[Clang][C++23] Implement P1774R8: Portable assumptions (#81014)

This implements the C++23 `[[assume]]` attribute.

Assumption information is lowered to a call to `@llvm.assume`, unless the expression has side-effects, in which case it is discarded and a warning is issued to tell the user that the assumption doesn’t do anything. A failed assumption at compile time is an error (unless we are in `MSVCCompat` mode, in which case we don’t check assumptions at compile time).

Due to performance regressions in LLVM, assumptions can be disabled with the `-fno-assumptions` flag. With it, assumptions will still be parsed and checked, but no calls to `@llvm.assume` will be emitted and assumptions will not be checked at compile time.

clang/docs/ReleaseNotes.rst

@@ -100,6 +100,7 @@ C++23 Feature Support
 
 - Implemented `P2718R0: Lifetime extension in range-based for loops <https://wg21.link/P2718R0>`_. Also
   materialize temporary object which is a prvalue in discarded-value expression.
+- Implemented `P1774R8: Portable assumptions <https://wg21.link/P1774R8>`_.
 
 - Implemented `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/P2448R2>`_.
 

clang/include/clang/Basic/Attr.td

@@ -1580,6 +1580,13 @@ def Unlikely : StmtAttr {
 }
 def : MutualExclusions<[Likely, Unlikely]>;
 
+def CXXAssume : StmtAttr {
+  let Spellings = [CXX11<"", "assume", 202207>];
+  let Subjects = SubjectList<[NullStmt], ErrorDiag, "empty statements">;
+  let Args = [ExprArgument<"Assumption">];
+  let Documentation = [CXXAssumeDocs];
+}
+
 def NoMerge : DeclOrStmtAttr {
   let Spellings = [Clang<"nomerge">];
   let Documentation = [NoMergeDocs];
@@ -4151,11 +4158,11 @@ def OMPDeclareVariant : InheritableAttr {
   }];
 }
 
-def Assumption : InheritableAttr {
+def OMPAssume : InheritableAttr {
   let Spellings = [Clang<"assume">];
   let Subjects = SubjectList<[Function, ObjCMethod]>;
   let InheritEvenIfAlreadyPresent = 1;
-  let Documentation = [AssumptionDocs];
+  let Documentation = [OMPAssumeDocs];
   let Args = [StringArgument<"Assumption">];
 }
 

clang/include/clang/Basic/AttrDocs.td

@@ -1996,6 +1996,34 @@ Here is an example:
   }];
 }
 
+def CXXAssumeDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "assume";
+  let Content = [{
+The ``assume`` attribute is used to indicate to the optimizer that a
+certain condition is assumed to be true at a certain point in the
+program. If this condition is violated at runtime, the behavior is
+undefined. ``assume`` can only be applied to a null statement.
+
+Different optimisers are likely to react differently to the presence of
+this attribute; in some cases, adding ``assume`` may affect performance
+negatively. It should be used with parsimony and care.
+
+Note that `clang::assume` is a different attribute. Always write ``assume``
+without a namespace if you intend to use the standard C++ attribute.
+
+Example:
+
+.. code-block:: c++
+
+  int f(int x, int y) {
+    [[assume(x == 27)]];
+    [[assume(x == y)]];
+    return y + 1; // May be optimised to `return 28`.
+  }
+  }];
+}
+
 def LikelihoodDocs : Documentation {
   let Category = DocCatStmt;
   let Heading = "likely and unlikely";
@@ -4629,7 +4657,7 @@ For more information see
 }];
 }
 
-def AssumptionDocs : Documentation {
+def OMPAssumeDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "assume";
   let Content = [{

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -399,6 +399,8 @@ def note_constexpr_unsupported_flexible_array : Note<
   "flexible array initialization is not yet supported">;
 def note_constexpr_non_const_vectorelements : Note<
   "cannot determine number of elements for sizeless vectors in a constant expression">;
+def note_constexpr_assumption_failed : Note<
+  "assumption evaluated to false">;
 def err_experimental_clang_interp_failed : Error<
   "the experimental clang interpreter failed to evaluate an expression">;
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1133,9 +1133,11 @@ def NonGCC : DiagGroup<"non-gcc",
 def CXX14Attrs : DiagGroup<"c++14-attribute-extensions">;
 def CXX17Attrs : DiagGroup<"c++17-attribute-extensions">;
 def CXX20Attrs : DiagGroup<"c++20-attribute-extensions">;
+def CXX23Attrs : DiagGroup<"c++23-attribute-extensions">;
 def FutureAttrs : DiagGroup<"future-attribute-extensions", [CXX14Attrs,
                                                             CXX17Attrs,
-                                                            CXX20Attrs]>;
+                                                            CXX20Attrs,
+                                                            CXX23Attrs]>;
 
 def CXX23AttrsOnLambda : DiagGroup<"c++23-lambda-attributes">;
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -786,6 +786,9 @@ def err_ms_property_expected_comma_or_rparen : Error<
 def err_ms_property_initializer : Error<
   "property declaration cannot have a default member initializer">;
 
