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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123596 91177308-0d34-0410-b5e6-96231b3b80d8

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;
 }

test/Transforms/DeadArgElim/deadexternal.ll

@@ -1,5 +1,4 @@
 ; RUN: opt -deadargelim -S %s | FileCheck %s
-; XFAIL: *
 
 define void @test(i32) {
   ret void
@@ -11,3 +10,30 @@ define void @foo() {
 ; CHECK: @foo
 ; CHECK: i32 undef
 }
+
+define void @f(i32 %X) {
+entry:
+  tail call void @sideeffect() nounwind
+  ret void
+}
+
+declare void @sideeffect()
+
+define void @g(i32 %n) {
+entry:
+  %add = add nsw i32 %n, 1
+; CHECK: tail call void @f(i32 undef)
+  tail call void @f(i32 %add)
+  ret void
+}
+
+define void @h() {
+entry:
+  %i = alloca i32, align 4
+  volatile store i32 10, i32* %i, align 4
+; CHECK: %tmp = volatile load i32* %i, align 4
+; CHECK-next: call void @f(i32 undef)
+  %tmp = volatile load i32* %i, align 4
+  call void @f(i32 %tmp)
+  ret void
+}