Infer norecurse for functions with external function call/library function callls if none of the user functions have had there address taken

Author: usha1830

llvm/include/llvm/Transforms/IPO/FunctionAttrs.h

@@ -49,8 +49,9 @@ LLVM_ABI bool thinLTOPropagateFunctionAttrs(
 /// attribute. It also discovers function arguments that are not captured by
 /// the function and marks them with the nocapture attribute.
 struct PostOrderFunctionAttrsPass : PassInfoMixin<PostOrderFunctionAttrsPass> {
-  PostOrderFunctionAttrsPass(bool SkipNonRecursive = false)
-      : SkipNonRecursive(SkipNonRecursive) {}
+  PostOrderFunctionAttrsPass(bool SkipNonRecursive = false,
+                             bool IsLTOPostLink = false)
+      : SkipNonRecursive(SkipNonRecursive), IsLTOPostLink(IsLTOPostLink) {}
   LLVM_ABI PreservedAnalyses run(LazyCallGraph::SCC &C,
                                  CGSCCAnalysisManager &AM, LazyCallGraph &CG,
                                  CGSCCUpdateResult &UR);
@@ -61,6 +62,7 @@ struct PostOrderFunctionAttrsPass : PassInfoMixin<PostOrderFunctionAttrsPass> {
 
 private:
   bool SkipNonRecursive;
+  bool IsLTOPostLink;
 };
 
 /// A pass to do RPO deduction and propagation of function attributes.

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -970,7 +970,8 @@ PassBuilder::buildInlinerPipeline(OptimizationLevel Level,
   // simplification pipeline, so this only needs to run when it could affect the
   // function simplification pipeline, which is only the case with recursive
   // functions.
-  MainCGPipeline.addPass(PostOrderFunctionAttrsPass(/*SkipNonRecursive*/ true));
+  MainCGPipeline.addPass(PostOrderFunctionAttrsPass(
+      /*SkipNonRecursive*/ true, Phase == ThinOrFullLTOPhase::FullLTOPostLink));
 
   // When at O3 add argument promotion to the pass pipeline.
   // FIXME: It isn't at all clear why this should be limited to O3.
@@ -992,7 +993,8 @@ PassBuilder::buildInlinerPipeline(OptimizationLevel Level,
 
   // Finally, deduce any function attributes based on the fully simplified
   // function.
-  MainCGPipeline.addPass(PostOrderFunctionAttrsPass());
+  MainCGPipeline.addPass(PostOrderFunctionAttrsPass(
+      false, Phase == ThinOrFullLTOPhase::FullLTOPostLink));
 
   // Mark that the function is fully simplified and that it shouldn't be
   // simplified again if we somehow revisit it due to CGSCC mutations unless
@@ -1914,7 +1916,7 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
     // Promoting by-reference arguments to by-value exposes more constants to
     // IPSCCP.
     CGSCCPassManager CGPM;
-    CGPM.addPass(PostOrderFunctionAttrsPass());
+    CGPM.addPass(PostOrderFunctionAttrsPass(false, true));
     CGPM.addPass(ArgumentPromotionPass());
     CGPM.addPass(
         createCGSCCToFunctionPassAdaptor(SROAPass(SROAOptions::ModifyCFG)));
@@ -2076,8 +2078,8 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   MPM.addPass(createModuleToFunctionPassAdaptor(std::move(FPM),
                                                 PTO.EagerlyInvalidateAnalyses));
 
-  MPM.addPass(
-      createModuleToPostOrderCGSCCPassAdaptor(PostOrderFunctionAttrsPass()));
+  MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(
+      PostOrderFunctionAttrsPass(false, true)));
 
   // Require the GlobalsAA analysis for the module so we can query it within
   // MainFPM.

llvm/lib/Transforms/IPO/FunctionAttrs.cpp

@@ -2068,7 +2068,9 @@ static void inferAttrsFromFunctionBodies(const SCCNodeSet &SCCNodes,
 }
 
 static void addNoRecurseAttrs(const SCCNodeSet &SCCNodes,
-                              SmallPtrSet<Function *, 8> &Changed) {
+                              SmallPtrSet<Function *, 8> &Changed,
+                              bool AnyFunctionsAddressIsTaken,
+                              bool IsLTOPostLink) {
   // Try and identify functions that do not recurse.
 