+def err_assume_attr_expects_cond_expr : Error<
+  "use of this expression in an %0 attribute requires parentheses">;
+
 def warn_cxx20_compat_explicit_bool : Warning<
   "this expression will be parsed as explicit(bool) in C++20">,
   InGroup<CXX20Compat>, DefaultIgnore;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -855,10 +855,10 @@ def note_strncat_wrong_size : Note<
 def warn_assume_side_effects : Warning<
   "the argument to %0 has side effects that will be discarded">,
   InGroup<DiagGroup<"assume">>;
-def warn_assume_attribute_string_unknown : Warning<
+def warn_omp_assume_attribute_string_unknown : Warning<
   "unknown assumption string '%0'; attribute is potentially ignored">,
   InGroup<UnknownAssumption>;
-def warn_assume_attribute_string_unknown_suggested : Warning<
+def warn_omp_assume_attribute_string_unknown_suggested : Warning<
   "unknown assumption string '%0' may be misspelled; attribute is potentially "
   "ignored, did you mean '%1'?">,
   InGroup<MisspelledAssumption>;
@@ -9115,6 +9115,8 @@ def ext_cxx17_attr : Extension<
   "use of the %0 attribute is a C++17 extension">, InGroup<CXX17Attrs>;
 def ext_cxx20_attr : Extension<
   "use of the %0 attribute is a C++20 extension">, InGroup<CXX20Attrs>;
+def ext_cxx23_attr : Extension<
+  "use of the %0 attribute is a C++23 extension">, InGroup<CXX23Attrs>;
 
 def warn_unused_comparison : Warning<
   "%select{equality|inequality|relational|three-way}0 comparison result unused">,
@@ -10169,6 +10171,9 @@ def err_fallthrough_attr_outside_switch : Error<
 def err_fallthrough_attr_invalid_placement : Error<
   "fallthrough annotation does not directly precede switch label">;
 
+def err_assume_attr_args : Error<
+  "attribute '%0' requires a single expression argument">;
+
 def warn_unreachable_default : Warning<
   "default label in switch which covers all enumeration values">,
   InGroup<CoveredSwitchDefault>, DefaultIgnore;

clang/include/clang/Basic/LangOptions.def

@@ -450,6 +450,8 @@ LANGOPT(RegCall4, 1, 0, "Set __regcall4 as a default calling convention to respe
 
 LANGOPT(MatrixTypes, 1, 0, "Enable or disable the builtin matrix type")
 
+LANGOPT(CXXAssumptions, 1, 1, "Enable or disable codegen and compile-time checks for C++23's [[assume]] attribute")
+
 ENUM_LANGOPT(StrictFlexArraysLevel, StrictFlexArraysLevelKind, 2,
              StrictFlexArraysLevelKind::Default,
              "Rely on strict definition of flexible arrays")

clang/include/clang/Driver/Options.td

@@ -3789,6 +3789,12 @@ def foptimization_record_passes_EQ : Joined<["-"], "foptimization-record-passes=
   HelpText<"Only include passes which match a specified regular expression in the generated optimization record (by default, include all passes)">,
   MetaVarName<"<regex>">;
 
+defm assumptions : BoolFOption<"assumptions",
+  LangOpts<"CXXAssumptions">, DefaultTrue,
+  NegFlag<SetFalse, [], [ClangOption, CC1Option],
+          "Disable codegen and compile-time checks for C++23's [[assume]] attribute">,
+  PosFlag<SetTrue>>;
+
 def fvectorize : Flag<["-"], "fvectorize">, Group<f_Group>,
   HelpText<"Enable the loop vectorization passes">;
 def fno_vectorize : Flag<["-"], "fno-vectorize">, Group<f_Group>;

clang/include/clang/Parse/Parser.h

@@ -1803,6 +1803,7 @@ public:
   ExprResult ParseConstraintLogicalOrExpression(bool IsTrailingRequiresClause);
   // Expr that doesn't include commas.
   ExprResult ParseAssignmentExpression(TypeCastState isTypeCast = NotTypeCast);
+  ExprResult ParseConditionalExpression();
 
   ExprResult ParseMSAsmIdentifier(llvm::SmallVectorImpl<Token> &LineToks,
                                   unsigned &NumLineToksConsumed,
@@ -2955,6 +2956,12 @@ private:
                                SourceLocation ScopeLoc,
                                CachedTokens &OpenMPTokens);
 
+  /// Parse a C++23 assume() attribute. Returns true on error.
+  bool ParseCXXAssumeAttributeArg(ParsedAttributes &Attrs,
+                                  IdentifierInfo *AttrName,
+                                  SourceLocation AttrNameLoc,
+                                  SourceLocation *EndLoc);
+
   IdentifierInfo *TryParseCXX11AttributeIdentifier(
       SourceLocation &Loc,
       Sema::AttributeCompletion Completion = Sema::AttributeCompletion::None,

clang/include/clang/Sema/Sema.h

@@ -9011,6 +9011,12 @@ public:
   void ProcessStmtAttributes(Stmt *Stmt, const ParsedAttributes &InAttrs,
                              SmallVectorImpl<const Attr *> &OutAttrs);
 
