Add CodeGen support for functions that always return arguments via a new parameter attribute 'returned', which is taken advantage of in target-independent tail call opportunity detection and in ARM call lowering (when placed on an integral first parameter).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179925 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 55 insertions, 6 deletions

diff --git a/lib/IR/Attributes.cpp b/lib/IR/Attributes.cpp
index 44fa78e4af..80968dbd1f 100644
--- a/lib/IR/Attributes.cpp
+++ b/lib/IR/Attributes.cpp
@@ -195,6 +195,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     return "readnone";
   if (hasAttribute(Attribute::ReadOnly))
     return "readonly";
+  if (hasAttribute(Attribute::Returned))
+    return "returned";
   if (hasAttribute(Attribute::ReturnsTwice))
     return "returns_twice";
   if (hasAttribute(Attribute::SExt))
@@ -393,6 +395,7 @@ uint64_t AttributeImpl::getAttrMask(Attribute::AttrKind Val) {
   case Attribute::SanitizeThread:  return 1ULL << 36;
   case Attribute::SanitizeMemory:  return 1ULL << 37;
   case Attribute::NoBuiltin:       return 1ULL << 38;
+  case Attribute::Returned:        return 1ULL << 39;
   }
   llvm_unreachable("Unsupported attribute type");
 }
diff --git a/lib/IR/Function.cpp b/lib/IR/Function.cpp
index 1e72b90a13..7f7efabf76 100644
--- a/lib/IR/Function.cpp
+++ b/lib/IR/Function.cpp
@@ -124,6 +124,13 @@ bool Argument::hasStructRetAttr() const {
     hasAttribute(1, Attribute::StructRet);
 }
 
+/// hasReturnedAttr - Return true if this argument has the returned attribute on
+/// it in its containing function.
+bool Argument::hasReturnedAttr() const {
+  return getParent()->getAttributes().
+    hasAttribute(getArgNo()+1, Attribute::Returned);
+}
+
 /// addAttr - Add attributes to an argument.
 void Argument::addAttr(AttributeSet AS) {
   assert(AS.getNumSlots() <= 1 &&
diff --git a/lib/IR/Verifier.cpp b/lib/IR/Verifier.cpp
index ec01edfb9a..69cb5dc381 100644
--- a/lib/IR/Verifier.cpp
+++ b/lib/IR/Verifier.cpp
@@ -694,8 +694,9 @@ void Verifier::VerifyParameterAttrs(AttributeSet Attrs, unsigned Idx, Type *Ty,
     Assert1(!Attrs.hasAttribute(Idx, Attribute::ByVal) &&
             !Attrs.hasAttribute(Idx, Attribute::Nest) &&
             !Attrs.hasAttribute(Idx, Attribute::StructRet) &&
-            !Attrs.hasAttribute(Idx, Attribute::NoCapture),
-            "Attribute 'byval', 'nest', 'sret', and 'nocapture' "
+            !Attrs.hasAttribute(Idx, Attribute::NoCapture) &&
+            !Attrs.hasAttribute(Idx, Attribute::Returned),
+            "Attribute 'byval', 'nest', 'sret', 'nocapture', and 'returned' "
             "do not apply to return values!", V);
 
   // Check for mutually incompatible attributes.
@@ -750,6 +751,7 @@ void Verifier::VerifyFunctionAttrs(FunctionType *FT, AttributeSet Attrs,
     return;
 
   bool SawNest = false;
+  bool SawReturned = false;
 
   for (unsigned i = 0, e = Attrs.getNumSlots(); i != e; ++i) {
     unsigned Idx = Attrs.getSlotIndex(i);
@@ -764,11 +766,22 @@ void Verifier::VerifyFunctionAttrs(FunctionType *FT, AttributeSet Attrs,
 
     VerifyParameterAttrs(Attrs, Idx, Ty, Idx == 0, V);
 
-    if (Attrs.hasAttribute(i, Attribute::Nest)) {
+    if (Idx == 0)
+      continue;
+
+    if (Attrs.hasAttribute(Idx, Attribute::Nest)) {
       Assert1(!SawNest, "More than one parameter has attribute nest!", V);
       SawNest = true;
     }
 
+    if (Attrs.hasAttribute(Idx, Attribute::Returned)) {
+      Assert1(!SawReturned, "More than one parameter has attribute returned!",
+              V);
+      Assert1(Ty->canLosslesslyBitCastTo(FT->getReturnType()), "Incompatible "
+              "argument and return types for 'returned' attribute", V);
+      SawReturned = true;
+    }
+
     if (Attrs.hasAttribute(Idx, Attribute::StructRet))
       Assert1(Idx == 1, "Attribute sret is not on first parameter!", V);
   }
@@ -1348,15 +1361,41 @@ void Verifier::VerifyCallSite(CallSite CS) {
   // Verify call attributes.
   VerifyFunctionAttrs(FTy, Attrs, I);
 
-  if (FTy->isVarArg())
+  if (FTy->isVarArg()) {
+    // FIXME? is 'nest' even legal here?
+    bool SawNest = false;
+    bool SawReturned = false;
+
+    for (unsigned Idx = 1; Idx < 1 + FTy->getNumParams(); ++Idx) {
+      if (Attrs.hasAttribute(Idx, Attribute::Nest))
+        SawNest = true;
+      if (Attrs.hasAttribute(Idx, Attribute::Returned))
+        SawReturned = true;
+    }
+
     // Check attributes on the varargs part.
     for (unsigned Idx = 1 + FTy->getNumParams(); Idx <= CS.arg_size(); ++Idx) {
-      VerifyParameterAttrs(Attrs, Idx, CS.getArgument(Idx-1)->getType(),
-                           false, I);
+      Type *Ty = CS.getArgument(Idx-1)->getType(); 
+      VerifyParameterAttrs(Attrs, Idx, Ty, false, I);
+      
+      if (Attrs.hasAttribute(Idx, Attribute::Nest)) {
+        Assert1(!SawNest, "More than one parameter has attribute nest!", I);
+        SawNest = true;
+      }
+
+      if (Attrs.hasAttribute(Idx, Attribute::Returned)) {
+        Assert1(!SawReturned, "More than one parameter has attribute returned!",
+                I);
+        Assert1(Ty->canLosslesslyBitCastTo(FTy->getReturnType()),
+                "Incompatible argument and return types for 'returned' "
+                "attribute", I);
+        SawReturned = true;
+      }
 
       Assert1(!Attrs.hasAttribute(Idx, Attribute::StructRet),
               "Attribute 'sret' cannot be used for vararg call arguments!", I);
     }
+  }
 
   // Verify that there's no metadata unless it's a direct call to an intrinsic.
   if (CS.getCalledFunction() == 0 ||