   // If the SCC contains multiple nodes we know for sure there is recursion.
@@ -2081,6 +2083,10 @@ static void addNoRecurseAttrs(const SCCNodeSet &SCCNodes,
 
   if (F->hasAddressTaken())
     return;
+
+  Module *M = F->getParent();
+  llvm::TargetLibraryInfoImpl TLII(llvm::Triple(M->getTargetTriple()));
+  llvm::TargetLibraryInfo TLI(TLII);
   // If all of the calls in F are identifiable and can be proven to not
   // callback F, F is norecurse. This check also detects self-recursion
   // as F is not currently marked norecurse, so any call from F to F
@@ -2096,11 +2102,21 @@ static void addNoRecurseAttrs(const SCCNodeSet &SCCNodes,
         if (Callee->doesNotRecurse())
           continue;
 
-        // External call with NoCallback attribute.
+        LibFunc LF;
         if (Callee->isDeclaration()) {
+          // External call with NoCallback attribute.
           if (Callee->hasFnAttribute(Attribute::NoCallback))
             continue;
-          return;
+          // We rely on this only in post link stage when all functions in the
+          // program are known.
+          if (IsLTOPostLink && !AnyFunctionsAddressIsTaken &&
+              TLI.getLibFunc(Callee->getName(), LF))
+            continue;
+          // Do not consider external functions safe unless we are in LTO
+          // post-link stage and have information about all functions in the
+          // program.
+          if (!IsLTOPostLink)
+            return;
         }
       }
     }
@@ -2252,7 +2268,8 @@ static SCCNodesResult createSCCNodeSet(ArrayRef<Function *> Functions) {
 template <typename AARGetterT>
 static SmallPtrSet<Function *, 8>
 deriveAttrsInPostOrder(ArrayRef<Function *> Functions, AARGetterT &&AARGetter,
-                       bool ArgAttrsOnly) {
+                       bool ArgAttrsOnly, bool AnyFunctionAddressTaken = false,
+                       bool IsLTOPostLink = false) {
   SCCNodesResult Nodes = createSCCNodeSet(Functions);
 
   // Bail if the SCC only contains optnone functions.
@@ -2282,7 +2299,8 @@ deriveAttrsInPostOrder(ArrayRef<Function *> Functions, AARGetterT &&AARGetter,
     addNoAliasAttrs(Nodes.SCCNodes, Changed);
     addNonNullAttrs(Nodes.SCCNodes, Changed);
     inferAttrsFromFunctionBodies(Nodes.SCCNodes, Changed);
-    addNoRecurseAttrs(Nodes.SCCNodes, Changed);
+    addNoRecurseAttrs(Nodes.SCCNodes, Changed, AnyFunctionAddressTaken,
+                      IsLTOPostLink);
 
   // Finally, infer the maximal set of attributes from the ones we've inferred
   // above.  This is handling the cases where one attribute on a signature
@@ -2324,8 +2342,29 @@ PreservedAnalyses PostOrderFunctionAttrsPass::run(LazyCallGraph::SCC &C,
     Functions.push_back(&N.getFunction());
   }
 
+  bool AnyFunctionsAddressIsTaken = false;
+  // Check if any function in the whole program has its address taken.
+  // We use this information when inferring norecurse attribute: If there is
+  // no function whose address is taken, we conclude that any external function
+  // cannot callback into any user function.
+  if (IsLTOPostLink) {
+    // Get the parent Module of the Function
+    Module &M = *C.begin()->getFunction().getParent();
+    for (Function &F : M) {
+      // We only care about functions defined in user program whose addresses
+      // escape, making them potential callback targets.
+      if (F.isDeclaration())
+        continue;
+
+      if (F.hasAddressTaken()) {
+        AnyFunctionsAddressIsTaken = true;
+        break; // break if we found one
+      }
+    }
+  }
   auto ChangedFunctions =
-      deriveAttrsInPostOrder(Functions, AARGetter, ArgAttrsOnly);
+      deriveAttrsInPostOrder(Functions, AARGetter, ArgAttrsOnly,
+                             AnyFunctionsAddressIsTaken, IsLTOPostLink);
   if (ChangedFunctions.empty())
     return PreservedAnalyses::all();
 

llvm/test/Other/new-pm-lto-defaults.ll

@@ -166,7 +166,9 @@
 ; CHECK-O-NEXT: Running pass: PrintModulePass
 
 ; Make sure we get the IR back out without changes when we print the module.
-; CHECK-O-LABEL: define void @foo(i32 %n) local_unnamed_addr {
+; CHECK-O23SZ: ; Function Attrs: norecurse
+; CHECK-O1-LABEL: define void @foo(i32 %n) local_unnamed_addr {
+; CHECK-O23SZ-LABEL: define void @foo(i32 %n) local_unnamed_addr #0 {
 ; CHECK-O-NEXT: entry:
 ; CHECK-O-NEXT:   br label %loop
 ; CHECK-O:      loop:
@@ -178,6 +180,7 @@
 ; CHECK-O:      exit:
 ; CHECK-O-NEXT:   ret void
 ; CHECK-O-NEXT: }
+; CHECK-O23SZ: attributes #0 = { norecurse }
 ;
 
 declare void @bar() local_unnamed_addr