+  ExprResult ActOnCXXAssumeAttr(Stmt *St, const ParsedAttr &A,
+                                SourceRange Range);
+  ExprResult BuildCXXAssumeExpr(Expr *Assumption,
+                                const IdentifierInfo *AttrName,
+                                SourceRange Range);
+
   ///@}
 
   //
@@ -14716,10 +14722,10 @@ private:
   SmallVector<OMPDeclareVariantScope, 4> OMPDeclareVariantScopes;
 
   /// The current `omp begin/end assumes` scopes.
-  SmallVector<AssumptionAttr *, 4> OMPAssumeScoped;
+  SmallVector<OMPAssumeAttr *, 4> OMPAssumeScoped;
 
   /// All `omp assumes` we encountered so far.
-  SmallVector<AssumptionAttr *, 4> OMPAssumeGlobal;
+  SmallVector<OMPAssumeAttr *, 4> OMPAssumeGlobal;
 
   /// OMPD_loop is mapped to OMPD_for, OMPD_distribute or OMPD_simd depending
   /// on the parameter of the bind clause. In the methods for the

clang/lib/AST/ExprConstant.cpp

@@ -5582,6 +5582,29 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
     MSConstexprContextRAII ConstexprContext(
         *Info.CurrentCall, hasSpecificAttr<MSConstexprAttr>(AS->getAttrs()) &&
                                isa<ReturnStmt>(SS));
+
+    auto LO = Info.getCtx().getLangOpts();
+    if (LO.CXXAssumptions && !LO.MSVCCompat) {
+      for (auto *Attr : AS->getAttrs()) {
+        auto *AA = dyn_cast<CXXAssumeAttr>(Attr);
+        if (!AA)
+          continue;
+
+        auto *Assumption = AA->getAssumption();
+        if (Assumption->isValueDependent())
+          return ESR_Failed;
+
+        bool Value;
+        if (!EvaluateAsBooleanCondition(Assumption, Value, Info))
+          return ESR_Failed;
+        if (!Value) {
+          Info.CCEDiag(Assumption->getExprLoc(),
+                       diag::note_constexpr_assumption_failed);
+          return ESR_Failed;
+        }
+      }
+    }
+
     return EvaluateStmt(Result, Info, SS, Case);
   }
 

clang/lib/CodeGen/CGCall.cpp

@@ -1796,14 +1796,14 @@ static void AddAttributesFromFunctionProtoType(ASTContext &Ctx,
     FuncAttrs.addAttribute("aarch64_inout_zt0");
 }
 
-static void AddAttributesFromAssumes(llvm::AttrBuilder &FuncAttrs,
-                                     const Decl *Callee) {
+static void AddAttributesFromOMPAssumes(llvm::AttrBuilder &FuncAttrs,
+                                        const Decl *Callee) {
   if (!Callee)
     return;
 
   SmallVector<StringRef, 4> Attrs;
 
-  for (const AssumptionAttr *AA : Callee->specific_attrs<AssumptionAttr>())
+  for (const OMPAssumeAttr *AA : Callee->specific_attrs<OMPAssumeAttr>())
     AA->getAssumption().split(Attrs, ",");
 
   if (!Attrs.empty())
@@ -2344,7 +2344,7 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
 
   // Attach assumption attributes to the declaration. If this is a call
   // site, attach assumptions from the caller to the call as well.
-  AddAttributesFromAssumes(FuncAttrs, TargetDecl);
+  AddAttributesFromOMPAssumes(FuncAttrs, TargetDecl);
 
   bool HasOptnone = false;
   // The NoBuiltinAttr attached to the target FunctionDecl.

clang/lib/CodeGen/CGStmt.cpp

@@ -728,11 +728,19 @@ void CodeGenFunction::EmitAttributedStmt(const AttributedStmt &S) {
     case attr::AlwaysInline:
       alwaysinline = true;
       break;
-    case attr::MustTail:
+    case attr::MustTail: {
       const Stmt *Sub = S.getSubStmt();
       const ReturnStmt *R = cast<ReturnStmt>(Sub);
       musttail = cast<CallExpr>(R->getRetValue()->IgnoreParens());
-      break;
+    } break;
+    case attr::CXXAssume: {
+      const Expr *Assumption = cast<CXXAssumeAttr>(A)->getAssumption();
+      if (getLangOpts().CXXAssumptions &&
+          !Assumption->HasSideEffects(getContext())) {
+        llvm::Value *AssumptionVal = EvaluateExprAsBool(Assumption);
+        Builder.CreateAssumption(AssumptionVal);
+      }
+    } break;
     }
   }
   SaveAndRestore save_nomerge(InNoMergeAttributedStmt, nomerge);

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6982,6 +6982,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                         (!IsWindowsMSVC || IsMSVC2015Compatible)))
     CmdArgs.push_back("-fno-threadsafe-statics");
 
