[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.

Author: Sirraide

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 @@ class Parser : public CodeCompletionHandler {
   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 @@ class Parser : public CodeCompletionHandler {
                                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,