Teach DAE to look for functions whose arguments are unused, and change all callers to pass in an undefvalue instead.

llvm-svn: 123596

1 files changed, 61 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
index a709f5d6a06..b42322116a9 100644
--- a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -39,7 +39,8 @@ using namespace llvm;
 
 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
 STATISTIC(NumRetValsEliminated  , "Number of unused return values removed");
-
+STATISTIC(NumArgumentsReplacedWithUndef, 
+          "Number of unread args replaced with undef");
 namespace {
   /// DAE - The dead argument elimination pass.
   ///
@@ -148,6 +149,7 @@ namespace {
     void PropagateLiveness(const RetOrArg &RA);
     bool RemoveDeadStuffFromFunction(Function *F);
     bool DeleteDeadVarargs(Function &Fn);
+    bool RemoveDeadArgumentsFromCallers(Function &Fn);
   };
 }
 
@@ -287,6 +289,55 @@ bool DAE::DeleteDeadVarargs(Function &Fn) {
   return true;
 }
 
+/// RemoveDeadArgumentsFromCallers - Checks if the given function has any 
+/// arguments that are unused, and changes the caller parameters to be undefined
+/// instead.
+bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
+{
+  if (Fn.isDeclaration())
+    return false;
+
+  // Functions with local linkage should already have been handled.
+  if (Fn.hasLocalLinkage())
+    return false;
+
+  if (Fn.use_empty())
+    return false;
+
+  llvm::SmallVector<unsigned, 8> UnusedArgs;
+  for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(); 
+       I != E; ++I) {
+    Argument *Arg = I;
+
+    if (Arg->use_empty() && !Arg->hasByValAttr())
+      UnusedArgs.push_back(Arg->getArgNo());
+  }
+
+  if (UnusedArgs.empty())
+    return false;
+
+  bool Changed = false;
+
+  for (Function::use_iterator I = Fn.use_begin(), E = Fn.use_end(); 
+       I != E; ++I) {
+    CallSite CS(*I);
+    if (!CS || !CS.isCallee(I))
+      continue;
+
+    // Now go through all unused args and replace them with "undef".
+    for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
+      unsigned ArgNo = UnusedArgs[I];
+
+      Value *Arg = CS.getArgument(ArgNo);
+      CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
+      ++NumArgumentsReplacedWithUndef;
+      Changed = true;
+    }
+  }
+
+  return Changed;
+}
+
 /// Convenience function that returns the number of return values. It returns 0
 /// for void functions and 1 for functions not returning a struct. It returns
 /// the number of struct elements for functions returning a struct.
@@ -939,5 +990,14 @@ bool DAE::runOnModule(Module &M) {
     Function *F = I++;
     Changed |= RemoveDeadStuffFromFunction(F);
   }
+
+  // Finally, look for any unused parameters in functions with non-local
+  // linkage and replace the passed in parameters with undef.
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
+    Function& F = *I;
+
+    Changed |= RemoveDeadArgumentsFromCallers(F);
+  }
+
   return Changed;
 }