+  // Add -fno-assumptions, if it was specified.
+  if (!Args.hasFlag(options::OPT_fassumptions, options::OPT_fno_assumptions,
+                    true))
+    CmdArgs.push_back("-fno-assumptions");
+
   // -fgnu-keywords default varies depending on language; only pass if
   // specified.
   Args.AddLastArg(CmdArgs, options::OPT_fgnu_keywords,

clang/lib/Parse/ParseDeclCXX.cpp

@@ -4528,6 +4528,61 @@ static bool IsBuiltInOrStandardCXX11Attribute(IdentifierInfo *AttrName,
   }
 }
 
+/// Parse the argument to C++23's [[assume()]] attribute.
+bool Parser::ParseCXXAssumeAttributeArg(ParsedAttributes &Attrs,
+                                        IdentifierInfo *AttrName,
+                                        SourceLocation AttrNameLoc,
+                                        SourceLocation *EndLoc) {
+  assert(Tok.is(tok::l_paren) && "Not a C++11 attribute argument list");
+  BalancedDelimiterTracker T(*this, tok::l_paren);
+  T.consumeOpen();
+
+  // [dcl.attr.assume]: The expression is potentially evaluated.
+  EnterExpressionEvaluationContext Unevaluated(
+      Actions, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
+
+  TentativeParsingAction TPA(*this);
+  ExprResult Res(
+      Actions.CorrectDelayedTyposInExpr(ParseConditionalExpression()));
+  if (Res.isInvalid()) {
+    TPA.Commit();
+    SkipUntil(tok::r_paren, tok::r_square, StopAtSemi | StopBeforeMatch);
+    if (Tok.is(tok::r_paren))
+      T.consumeClose();
+    return true;
+  }
+
+  if (!Tok.isOneOf(tok::r_paren, tok::r_square)) {
+    // Emit a better diagnostic if this is an otherwise valid expression that
+    // is not allowed here.
+    TPA.Revert();
+    Res = ParseExpression();
+    if (!Res.isInvalid()) {
+      auto *E = Res.get();
+      Diag(E->getExprLoc(), diag::err_assume_attr_expects_cond_expr)
+          << AttrName << FixItHint::CreateInsertion(E->getBeginLoc(), "(")
+          << FixItHint::CreateInsertion(PP.getLocForEndOfToken(E->getEndLoc()),
+                                        ")")
+          << E->getSourceRange();
+    }
+
+    T.consumeClose();
+    return true;
+  }
+
+  TPA.Commit();
+  ArgsUnion Assumption = Res.get();
+  auto RParen = Tok.getLocation();
+  T.consumeClose();
+  Attrs.addNew(AttrName, SourceRange(AttrNameLoc, RParen), nullptr,
+               SourceLocation(), &Assumption, 1, ParsedAttr::Form::CXX11());
+
+  if (EndLoc)
+    *EndLoc = RParen;
+
+  return false;
+}
+
 /// ParseCXX11AttributeArgs -- Parse a C++11 attribute-argument-clause.
 ///
 /// [C++11] attribute-argument-clause:
@@ -4596,7 +4651,12 @@ bool Parser::ParseCXX11AttributeArgs(
   if (ScopeName && (ScopeName->isStr("clang") || ScopeName->isStr("_Clang")))
     NumArgs = ParseClangAttributeArgs(AttrName, AttrNameLoc, Attrs, EndLoc,
                                       ScopeName, ScopeLoc, Form);
-  else
+  // So does C++23's assume() attribute.
+  else if (!ScopeName && AttrName->isStr("assume")) {
+    if (ParseCXXAssumeAttributeArg(Attrs, AttrName, AttrNameLoc, EndLoc))
+      return true;
+    NumArgs = 1;
+  } else
     NumArgs = ParseAttributeArgsCommon(AttrName, AttrNameLoc, Attrs, EndLoc,
                                        ScopeName, ScopeLoc, Form);
 

clang/lib/Parse/ParseExpr.cpp

@@ -179,6 +179,19 @@ ExprResult Parser::ParseAssignmentExpression(TypeCastState isTypeCast) {
   return ParseRHSOfBinaryExpression(LHS, prec::Assignment);
 }
 
+ExprResult Parser::ParseConditionalExpression() {
+  if (Tok.is(tok::code_completion)) {
+    cutOffParsing();
+    Actions.CodeCompleteExpression(getCurScope(),
+                                   PreferredType.get(Tok.getLocation()));
+    return ExprError();
+  }
+
+  ExprResult LHS = ParseCastExpression(
+      AnyCastExpr, /*isAddressOfOperand=*/false, NotTypeCast);
+  return ParseRHSOfBinaryExpression(LHS, prec::Conditional);
+}
+
 /// Parse an assignment expression where part of an Objective-C message
 /// send has already been parsed.
 ///

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1771,8 +1771,8 @@ void Sema::AddAllocAlignAttr(Decl *D, const AttributeCommonInfo &CI,
 }
 
 /// Check if \p AssumptionStr is a known assumption and warn if not.
-static void checkAssumptionAttr(Sema &S, SourceLocation Loc,
-                                StringRef AssumptionStr) {
+static void checkOMPAssumeAttr(Sema &S, SourceLocation Loc,
+                               StringRef AssumptionStr) {
   if (llvm::KnownAssumptionStrings.count(AssumptionStr))
     return;
 
