Revert "[Clang][Attribute] Introduce maybe_undef attribute for function arguments which accepts undef values"

This reverts commit a35c64c.

Reverting this commit as it causes various failures on LE and BE PPC bots.

Author: amy-kwan

clang/include/clang/Basic/Attr.td

@@ -2023,13 +2023,6 @@ def NoEscape : Attr {
   let Documentation = [NoEscapeDocs];
 }
 
-def MaybeUndef : InheritableAttr {
-  let Spellings = [Clang<"maybe_undef">];
-  let Subjects = SubjectList<[ParmVar]>;
-  let Documentation = [MaybeUndefDocs];
-  let SimpleHandler = 1;
-}
-
 def AssumeAligned : InheritableAttr {
   let Spellings = [GCC<"assume_aligned">];
   let Subjects = SubjectList<[ObjCMethod, Function]>;

clang/include/clang/Basic/AttrDocs.td

@@ -257,28 +257,6 @@ applies to copies of the block. For example:
   }];
 }
 
-def MaybeUndefDocs : Documentation {
-  let Category = DocCatVariable;
-  let Content = [{
-The ``maybe_undef`` attribute can be placed on a function parameter. It indicates
-that the parameter is allowed to use undef values. It informs the compiler
-to insert a freeze LLVM IR instruction on the function parameter.
-Please note that this is an attribute that is used as an internal
-implementation detail and not intended to be used by external users.
-
-In languages HIP, CUDA etc., some functions have multi-threaded semantics and
-it is enough for only one or some threads to provide defined arguments.
-Depending on semantics, undef arguments in some threads don't produce
-undefined results in the function call. Since, these functions accept undefined
-arguments, ``maybe_undef`` attribute can be placed.
-
-Sample usage:
-.. code-block:: c
-
-  void maybeundeffunc(int __attribute__((maybe_undef))param);
-  }];
-}
-
 def CarriesDependencyDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/lib/CodeGen/CGCall.cpp

@@ -2046,27 +2046,6 @@ static bool DetermineNoUndef(QualType QTy, CodeGenTypes &Types,
   return false;
 }
 
-/// Check if the argument of a function has maybe_undef attribute.
-static bool IsArgumentMaybeUndef(const Decl *TargetDecl,
-                                 unsigned NumRequiredArgs, unsigned ArgNo) {
-  const auto *FD = dyn_cast_or_null<FunctionDecl>(TargetDecl);
-  if (!FD)
-    return false;
-
-  // Assume variadic arguments do not have maybe_undef attribute.
-  if (ArgNo >= NumRequiredArgs)
-    return false;
-
-  // Check if argument has maybe_undef attribute.
-  if (ArgNo < FD->getNumParams()) {
-    const ParmVarDecl *Param = FD->getParamDecl(ArgNo);
-    if (Param && Param->hasAttr<MaybeUndefAttr>())
-      return true;
-  }
-
-  return false;
-}
-
 /// Construct the IR attribute list of a function or call.
 ///
 /// When adding an attribute, please consider where it should be handled:
@@ -4842,9 +4821,6 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
     unsigned FirstIRArg, NumIRArgs;
     std::tie(FirstIRArg, NumIRArgs) = IRFunctionArgs.getIRArgs(ArgNo);
 
-    bool ArgHasMaybeUndefAttr =
-        IsArgumentMaybeUndef(TargetDecl, CallInfo.getNumRequiredArgs(), ArgNo);
-
     switch (ArgInfo.getKind()) {
     case ABIArgInfo::InAlloca: {
       assert(NumIRArgs == 0);
@@ -4903,11 +4879,7 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
         // Make a temporary alloca to pass the argument.
         Address Addr = CreateMemTempWithoutCast(
             I->Ty, ArgInfo.getIndirectAlign(), "indirect-arg-temp");
-
-        llvm::Value *Val = Addr.getPointer();
-        if (ArgHasMaybeUndefAttr)
-          Val = Builder.CreateFreeze(Addr.getPointer());
-        IRCallArgs[FirstIRArg] = Val;
+        IRCallArgs[FirstIRArg] = Addr.getPointer();
 
