[Linker] Remove nocallback attribute while linking

GCC's leaf attribute is lowered to LLVM IR nocallback attribute.
Clang conservatively treats this function attribute as a hint on the
module level, and removes it while linking modules. More context can
be found in: https://reviews.llvm.org/D131628.

Differential Revision: https://reviews.llvm.org/D137360

llvm/lib/Linker/IRMover.cpp

@@ -19,6 +19,8 @@
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GVMaterializer.h"
 #include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PseudoProbe.h"
@@ -526,6 +528,9 @@ class IRLinker {
   void prepareCompileUnitsForImport();
   void linkNamedMDNodes();
 
+  ///  Update attributes while linking.
+  void updateAttributes(GlobalValue &GV);
+
 public:
   IRLinker(Module &DstM, MDMapT &SharedMDs,
            IRMover::IdentifiedStructTypeSet &Set, std::unique_ptr<Module> SrcM,
@@ -636,6 +641,7 @@ Value *IRLinker::materialize(Value *V, bool ForIndirectSymbol) {
   if (ForIndirectSymbol || shouldLink(New, *SGV))
     setError(linkGlobalValueBody(*New, *SGV));
 
+  updateAttributes(*New);
   return New;
 }
 
@@ -1527,6 +1533,33 @@ static std::string adjustInlineAsm(const std::string &InlineAsm,
   return InlineAsm;
 }
 
+void IRLinker::updateAttributes(GlobalValue &GV) {
+  /// Remove nocallback attribute while linking, because nocallback attribute
+  /// indicates that the function is only allowed to jump back into caller's
+  /// module only by a return or an exception. When modules are linked, this
+  /// property cannot be guaranteed anymore. For example, the nocallback
+  /// function may contain a call to another module. But if we merge its caller
+  /// and callee module here, and not the module containing the nocallback
+  /// function definition itself, the nocallback property will be violated
+  /// (since the nocallback function will call back into the newly merged module
+  /// containing both its caller and callee). This could happen if the module
+  /// containing the nocallback function definition is native code, so it does
+  /// not participate in the LTO link. Note if the nocallback function does
+  /// participate in the LTO link, and thus ends up in the merged module
+  /// containing its caller and callee, removing the attribute doesn't hurt as
+  /// it has no effect on definitions in the same module.
+  if (auto *F = dyn_cast<Function>(&GV)) {
+    if (!F->isIntrinsic())
+      F->removeFnAttr(llvm::Attribute::NoCallback);
+
+    // Remove nocallback attribute when it is on a call-site.
+    for (BasicBlock &BB : *F)
+      for (Instruction &I : BB)
+        if (CallInst *CI = dyn_cast<CallInst>(&I))
+          CI->removeFnAttr(Attribute::NoCallback);
+  }
+}
+
 Error IRLinker::run() {
   // Ensure metadata materialized before value mapping.
   if (SrcM->getMaterializer())

llvm/test/Linker/Inputs/drop-attribute.ll

@@ -0,0 +1,4 @@
+define void @test_nocallback_declaration_definition_linked_in() {
+entry:
+  ret void
+}

llvm/test/Linker/drop-attribute.ll

@@ -0,0 +1,44 @@
+; RUN: llvm-link %s %p/Inputs/drop-attribute.ll -S -o - | FileCheck %s
+
+; Test case that checks that nocallback attribute is dropped during linking.
+
+; CHECK: define i32 @main()
+; CHECK-NEXT: entry:
+; CHECK-NEXT: call void @test_nocallback_definition()
+; Test that checks that nocallback attribute on a call-site is dropped.
+; CHECK-NEXT: call void @test_nocallback_call_site(){{$}}
+; CHECK-NEXT: %0 = call float @llvm.sqrt.f32(float undef)
+; CHECK-NEXT: call void @test_nocallback_declaration_definition_not_linked_in()
+; CHECK-NEXT: call void @test_nocallback_declaration_definition_linked_in()
+define i32 @main() {
+entry:
+  call void @test_nocallback_definition()
+  call void @test_nocallback_call_site() nocallback
+  call float @llvm.sqrt.f32(float undef)
+  call void @test_nocallback_declaration_definition_not_linked_in()
+  call void @test_nocallback_declaration_definition_linked_in()
+  ret i32 0
+}
+
+; Test that checks that nocallback attribute on a definition is dropped.
+; CHECK: define void @test_nocallback_definition()
+define void @test_nocallback_definition() nocallback {
+  ret void
+}
+
+; Test that checks that nocallback attribute on a declaration when a definition is linked in is dropped.
+; CHECK: declare void @test_nocallback_call_site(){{$}}
+declare void @test_nocallback_call_site()
+
+; Test that checks that nocallback attribute on an intrinsic is NOT dropped.
+; CHECK: ; Function Attrs: nocallback nofree nosync nounwind speculatable willreturn
+; CHECK-NEXT: declare float @llvm.sqrt.f32(float) #0
+declare float @llvm.sqrt.f32(float) nocallback
+
+; Test that checks that nocallback attribute on a declaration when a definition is not linked in is dropped.
+; CHECK: declare void @test_nocallback_declaration_definition_not_linked_in(){{$}}
+declare void @test_nocallback_declaration_definition_not_linked_in() nocallback
+
+; Test that checks that nocallback attribute on a declaration when a definition is linked in is dropped.
+; CHECK: define void @test_nocallback_declaration_definition_linked_in() {{{$}}
+declare void @test_nocallback_declaration_definition_linked_in() nocallback