@@ -1788,22 +1788,23 @@ static void checkAssumptionAttr(Sema &S, SourceLocation Loc,
   }
 
   if (!Suggestion.empty())
-    S.Diag(Loc, diag::warn_assume_attribute_string_unknown_suggested)
+    S.Diag(Loc, diag::warn_omp_assume_attribute_string_unknown_suggested)
         << AssumptionStr << Suggestion;
   else
-    S.Diag(Loc, diag::warn_assume_attribute_string_unknown) << AssumptionStr;
+    S.Diag(Loc, diag::warn_omp_assume_attribute_string_unknown)
+        << AssumptionStr;
 }
 
-static void handleAssumumptionAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+static void handleOMPAssumeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Handle the case where the attribute has a text message.
   StringRef Str;
   SourceLocation AttrStrLoc;
   if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, &AttrStrLoc))
     return;
 
-  checkAssumptionAttr(S, AttrStrLoc, Str);
+  checkOMPAssumeAttr(S, AttrStrLoc, Str);
 
-  D->addAttr(::new (S.Context) AssumptionAttr(S.Context, AL, Str));
+  D->addAttr(::new (S.Context) OMPAssumeAttr(S.Context, AL, Str));
 }
 
 /// Normalize the attribute, __foo__ becomes foo.
@@ -9491,8 +9492,8 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_Unavailable:
     handleAttrWithMessage<UnavailableAttr>(S, D, AL);
     break;
-  case ParsedAttr::AT_Assumption:
-    handleAssumumptionAttr(S, D, AL);
+  case ParsedAttr::AT_OMPAssume:
+    handleOMPAssumeAttr(S, D, AL);
     break;
   case ParsedAttr::AT_ObjCDirect:
     handleObjCDirectAttr(S, D, AL);

clang/lib/Sema/SemaOpenMP.cpp

@@ -3496,7 +3496,7 @@ void Sema::ActOnOpenMPAssumesDirective(SourceLocation Loc,
         << llvm::omp::getAllAssumeClauseOptions()
         << llvm::omp::getOpenMPDirectiveName(DKind);
 
-  auto *AA = AssumptionAttr::Create(Context, llvm::join(Assumptions, ","), Loc);
+  auto *AA = OMPAssumeAttr::Create(Context, llvm::join(Assumptions, ","), Loc);
   if (DKind == llvm::omp::Directive::OMPD_begin_assumes) {
     OMPAssumeScoped.push_back(AA);
     return;
@@ -7275,10 +7275,10 @@ void Sema::ActOnFinishedFunctionDefinitionInOpenMPAssumeScope(Decl *D) {
   // only global ones. We apply scoped assumption to the template definition
   // though.
   if (!inTemplateInstantiation()) {
-    for (AssumptionAttr *AA : OMPAssumeScoped)
+    for (OMPAssumeAttr *AA : OMPAssumeScoped)
       FD->addAttr(AA);
   }
-  for (AssumptionAttr *AA : OMPAssumeGlobal)
+  for (OMPAssumeAttr *AA : OMPAssumeGlobal)
     FD->addAttr(AA);
 }
 

clang/lib/Sema/SemaStmtAttr.cpp

@@ -303,6 +303,15 @@ static Attr *handleAlwaysInlineAttr(Sema &S, Stmt *St, const ParsedAttr &A,
   return ::new (S.Context) AlwaysInlineAttr(S.Context, A);
 }
 
+static Attr *handleCXXAssumeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                                 SourceRange Range) {
+  ExprResult Res = S.ActOnCXXAssumeAttr(St, A, Range);
+  if (!Res.isUsable())
+    return nullptr;
+
+  return ::new (S.Context) CXXAssumeAttr(S.Context, A, Res.get());
+}
+
 static Attr *handleMustTailAttr(Sema &S, Stmt *St, const ParsedAttr &A,
                                 SourceRange Range) {
   // Validation is in Sema::ActOnAttributedStmt().
@@ -594,6 +603,8 @@ static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
   switch (A.getKind()) {
   case ParsedAttr::AT_AlwaysInline:
     return handleAlwaysInlineAttr(S, St, A, Range);
+  case ParsedAttr::AT_CXXAssume:
+    return handleCXXAssumeAttr(S, St, A, Range);
   case ParsedAttr::AT_FallThrough:
     return handleFallThroughAttr(S, St, A, Range);
   case ParsedAttr::AT_LoopHint:
@@ -641,3 +652,47 @@ bool Sema::CheckRebuiltStmtAttributes(ArrayRef<const Attr *> Attrs) {
   CheckForDuplicateLoopAttrs<CodeAlignAttr>(*this, Attrs);
   return false;
 }
+
+ExprResult Sema::ActOnCXXAssumeAttr(Stmt *St, const ParsedAttr &A,
+                                    SourceRange Range) {
+  if (A.getNumArgs() != 1 || !A.getArgAsExpr(0)) {
+    Diag(A.getLoc(), diag::err_assume_attr_args) << A.getAttrName() << Range;
+    return ExprError();
+  }
+
+  auto *Assumption = A.getArgAsExpr(0);
+  if (Assumption->getDependence() == ExprDependence::None) {
+    ExprResult Res = BuildCXXAssumeExpr(Assumption, A.getAttrName(), Range);
+    if (Res.isInvalid())
+      return ExprError();
+    Assumption = Res.get();
+  }
+
+  if (!getLangOpts().CPlusPlus23)
+    Diag(A.getLoc(), diag::ext_cxx23_attr) << A << Range;
+
+  return Assumption;
+}
+
+ExprResult Sema::BuildCXXAssumeExpr(Expr *Assumption,
+                                    const IdentifierInfo *AttrName,
+                                    SourceRange Range) {
+  ExprResult Res = CorrectDelayedTyposInExpr(Assumption);
+  if (Res.isInvalid())
+    return ExprError();
+
+  Res = CheckPlaceholderExpr(Res.get());
+  if (Res.isInvalid())
+    return ExprError();
+
+  Res = PerformContextuallyConvertToBool(Res.get());
+  if (Res.isInvalid())
+    return ExprError();
+
+  Assumption = Res.get();
+  if (Assumption->HasSideEffects(Context))
+    Diag(Assumption->getBeginLoc(), diag::warn_assume_side_effects)
+        << AttrName << Range;
+
+  return Assumption;
+}

