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

via cfe-commits cfe-commits at lists.llvm.org
Wed Feb 7 09:41:03 PST 2024


https://github.com/Sirraide updated https://github.com/llvm/llvm-project/pull/81014

>From 5ba669011752b6ea6e6fe5f6141c4af66ca7ccbf Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 17:25:10 +0100
Subject: [PATCH 1/9] [Clang] Parsing and Sema for C++23's `assume` attribute

---
 clang/include/clang/Basic/Attr.td             |  9 ++++
 clang/include/clang/Basic/DiagnosticGroups.td |  4 +-
 .../clang/Basic/DiagnosticParseKinds.td       |  3 ++
 .../clang/Basic/DiagnosticSemaKinds.td        |  7 +++
 clang/include/clang/Parse/Parser.h            |  7 +++
 clang/lib/Parse/ParseDeclCXX.cpp              | 54 ++++++++++++++++++-
 clang/lib/Parse/ParseExpr.cpp                 | 13 +++++
 clang/lib/Sema/SemaStmtAttr.cpp               | 34 ++++++++++++
 clang/test/Parser/cxx23-assume.cpp            | 27 ++++++++++
 9 files changed, 156 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/Parser/cxx23-assume.cpp

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index b2d5309e142c1..2804ff1aea005 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1564,6 +1564,15 @@ def Unlikely : StmtAttr {
 }
 def : MutualExclusions<[Likely, Unlikely]>;
 
+def Assume : StmtAttr {
+  let Spellings = [CXX11<"", "assume", 202302>];
+  let Subjects = SubjectList<[NullStmt], ErrorDiag, "empty statements">;
+  // The standard only allows a conditional-expression here, but we ought
+  // to get better results by handling that in Sema.
+  let Args = [ExprArgument<"Assumption">];
+  let Documentation = [AssumptionDocs];
+}
+
 def NoMerge : DeclOrStmtAttr {
   let Spellings = [Clang<"nomerge">];
   let Documentation = [NoMergeDocs];
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 6765721ae7002..192b081404a82 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1124,9 +1124,11 @@ 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">;
 
diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index a30ab27566ec3..9ecfdab3617e0 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -783,6 +783,9 @@ 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 'assume' attribute requires parentheses">;
+
 def warn_cxx20_compat_explicit_bool : Warning<
   "this expression will be parsed as explicit(bool) in C++20">,
   InGroup<CXX20Compat>, DefaultIgnore;
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index b4dc4feee8e63..847168af28862 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9083,6 +9083,8 @@ 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">,
@@ -10149,6 +10151,11 @@ 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 'assume' requires a single expression argument">;
+def err_assume_attr_wrong_target : Error<
+  "'assume' attribute is only allowed on empty statements">;
+
 def warn_unreachable_default : Warning<
   "default label in switch which covers all enumeration values">,
   InGroup<CoveredSwitchDefault>, DefaultIgnore;
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index da18cf88edcc9..0f982dbb67b41 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -1801,6 +1801,7 @@ 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,
@@ -2953,6 +2954,12 @@ class Parser : public CodeCompletionHandler {
                                SourceLocation ScopeLoc,
                                CachedTokens &OpenMPTokens);
 
+  /// Parse a C++23 assume() attribute. Returns true on error.
+  bool ParseAssumeAttributeArg(ParsedAttributes &Attrs,
+                               IdentifierInfo *AttrName,
+                               SourceLocation AttrNameLoc,
+                               SourceLocation *EndLoc);
+
   IdentifierInfo *TryParseCXX11AttributeIdentifier(
       SourceLocation &Loc,
       Sema::AttributeCompletion Completion = Sema::AttributeCompletion::None,
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 79928ddb5af59..768aca68030fe 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -4528,6 +4528,53 @@ static bool IsBuiltInOrStandardCXX11Attribute(IdentifierInfo *AttrName,
   }
 }
 
