Add nomerge function attribute to clang

clang/include/clang/Basic/Attr.td

@@ -1275,6 +1275,11 @@ def FallThrough : StmtAttr {
   let Documentation = [FallthroughDocs];
 }
 
+def NoMerge : StmtAttr {
+  let Spellings = [Clang<"nomerge">];
+  let Documentation = [NoMergeDocs];
+}
+
 def FastCall : DeclOrTypeAttr {
   let Spellings = [GCC<"fastcall">, Keyword<"__fastcall">,
                    Keyword<"_fastcall">];

clang/include/clang/Basic/AttrDocs.td

@@ -350,6 +350,20 @@ that appears to be capable of returning to its caller.
   }];
 }
 
+def NoMergeDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+If a statement is marked ``nomerge`` and contains call experessions, those call
+expressions inside the statement will not be merged during optimization. This 
+attribute can be used to prevent the optimizer from obscuring the source
+location of certain calls. For example, it will prevent tail merging otherwise
+identical code sequences that raise an exception or terminate the program. Tail
+merging normally reduces the precision of source location information, making
+stack traces less useful for debugging. This attribute gives the user control
+over the tradeoff between code size and debug information precision.
+  }];
+}
+
 def AssertCapabilityDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "assert_capability, assert_shared_capability";

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2762,6 +2762,10 @@ def warn_auto_var_is_id : Warning<
   InGroup<DiagGroup<"auto-var-id">>;
 
 // Attributes
+def warn_nomerge_attribute_ignored_in_stmt: Warning<
+  "%0 attribute is ignored because there exists no call expression inside the "
+  "statement">, 
+  InGroup<IgnoredAttributes>;
 def err_nsobject_attribute : Error<
   "'NSObject' attribute is for pointer types only">;
 def err_attributes_are_not_compatible : Error<

clang/lib/CodeGen/CGCall.cpp

@@ -4822,6 +4822,12 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
         Attrs.addAttribute(getLLVMContext(), llvm::AttributeList::FunctionIndex,
                            llvm::Attribute::StrictFP);
 
+  // Add call-site nomerge attribute if exists.
+  if (InNoMergeAttributedStmt)
+    Attrs =
+      Attrs.addAttribute(getLLVMContext(), llvm::AttributeList::FunctionIndex,
+                         llvm::Attribute::NoMerge);
+
   // Apply some call-site-specific attributes.
   // TODO: work this into building the attribute set.
 

clang/lib/CodeGen/CGStmt.cpp

@@ -25,6 +25,7 @@
 #include "llvm/IR/InlineAsm.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/MDBuilder.h"
+#include "llvm/Support/SaveAndRestore.h"
 
 using namespace clang;
 using namespace CodeGen;
@@ -608,6 +609,13 @@ void CodeGenFunction::EmitLabelStmt(const LabelStmt &S) {
 }
 
 void CodeGenFunction::EmitAttributedStmt(const AttributedStmt &S) {
+  bool nomerge = false;
+  for (const auto *A: S.getAttrs())
+    if (A->getKind() == attr::NoMerge) {
+      nomerge = true;
+      break;
+    }
+  SaveAndRestore<bool> save_nomerge(InNoMergeAttributedStmt, nomerge);
   EmitStmt(S.getSubStmt(), S.getAttrs());
 }
 

clang/lib/CodeGen/CodeGenFunction.h

@@ -595,6 +595,9 @@ public:
   /// region.
   bool IsInPreservedAIRegion = false;
 
+  /// True if the current statement has nomerge attribute.
+  bool InNoMergeAttributedStmt = false;
+
   const CodeGen::CGBlockInfo *BlockInfo = nullptr;
   llvm::Value *BlockPointer = nullptr;
 

clang/lib/Sema/SemaStmtAttr.cpp

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "clang/AST/EvaluatedExprVisitor.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/Basic/SourceManager.h"
@@ -170,6 +171,45 @@ static Attr *handleLoopHintAttr(Sema &S, Stmt *St, const ParsedAttr &A,
   return LoopHintAttr::CreateImplicit(S.Context, Option, State, ValueExpr, A);
 }
 