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1411,6 +1411,7 @@ namespace {
                           NamedDecl *FirstQualifierInScope = nullptr,
                           bool AllowInjectedClassName = false);
 
+    const CXXAssumeAttr *TransformCXXAssumeAttr(const CXXAssumeAttr *AA);
     const LoopHintAttr *TransformLoopHintAttr(const LoopHintAttr *LH);
     const NoInlineAttr *TransformStmtNoInlineAttr(const Stmt *OrigS,
                                                   const Stmt *InstS,
@@ -1980,6 +1981,21 @@ TemplateInstantiator::TransformTemplateParmRefExpr(DeclRefExpr *E,
                                          Arg, PackIndex);
 }
 
+const CXXAssumeAttr *
+TemplateInstantiator::TransformCXXAssumeAttr(const CXXAssumeAttr *AA) {
+  ExprResult Res = getDerived().TransformExpr(AA->getAssumption());
+  if (!Res.isUsable())
+    return AA;
+
+  Res = getSema().BuildCXXAssumeExpr(Res.get(), AA->getAttrName(),
+                                     AA->getRange());
+  if (!Res.isUsable())
+    return AA;
+
+  return CXXAssumeAttr::CreateImplicit(getSema().Context, Res.get(),
+                                       AA->getRange());
+}
+
 const LoopHintAttr *
 TemplateInstantiator::TransformLoopHintAttr(const LoopHintAttr *LH) {
   Expr *TransformedExpr = getDerived().TransformExpr(LH->getValue()).get();

clang/test/CodeGenCXX/cxx23-assume.cpp

@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++23 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++23 -fno-assumptions %s -emit-llvm -o - | FileCheck %s --check-prefix=DISABLED
+
+// DISABLED-NOT: @llvm.assume
+
+bool f();
+
+template <typename T>
+void f2() {
+  [[assume(sizeof(T) == sizeof(int))]];
+}
+
+// CHECK: @_Z1gii(i32 noundef [[X:%.*]], i32 noundef [[Y:%.*]])
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[X_ADDR:%.*]] = alloca i32
+// CHECK-NEXT: [[Y_ADDR:%.*]] = alloca i32
+// CHECK-NEXT: store i32 [[X]], ptr [[X_ADDR]]
+// CHECK-NEXT: store i32 [[Y]], ptr [[Y_ADDR]]
+void g(int x, int y) {
+  // Not emitted because it has side-effects.
+  [[assume(f())]];
+
+  // CHECK-NEXT: call void @llvm.assume(i1 true)
+  [[assume((1, 2))]];
+
+  // CHECK-NEXT: [[X1:%.*]] = load i32, ptr [[X_ADDR]]
+  // CHECK-NEXT: [[CMP1:%.*]] = icmp ne i32 [[X1]], 27
+  // CHECK-NEXT: call void @llvm.assume(i1 [[CMP1]])
+  [[assume(x != 27)]];
+
+  // CHECK-NEXT: [[X2:%.*]] = load i32, ptr [[X_ADDR]]
+  // CHECK-NEXT: [[Y2:%.*]] = load i32, ptr [[Y_ADDR]]
+  // CHECK-NEXT: [[CMP2:%.*]] = icmp eq i32 [[X2]], [[Y2]]
+  // CHECK-NEXT: call void @llvm.assume(i1 [[CMP2]])
+  [[assume(x == y)]];
+
+  // CHECK-NEXT: call void @_Z2f2IiEvv()
+  f2<int>();
+
+  // CHECK-NEXT: call void @_Z2f2IdEvv()
+  f2<double>();
+}
+
+// CHECK: void @_Z2f2IiEvv()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: call void @llvm.assume(i1 true)
+
+// CHECK: void @_Z2f2IdEvv()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: call void @llvm.assume(i1 false)

