Revert "Revert r275027 - Let FuncAttrs infer the 'returned' argument attribute"

This reverts commit r275042; the initial commit triggered self-hosting failures
on ARM/AArch64. James Molloy identified the problematic backend code, which has
been disabled in r275677. Trying again...

Original commit message:

Let FuncAttrs infer the 'returned' argument attribute

A function can have one argument with the 'returned' attribute, indicating that
the associated argument is always the return value of the function. Add
FuncAttrs inference logic.

llvm-svn: 275678

1 files changed, 50 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index fff5440..9c9e482 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -42,6 +42,7 @@ using namespace llvm;
 STATISTIC(NumReadNone, "Number of functions marked readnone");
 STATISTIC(NumReadOnly, "Number of functions marked readonly");
 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
+STATISTIC(NumReturned, "Number of arguments marked returned");
 STATISTIC(NumReadNoneArg, "Number of arguments marked readnone");
 STATISTIC(NumReadOnlyArg, "Number of arguments marked readonly");
 STATISTIC(NumNoAlias, "Number of function returns marked noalias");
@@ -483,6 +484,53 @@ determinePointerReadAttrs(Argument *A,
   return IsRead ? Attribute::ReadOnly : Attribute::ReadNone;
 }
 
+/// Deduce returned attributes for the SCC.
+static bool addArgumentReturnedAttrs(const SCCNodeSet &SCCNodes) {
+  bool Changed = false;
+
+  AttrBuilder B;
+  B.addAttribute(Attribute::Returned);
+
+  // Check each function in turn, determining if an argument is always returned.
+  for (Function *F : SCCNodes) {
+    // We can infer and propagate function attributes only when we know that the
+    // definition we'll get at link time is *exactly* the definition we see now.
+    // For more details, see GlobalValue::mayBeDerefined.
+    if (!F->hasExactDefinition())
+      continue;
+
+    if (F->getReturnType()->isVoidTy())
+      continue;
+
+    auto FindRetArg = [&]() -> Value * {
+      Value *RetArg = nullptr;
+      for (BasicBlock &BB : *F)
+        if (auto *Ret = dyn_cast<ReturnInst>(BB.getTerminator())) {
+          // Note that stripPointerCasts should look through functions with
+          // returned arguments.
+          Value *RetVal = Ret->getReturnValue()->stripPointerCasts();
+          if (RetVal->getType() == F->getReturnType() && isa<Argument>(RetVal)) {
+            if (!RetArg)
+              RetArg = RetVal;
+            else if (RetArg != RetVal)
+              return nullptr;
+          }
+        }
+
+      return RetArg;
+    };
+
+    if (Value *RetArg = FindRetArg()) {
+      auto *A = cast<Argument>(RetArg);
+      A->addAttr(AttributeSet::get(F->getContext(), A->getArgNo() + 1, B));
+      ++NumReturned;
+      Changed = true;
+    }
+  }
+
+  return Changed;
+}
+
 /// Deduce nocapture attributes for the SCC.
 static bool addArgumentAttrs(const SCCNodeSet &SCCNodes) {
   bool Changed = false;
@@ -1024,6 +1072,7 @@ PreservedAnalyses PostOrderFunctionAttrsPass::run(LazyCallGraph::SCC &C,
   }
 
   bool Changed = false;
+  Changed |= addArgumentReturnedAttrs(SCCNodes);
   Changed |= addReadAttrs(SCCNodes, AARGetter);
   Changed |= addArgumentAttrs(SCCNodes);
 
@@ -1089,6 +1138,7 @@ static bool runImpl(CallGraphSCC &SCC, AARGetterT AARGetter) {
     SCCNodes.insert(F);
   }
 
+  Changed |= addArgumentReturnedAttrs(SCCNodes);
   Changed |= addReadAttrs(SCCNodes, AARGetter);
   Changed |= addArgumentAttrs(SCCNodes);