+/// Parse the argument to C++23's [[assume()]] attribute.
+bool Parser::ParseAssumeAttributeArg(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();
+
+  EnterExpressionEvaluationContext Unevaluated(
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  TentativeParsingAction TPA(*this);
+  ExprResult Res(
+      Actions.CorrectDelayedTyposInExpr(ParseConditionalExpression()));
+  if (Res.isInvalid() || !Tok.is(tok::r_paren)) {
+    // Emit a better diagnostic if this is an otherwise valid expression that
+    // is not allowed here.
+    TPA.Revert();
+    Sema::TentativeAnalysisScope Scope(Actions);
+    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 +4643,12 @@ 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 (ParseAssumeAttributeArg(Attrs, AttrName, AttrNameLoc, EndLoc))
+      return true;
+    NumArgs = 1;
+  } else
     NumArgs = ParseAttributeArgsCommon(AttrName, AttrNameLoc, Attrs, EndLoc,
                                        ScopeName, ScopeLoc, Form);
 
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 52cebdb6f64ba..cf2a7bd026c5d 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/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.
 ///
diff --git a/clang/lib/Sema/SemaStmtAttr.cpp b/clang/lib/Sema/SemaStmtAttr.cpp
index e6a4d3e63e4aa..41f6461e183e7 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -303,6 +303,38 @@ static Attr *handleAlwaysInlineAttr(Sema &S, Stmt *St, const ParsedAttr &A,
   return ::new (S.Context) AlwaysInlineAttr(S.Context, A);
 }
 
+static Attr *handleAssumeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                              SourceRange Range) {
+  if (!S.getLangOpts().CPlusPlus23)
+    S.Diag(A.getLoc(), diag::ext_cxx23_attr) << A << Range;
+
+  if (A.getNumArgs() != 1 || !A.getArgAsExpr(0)) {
+    S.Diag(A.getLoc(), diag::err_assume_attr_args) << Range;
+    return nullptr;
+  }
+
+  if (!isa<NullStmt>(St)) {
+    S.Diag(A.getLoc(), diag::err_assume_attr_wrong_target) << Range;
+    return nullptr;
+  }
+
+  auto *Assumption = A.getArgAsExpr(0);
+  if (Assumption->getDependence() == ExprDependence::None) {
+    ExprResult Res = S.CorrectDelayedTyposInExpr(Assumption);
+    if (Res.isInvalid())
+      return nullptr;
+    Res = S.CheckPlaceholderExpr(Res.get());
+    if (Res.isInvalid())
+      return nullptr;
+    Res = S.PerformContextuallyConvertToBool(Res.get());
+    if (Res.isInvalid())
+      return nullptr;
+    Assumption = Res.get();
+  }
+
+  return ::new (S.Context) AssumeAttr(S.Context, A, Assumption);
+}
+
 static Attr *handleMustTailAttr(Sema &S, Stmt *St, const ParsedAttr &A,
                                 SourceRange Range) {
   // Validation is in Sema::ActOnAttributedStmt().
@@ -594,6 +626,8 @@ 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_Assume:
+    return handleAssumeAttr(S, St, A, Range);
   case ParsedAttr::AT_FallThrough:
     return handleFallThroughAttr(S, St, A, Range);
   case ParsedAttr::AT_LoopHint:
diff --git a/clang/test/Parser/cxx23-assume.cpp b/clang/test/Parser/cxx23-assume.cpp
new file mode 100644
index 0000000000000..ecf6754600ccb
--- /dev/null
+++ b/clang/test/Parser/cxx23-assume.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -std=c++23 -x c++ %s -verify
+
+struct A{};
+struct B{ explicit operator bool() { return true; } };
+
+void f(int x, int y) {
+  [[assume(true)]];
+  [[assume(1)]];
+  [[assume(1.0)]];
+  [[assume(1 + 2 == 3)]];
+  [[assume((1, 2, 3))]];
+  [[assume((x = 3))]];
+  [[assume(x ? 1 : 2)]];
+  [[assume(x && y)]];
+  [[assume(x++)]];
+  [[assume(++x)]];
+  [[assume([]{ return true; }())]];
+  [[assume(B{})]];
+  [[assume(true)]] [[assume(true)]];
+
+  [[assume]]; // expected-error {{takes one argument}}
+  [[assume(z)]]; // expected-error {{undeclared identifier}}
+  [[assume(x = 2)]]; // expected-error {{requires parentheses}}
+  [[assume(2, 3)]]; // expected-error {{requires parentheses}}
+  [[assume(A{})]]; // expected-error {{not contextually convertible to 'bool'}}
+  [[assume(true)]] if (true) {} // expected-error {{only applies to empty statements}}
+}
\ No newline at end of file