         I->copyInto(*this, Addr);
       } else {
@@ -4965,10 +4937,7 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
           // Create an aligned temporary, and copy to it.
           Address AI = CreateMemTempWithoutCast(
               I->Ty, ArgInfo.getIndirectAlign(), "byval-temp");
-          llvm::Value *Val = AI.getPointer();
-          if (ArgHasMaybeUndefAttr)
-            Val = Builder.CreateFreeze(AI.getPointer());
-          IRCallArgs[FirstIRArg] = Val;
+          IRCallArgs[FirstIRArg] = AI.getPointer();
 
           // Emit lifetime markers for the temporary alloca.
           llvm::TypeSize ByvalTempElementSize =
@@ -4987,13 +4956,9 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
           auto *T = llvm::PointerType::getWithSamePointeeType(
               cast<llvm::PointerType>(V->getType()),
               CGM.getDataLayout().getAllocaAddrSpace());
-
-          llvm::Value *Val = getTargetHooks().performAddrSpaceCast(
+          IRCallArgs[FirstIRArg] = getTargetHooks().performAddrSpaceCast(
               *this, V, LangAS::Default, CGM.getASTAllocaAddressSpace(), T,
               true);
-          if (ArgHasMaybeUndefAttr)
-            Val = Builder.CreateFreeze(Val);
-          IRCallArgs[FirstIRArg] = Val;
         }
       }
       break;
@@ -5047,8 +5012,6 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
             V->getType() != IRFuncTy->getParamType(FirstIRArg))
           V = Builder.CreateBitCast(V, IRFuncTy->getParamType(FirstIRArg));
 
-        if (ArgHasMaybeUndefAttr)
-          V = Builder.CreateFreeze(V);
         IRCallArgs[FirstIRArg] = V;
         break;
       }
@@ -5093,8 +5056,6 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
         for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
           Address EltPtr = Builder.CreateStructGEP(Src, i);
           llvm::Value *LI = Builder.CreateLoad(EltPtr);
-          if (ArgHasMaybeUndefAttr)
-            LI = Builder.CreateFreeze(LI);
           IRCallArgs[FirstIRArg + i] = LI;
         }
       } else {
@@ -5111,9 +5072,6 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
           if (ATy != nullptr && isa<RecordType>(I->Ty.getCanonicalType()))
             Load = EmitCMSEClearRecord(Load, ATy, I->Ty);
         }
-
-        if (ArgHasMaybeUndefAttr)
-          Load = Builder.CreateFreeze(Load);
         IRCallArgs[FirstIRArg] = Load;
       }
 
@@ -5159,8 +5117,6 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
         if (ABIArgInfo::isPaddingForCoerceAndExpand(eltType)) continue;
         Address eltAddr = Builder.CreateStructGEP(addr, i);
         llvm::Value *elt = Builder.CreateLoad(eltAddr);
-        if (ArgHasMaybeUndefAttr)
-          elt = Builder.CreateFreeze(elt);
         IRCallArgs[IRArgPos++] = elt;
       }
       assert(IRArgPos == FirstIRArg + NumIRArgs);

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8634,9 +8634,6 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_NoEscape:
     handleNoEscapeAttr(S, D, AL);
     break;
-  case ParsedAttr::AT_MaybeUndef:
-    handleSimpleAttribute<MaybeUndefAttr>(S, D, AL);
-    break;
   case ParsedAttr::AT_AssumeAligned:
     handleAssumeAlignedAttr(S, D, AL);
     break;

clang/test/CodeGen/attr-maybeundef-template.cpp

@@ -83,7 +83,6 @@
 // CHECK-NEXT: Lockable (SubjectMatchRule_record)
 // CHECK-NEXT: MIGServerRoutine (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_block)
 // CHECK-NEXT: MSStruct (SubjectMatchRule_record)
-// CHECK-NEXT: MaybeUndef (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: MicroMips (SubjectMatchRule_function)
 // CHECK-NEXT: MinSize (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: MinVectorWidth (SubjectMatchRule_function)