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.

Differential Revision: http://reviews.llvm.org/D22202

llvm-svn: 275027

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 +484,45 @@ 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;
+
+    SmallPtrSet<Value *, 2> RetArgs;
+    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))
+          RetArgs.insert(RetVal);
+      }
+
+    if (RetArgs.size() == 1) {
+      auto *A = cast<Argument>(*RetArgs.begin());
+      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 +1064,7 @@ PreservedAnalyses PostOrderFunctionAttrsPass::run(LazyCallGraph::SCC &C,
   }
 
   bool Changed = false;
+  Changed |= addArgumentReturnedAttrs(SCCNodes);
   Changed |= addReadAttrs(SCCNodes, AARGetter);
   Changed |= addArgumentAttrs(SCCNodes);
 
@@ -1089,6 +1130,7 @@ static bool runImpl(CallGraphSCC &SCC, AARGetterT AARGetter) {
     SCCNodes.insert(F);
   }
 
+  Changed |= addArgumentReturnedAttrs(SCCNodes);
   Changed |= addReadAttrs(SCCNodes, AARGetter);
   Changed |= addArgumentAttrs(SCCNodes);
 

llvm/test/Transforms/FunctionAttrs/2009-01-02-LocalStores.ll

@@ -14,7 +14,7 @@ define i32* @b(i32 *%q) {
 	ret i32* %tmp
 }
 
-; CHECK: define i32* @c(i32* readnone %r)
+; CHECK: define i32* @c(i32* readnone returned %r)
 @g = global i32 0
 define i32* @c(i32 *%r) {
 	%a = icmp eq i32* %r, null

llvm/test/Transforms/FunctionAttrs/nocapture.ll

@@ -1,7 +1,7 @@
 ; RUN: opt < %s -functionattrs -S | FileCheck %s
 @g = global i32* null		; <i32**> [#uses=1]
 
-; CHECK: define i32* @c1(i32* readnone %q)
+; CHECK: define i32* @c1(i32* readnone returned %q)
 define i32* @c1(i32* %q) {
 	ret i32* %q
 }
@@ -140,7 +140,7 @@ define void @test1_1(i8* %x1_1, i8* %y1_1) {
   ret void
 }
 
-; CHECK: define i8* @test1_2(i8* nocapture readnone %x1_2, i8* %y1_2)
+; CHECK: define i8* @test1_2(i8* nocapture readnone %x1_2, i8* returned %y1_2)
 define i8* @test1_2(i8* %x1_2, i8* %y1_2) {
   call void @test1_1(i8* %x1_2, i8* %y1_2)
   store i32* null, i32** @g
@@ -168,7 +168,7 @@ define void @test4_1(i8* %x4_1) {
   ret void
 }
 
-; CHECK: define i8* @test4_2(i8* nocapture readnone %x4_2, i8* readnone %y4_2, i8* nocapture readnone %z4_2)
+; CHECK: define i8* @test4_2(i8* nocapture readnone %x4_2, i8* readnone returned %y4_2, i8* nocapture readnone %z4_2)
 define i8* @test4_2(i8* %x4_2, i8* %y4_2, i8* %z4_2) {
   call void @test4_1(i8* null)
   store i32* null, i32** @g

llvm/test/Transforms/FunctionAttrs/readattrs.ll

@@ -11,7 +11,7 @@ define void @test1_2(i8* %x1_2, i8* %y1_2, i8* %z1_2) {
   ret void
 }
 
-; CHECK: define i8* @test2(i8* readnone %p)
+; CHECK: define i8* @test2(i8* readnone returned %p)
 define i8* @test2(i8* %p) {
   store i32 0, i32* @x
   ret i8* %p
@@ -53,7 +53,7 @@ define void @test7_1(i32* inalloca %a) {
   ret void
 }
 
-; CHECK: define i32* @test8_1(i32* readnone %p)
+; CHECK: define i32* @test8_1(i32* readnone returned %p)
 define i32* @test8_1(i32* %p) {
 entry:
   ret i32* %p