>From df08a1468bda9337dcd602472fee32f1bd953f87 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 17:42:30 +0100
Subject: [PATCH 2/9] [Clang] Make argument of assume() potentially evaluated

---
 clang/lib/Parse/ParseDeclCXX.cpp    |  3 ++-
 clang/test/Parser/cxx23-assume.cpp  |  8 +++++++-
 clang/test/SemaCXX/cxx23-assume.cpp | 26 ++++++++++++++++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/SemaCXX/cxx23-assume.cpp

diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 768aca68030fe..7f2762d4fe22b 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -4537,8 +4537,9 @@ bool Parser::ParseAssumeAttributeArg(ParsedAttributes &Attrs,
   BalancedDelimiterTracker T(*this, tok::l_paren);
   T.consumeOpen();
 
+  // [dcl.attr.assume]: The expression is potentially evaluated.
   EnterExpressionEvaluationContext Unevaluated(
-      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
+      Actions, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
 
   TentativeParsingAction TPA(*this);
   ExprResult Res(
diff --git a/clang/test/Parser/cxx23-assume.cpp b/clang/test/Parser/cxx23-assume.cpp
index ecf6754600ccb..5f14a35144554 100644
--- a/clang/test/Parser/cxx23-assume.cpp
+++ b/clang/test/Parser/cxx23-assume.cpp
@@ -8,7 +8,6 @@ void f(int x, int y) {
   [[assume(1)]];
   [[assume(1.0)]];
   [[assume(1 + 2 == 3)]];
-  [[assume((1, 2, 3))]];
   [[assume((x = 3))]];
   [[assume(x ? 1 : 2)]];
   [[assume(x && y)]];
@@ -18,10 +17,17 @@ void f(int x, int y) {
   [[assume(B{})]];
   [[assume(true)]] [[assume(true)]];
 
+  [[assume((1, 2))]]; // expected-warning {{has no effect}}
+
   [[assume]]; // expected-error {{takes one argument}}
   [[assume(z)]]; // expected-error {{undeclared identifier}}
   [[assume(x = 2)]]; // expected-error {{requires parentheses}}
   [[assume(2, 3)]]; // expected-error {{requires parentheses}}
   [[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}}
 }
\ No newline at end of file
diff --git a/clang/test/SemaCXX/cxx23-assume.cpp b/clang/test/SemaCXX/cxx23-assume.cpp
new file mode 100644
index 0000000000000..9fdcda31947e7
--- /dev/null
+++ b/clang/test/SemaCXX/cxx23-assume.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -std=c++23 -x c++ %s -verify
+// RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext
+// expected-no-diagnostics
+
+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}}
+  }
+};
+
+void g() {
+  f<true>();
+  f<false>();
+  S<true>{}.f();
+  S<false>{}.f();
+}
+
+// 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));

>From 64345e281cbd0fa0029b36a4ada40dc311b1ba94 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:01:36 +0100
Subject: [PATCH 3/9] [Clang] Move most [[assume()]] diags to test/SemaCXX

---
 clang/lib/Sema/SemaStmtAttr.cpp     |  9 ++++++---
 clang/test/Parser/cxx23-assume.cpp  | 19 -------------------
 clang/test/SemaCXX/cxx23-assume.cpp | 28 +++++++++++++++++++++++++---
 3 files changed, 31 insertions(+), 25 deletions(-)

diff --git a/clang/lib/Sema/SemaStmtAttr.cpp b/clang/lib/Sema/SemaStmtAttr.cpp
index 41f6461e183e7..e712745a237c3 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -305,9 +305,6 @@ static Attr *handleAlwaysInlineAttr(Sema &S, Stmt *St, const ParsedAttr &A,
 