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -19,7 +19,6 @@
 // CHECK-NEXT: ArcWeakrefUnavailable (SubjectMatchRule_objc_interface)
 // CHECK-NEXT: ArmBuiltinAlias (SubjectMatchRule_function)
 // CHECK-NEXT: AssumeAligned (SubjectMatchRule_objc_method, SubjectMatchRule_function)
-// CHECK-NEXT: Assumption (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: Availability ((SubjectMatchRule_record, SubjectMatchRule_enum, SubjectMatchRule_enum_constant, SubjectMatchRule_field, SubjectMatchRule_function, SubjectMatchRule_namespace, SubjectMatchRule_objc_category, SubjectMatchRule_objc_implementation, SubjectMatchRule_objc_interface, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_objc_protocol, SubjectMatchRule_record, SubjectMatchRule_type_alias, SubjectMatchRule_variable))
 // CHECK-NEXT: AvailableOnlyInDefaultEvalMethod (SubjectMatchRule_type_alias)
 // CHECK-NEXT: BPFPreserveAccessIndex (SubjectMatchRule_record)
@@ -127,6 +126,7 @@
 // CHECK-NEXT: NoThrow (SubjectMatchRule_hasType_functionType)
 // CHECK-NEXT: NoUwtable (SubjectMatchRule_hasType_functionType)
 // CHECK-NEXT: NotTailCalled (SubjectMatchRule_function)
+// CHECK-NEXT: OMPAssume (SubjectMatchRule_function, SubjectMatchRule_objc_method)
 // CHECK-NEXT: OSConsumed (SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsNotRetained (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_variable_is_parameter)
 // CHECK-NEXT: OSReturnsRetained (SubjectMatchRule_function, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_variable_is_parameter)

clang/test/Parser/cxx23-assume.cpp

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -std=c++23 -x c++ %s -verify
+
+void f(int x, int y) {
+  [[assume(true)]];
+  [[assume(1)]];
+  [[assume(1.0)]];
+  [[assume(1 + 2 == 3)]];
+  [[assume(x ? 1 : 2)]];
+  [[assume(x && y)]];
+  [[assume(true)]] [[assume(true)]];
+
+  [[assume]]; // expected-error {{takes one argument}}
+  [[assume(]]; // expected-error {{expected expression}}
+  [[assume()]]; // expected-error {{expected expression}}
+  [[assume(2]]; // expected-error {{expected ')'}} expected-note {{to match this '('}}
+  [[assume(x = 2)]]; // expected-error {{requires parentheses}}
+  [[assume(2, 3)]]; // expected-error {{requires parentheses}} expected-warning {{has no effect}}
+}

clang/test/SemaCXX/cxx23-assume-disabled.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c++23 -x c++ %s -fno-assumptions -verify
+// RUN: %clang_cc1 -std=c++23 -x c++ %s -fms-compatibility -verify
+// expected-no-diagnostics
+
+// We don't check assumptions at compile time if '-fno-assumptions' is passed,
+// or if we're in MSVCCompat mode
+
+constexpr bool f(bool x) {
+  [[assume(x)]];
+  return true;
+}
+
+static_assert(f(false));
+

clang/test/SemaCXX/cxx23-assume-print.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++23 -ast-print %s | FileCheck %s
+
+// CHECK: void f(int x, int y) {
+void f(int x, int y) {
+  // CHECK-NEXT: {{\[}}[assume(true)]]
+  [[assume(true)]];
+
+  // CHECK-NEXT: {{\[}}[assume(2 + 4)]]
+  [[assume(2 + 4)]];
+
+  // CHECK-NEXT: {{\[}}[assume(x == y)]]
+  [[assume(x == y)]];
+}