+namespace {
+class CallExprFinder : public ConstEvaluatedExprVisitor<CallExprFinder> {
+  bool FoundCallExpr = false;
+
+public:
+  typedef ConstEvaluatedExprVisitor<CallExprFinder> Inherited;
+
+  CallExprFinder(Sema &S, const Stmt* St) : Inherited(S.Context) {
+    Visit(St);
+  }
+
+  bool foundCallExpr() { return FoundCallExpr; }
+
+  void VisitCallExpr(const CallExpr *E) { FoundCallExpr = true; }
+
+  void Visit(const Stmt *St) {
+    if (!St) return;
+    ConstEvaluatedExprVisitor<CallExprFinder>::Visit(St);
+  }
+};
+} // namespace
+
+static Attr *handleNoMergeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                               SourceRange Range) {
+  NoMergeAttr NMA(S.Context, A);
+  if (S.CheckAttrNoArgs(A))
+    return nullptr;
+  
+  CallExprFinder CEF(S, St);
+
+  if (!CEF.foundCallExpr()) {
+    S.Diag(St->getBeginLoc(), diag::warn_nomerge_attribute_ignored_in_stmt)
+        << NMA.getSpelling();
+    return nullptr;
+  }
+
+  return ::new (S.Context) NoMergeAttr(S.Context, A);
+}
+
 static void
 CheckForIncompatibleAttributes(Sema &S,
                                const SmallVectorImpl<const Attr *> &Attrs) {
@@ -335,6 +375,8 @@ static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
     return handleOpenCLUnrollHint(S, St, A, Range);
   case ParsedAttr::AT_Suppress:
     return handleSuppressAttr(S, St, A, Range);
+  case ParsedAttr::AT_NoMerge:
+    return handleNoMergeAttr(S, St, A, Range);
   default:
     // if we're here, then we parsed a known attribute, but didn't recognize
     // it as a statement attribute => it is declaration attribute

clang/test/CodeGen/attr-nomerge.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -S -emit-llvm %s -o - | FileCheck %s
+
+bool bar();
+void f(bool, bool);
+
+void foo(int i) {
+  [[clang::nomerge]] bar();
+  [[clang::nomerge]] (i = 4, bar());
+  [[clang::nomerge]] (void)(bar());
+  [[clang::nomerge]] f(bar(), bar());
+  [[clang::nomerge]] [] { bar(); bar(); }(); // nomerge only applies to the anonymous function call
+  [[clang::nomerge]] for (bar(); bar(); bar()) {}
+  bar();
+}
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR:[0-9]+]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call void @_Z1fbb({{.*}}) #[[NOMERGEATTR]]
+// CHECK: call void @"_ZZ3fooiENK3$_0clEv"(%class.anon* %ref.tmp) #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv() #[[NOMERGEATTR]]
+// CHECK: call zeroext i1 @_Z3barv()
+// CHECK: attributes #[[NOMERGEATTR]] = { nomerge }

clang/test/Sema/attr-nomerge.cpp

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+
+void bar();
+
+void foo() {
+  [[clang::nomerge]] bar();
+  [[clang::nomerge(1, 2)]] bar(); // expected-error {{'nomerge' attribute takes no arguments}}
+  int x;
+  [[clang::nomerge]] x = 10; // expected-warning {{nomerge attribute is ignored because there exists no call expression inside the statement}}
+
+  [[clang::nomerge]] label: bar(); // expected-error {{'nomerge' attribute cannot be applied to a declaration}}
+
+}
+
+int f();
+
+[[clang::nomerge]] static int i = f(); // expected-error {{'nomerge' attribute cannot be applied to a declaration}}