instruction *I) {
   return F->use_empty();
 }
 
+
 /// isStoredThrough - Return false if the specified pointer is provably never
 /// stored through.  If we can't tell, we must conservatively assume it might.
 ///
@@ -68,7 +71,16 @@ static bool isStoredThrough(Value *V, std::set<PHINode*> &PHIUsers) {
           // have to be careful about infinite recursion.
           if (PHIUsers.insert(PN).second)  // Not already visited.
             if (isStoredThrough(I, PHIUsers)) return true;
-
+        } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
+          // If this store is just storing the initializer into a global
+          // (i.e. not changing the value), ignore it.  For now we just handle
+          // direct stores, no stores to fields of aggregates.
+          if (!isa<GlobalVariable>(SI->getOperand(1)))
+            return true;
+          Constant *GVInit =
+            cast<GlobalVariable>(SI->getOperand(1))->getInitializer();
+          if (SI->getOperand(0) != GVInit)
+            return true;
         } else if (!isa<LoadInst>(I) && !isa<SetCondInst>(I)) {
           return true;  // Any other non-load instruction might store!
         }
@@ -81,18 +93,55 @@ static bool isStoredThrough(Value *V, std::set<PHINode*> &PHIUsers) {
   return false;
 }
 
+/// CleanupConstantGlobalUsers - We just marked GV constant.  Loop over all
+/// users of the global, cleaning up the obvious ones.  This is largely just a
+/// quick scan over the use list to clean up the easy and obvious cruft.
+static void CleanupConstantGlobalUsers(GlobalVariable *GV) {
+  Constant *Init = GV->getInitializer();
+  if (!Init->getType()->isFirstClassType())
+    return;  // We can't simplify aggregates yet!
+
+  std::vector<User*> Users(GV->use_begin(), GV->use_end());
+
+  std::sort(Users.begin(), Users.end());
+  Users.erase(std::unique(Users.begin(), Users.end()), Users.end());
+  for (unsigned i = 0, e = Users.size(); i != e; ++i) {
+    if (LoadInst *LI = dyn_cast<LoadInst>(Users[i])) {
+      // Replace the load with the initializer.
+      LI->replaceAllUsesWith(Init);
+      LI->getParent()->getInstList().erase(LI);
+    } else if (StoreInst *SI = dyn_cast<StoreInst>(Users[i])) {
+      // Store must be unreachable or storing Init into the global.
+      SI->getParent()->getInstList().erase(SI);
+    }
+  }
+}
+
+
 bool Constifier::runOnModule(Module &M) {
   bool Changed = false;
   std::set<PHINode*> PHIUsers;
-  for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
+  for (Module::giterator GVI = M.gbegin(), E = M.gend(); GVI != E;) {
+    GlobalVariable *GV = GVI++;
     if (!GV->isConstant() && GV->hasInternalLinkage() && GV->hasInitializer()) {
       if (!isStoredThrough(GV, PHIUsers)) {
         DEBUG(std::cerr << "MARKING CONSTANT: " << *GV << "\n");
         GV->setConstant(true);
+        
+        // Clean up any obviously simplifiable users now.
+        CleanupConstantGlobalUsers(GV);
+
+        // If the global is dead now, just nuke it.
+        if (GV->use_empty()) {
+          M.getGlobalList().erase(GV);
+          ++NumDeleted;
+        }
+
         ++NumMarked;
         Changed = true;
       }
       PHIUsers.clear();
     }
+  }
   return Changed;
 }