clang/test/SemaCXX/cxx23-assume.cpp

@@ -0,0 +1,128 @@
+// RUN: %clang_cc1 -std=c++23  -x c++ %s -verify
+// RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext,expected
+
+struct A{};
+struct B{ explicit operator bool() { return true; } };
+
+template <bool cond>
+void f() {
+  [[assume(cond)]]; // ext-warning {{C++23 extension}}
+}
+
+template <bool cond>
+struct S {
+  void f() {
+    [[assume(cond)]]; // ext-warning {{C++23 extension}}
+  }
+
+  template <typename T>
+  constexpr bool g() {
+    [[assume(cond == sizeof(T))]]; // expected-note {{assumption evaluated to false}} ext-warning {{C++23 extension}}
+    return true;
+  }
+};
+
+bool f2();
+
+template <typename T>
+constexpr void f3() {
+  [[assume(T{})]]; // expected-error {{not contextually convertible to 'bool'}} expected-warning {{has side effects that will be discarded}} ext-warning {{C++23 extension}}
+}
+
+void g(int x) {
+  f<true>();
+  f<false>();
+  S<true>{}.f();
+  S<false>{}.f();
+  S<true>{}.g<char>();
+  S<true>{}.g<int>();
+  [[assume(f2())]]; // expected-warning {{side effects that will be discarded}} ext-warning {{C++23 extension}}
+
+  [[assume((x = 3))]]; // expected-warning {{has side effects that will be discarded}} // ext-warning {{C++23 extension}}
+  [[assume(x++)]]; // expected-warning {{has side effects that will be discarded}} // ext-warning {{C++23 extension}}
+  [[assume(++x)]]; // expected-warning {{has side effects that will be discarded}} // ext-warning {{C++23 extension}}
+  [[assume([]{ return true; }())]]; // expected-warning {{has side effects that will be discarded}} // ext-warning {{C++23 extension}}
+  [[assume(B{})]]; // expected-warning {{has side effects that will be discarded}} // ext-warning {{C++23 extension}}
+  [[assume((1, 2))]]; // expected-warning {{has no effect}} // ext-warning {{C++23 extension}}
+
+  f3<A>(); // expected-note {{in instantiation of}}
+  f3<B>(); // expected-note {{in instantiation of}}
+  [[assume]]; // expected-error {{takes one argument}}
+  [[assume(z)]]; // expected-error {{undeclared identifier}}
+  [[assume(A{})]]; // expected-error {{not contextually convertible to 'bool'}}
+  [[assume(true)]] if (true) {} // expected-error {{only applies to empty statements}}
+  [[assume(true)]] {} // expected-error {{only applies to empty statements}}
+  [[assume(true)]] for (;false;) {} // expected-error {{only applies to empty statements}}
+  [[assume(true)]] while (false) {} // expected-error {{only applies to empty statements}}
+  [[assume(true)]] label:; // expected-error {{cannot be applied to a declaration}}
+  [[assume(true)]] goto label; // expected-error {{only applies to empty statements}}
+}
+
+// Check that 'x' is ODR-used here.
+constexpr int h(int x) { return sizeof([=] { [[assume(x)]]; }); } // ext-warning {{C++23 extension}}
+static_assert(h(4) == sizeof(int));
+
+static_assert(__has_cpp_attribute(assume) == 202207L);
+static_assert(__has_attribute(assume));
+
+constexpr bool i() { // expected-error {{never produces a constant expression}}
+  [[assume(false)]]; // expected-note {{assumption evaluated to false}} expected-note {{assumption evaluated to false}} ext-warning {{C++23 extension}}
+  return true;
+}
+
+constexpr bool j(bool b) {
+  [[assume(b)]]; // expected-note {{assumption evaluated to false}} ext-warning {{C++23 extension}}
+  return true;
+}
+
+static_assert(i()); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+static_assert(j(true));
+static_assert(j(false)); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+static_assert(S<true>{}.g<char>());
+static_assert(S<false>{}.g<A>()); // expected-error {{not an integral constant expression}} expected-note {{in call to}}
+
+
+template <typename T>
+constexpr bool f4() {
+  [[assume(!T{})]]; // expected-error {{invalid argument type 'D'}} // expected-warning 2 {{side effects}} ext-warning {{C++23 extension}}
+  return sizeof(T) == sizeof(int);
+}
+
+template <typename T>
+concept C = f4<T>(); // expected-note 3 {{in instantiation of}}
+                     // expected-note@-1 3 {{while substituting}}
+                     // expected-error@-2 2 {{resulted in a non-constant expression}}
+
+struct D {
+  int x;
+};
+
+struct E {
+  int x;
+  constexpr explicit operator bool() { return false; }
+};
+
+struct F {
+  int x;
+  int y;
+  constexpr explicit operator bool() { return false; }
+};
+
+template <typename T>
+constexpr int f5() requires C<T> { return 1; } // expected-note {{while checking the satisfaction}}
+                                               // expected-note@-1 {{while substituting template arguments}}
+                                               // expected-note@-2 {{candidate template ignored}}
+
+template <typename T>
+constexpr int f5() requires (!C<T>) { return 2; } // expected-note 4 {{while checking the satisfaction}}
+                                                  // expected-note@-1 4 {{while substituting template arguments}}
+                                                  // expected-note@-2 {{candidate template ignored}}
+
+static_assert(f5<int>() == 1);
+static_assert(f5<D>() == 1); // expected-note 3 {{while checking constraint satisfaction}}
+                             // expected-note@-1 3 {{in instantiation of}}
+                             // expected-error@-2 {{no matching function for call}}
+
+static_assert(f5<double>() == 2);
+static_assert(f5<E>() == 1); // expected-note {{while checking constraint satisfaction}} expected-note {{in instantiation of}}
+static_assert(f5<F>() == 2); // expected-note {{while checking constraint satisfaction}} expected-note {{in instantiation of}}

clang/www/cxx_status.html

@@ -381,7 +381,7 @@ C++23, informally referred to as C++26.</p>
     <tr>
       <td>Portable assumptions</td>
       <td><a href="https://wg21.link/P1774R8">P1774R8</a></td>
-      <td class="none" align="center">No</td>
+      <td class="full" align="center">Clang 19</td>
     </tr>
     <tr>
       <td>Support for UTF-8 as a portable source file encoding</td>