 static Attr *handleAssumeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
                               SourceRange Range) {
-  if (!S.getLangOpts().CPlusPlus23)
-    S.Diag(A.getLoc(), diag::ext_cxx23_attr) << A << Range;
-
   if (A.getNumArgs() != 1 || !A.getArgAsExpr(0)) {
     S.Diag(A.getLoc(), diag::err_assume_attr_args) << Range;
     return nullptr;
@@ -330,8 +327,14 @@ static Attr *handleAssumeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
     if (Res.isInvalid())
       return nullptr;
     Assumption = Res.get();
+
+    if (Assumption->HasSideEffects(S.Context, /*IncludePossibleEffects=*/true))
+      S.Diag(A.getLoc(), diag::warn_assume_side_effects) << A.getAttrName() << Range;
   }
 
+  if (!S.getLangOpts().CPlusPlus23)
+    S.Diag(A.getLoc(), diag::ext_cxx23_attr) << A << Range;
+
   return ::new (S.Context) AssumeAttr(S.Context, A, Assumption);
 }
 
diff --git a/clang/test/Parser/cxx23-assume.cpp b/clang/test/Parser/cxx23-assume.cpp
index 5f14a35144554..968caae7dab9a 100644
--- a/clang/test/Parser/cxx23-assume.cpp
+++ b/clang/test/Parser/cxx23-assume.cpp
@@ -1,33 +1,14 @@
 // RUN: %clang_cc1 -std=c++23 -x c++ %s -verify
 
-struct A{};
-struct B{ explicit operator bool() { return true; } };
-
 void f(int x, int y) {
   [[assume(true)]];
   [[assume(1)]];
   [[assume(1.0)]];
   [[assume(1 + 2 == 3)]];
-  [[assume((x = 3))]];
   [[assume(x ? 1 : 2)]];
   [[assume(x && y)]];
-  [[assume(x++)]];
-  [[assume(++x)]];
-  [[assume([]{ return true; }())]];
-  [[assume(B{})]];
   [[assume(true)]] [[assume(true)]];
 
-  [[assume((1, 2))]]; // expected-warning {{has no effect}}
-
-  [[assume]]; // expected-error {{takes one argument}}
-  [[assume(z)]]; // expected-error {{undeclared identifier}}
   [[assume(x = 2)]]; // expected-error {{requires parentheses}}
   [[assume(2, 3)]]; // expected-error {{requires parentheses}}
-  [[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}}
 }
\ No newline at end of file
diff --git a/clang/test/SemaCXX/cxx23-assume.cpp b/clang/test/SemaCXX/cxx23-assume.cpp
index 9fdcda31947e7..d3132c55f4b47 100644
--- a/clang/test/SemaCXX/cxx23-assume.cpp
+++ b/clang/test/SemaCXX/cxx23-assume.cpp
@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -std=c++23 -x c++ %s -verify
-// RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext
-// expected-no-diagnostics
+// 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() {
@@ -14,11 +16,31 @@ struct S {
   }
 };
 
-void g() {
+bool f2();
+
+void g(int x) {
   f<true>();
   f<false>();
   S<true>{}.f();
   S<false>{}.f();
+  [[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}}
+
+  [[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.

>From 6c44d2097b2e1594857f29437f09def1c3e1b0d9 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:11:10 +0100
Subject: [PATCH 4/9] [Clang] Codegen for [[assume()]]

---
 clang/lib/CodeGen/CGStmt.cpp           | 11 ++++++++--
 clang/test/CodeGenCXX/cxx23-assume.cpp | 28 ++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CodeGenCXX/cxx23-assume.cpp

diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index beff0ad9da270..66be305550b1b 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -721,11 +721,18 @@ 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::Assume: {
+      const Expr *Assumption = cast<AssumeAttr>(A)->getAssumption();
+      if (!Assumption->HasSideEffects(getContext())) {
+        llvm::Value *AssumptionVal = EvaluateExprAsBool(Assumption);
+        Builder.CreateAssumption(AssumptionVal);
+      }
+    } break;
     }
   }
   SaveAndRestore save_nomerge(InNoMergeAttributedStmt, nomerge);
diff --git a/clang/test/CodeGenCXX/cxx23-assume.cpp b/clang/test/CodeGenCXX/cxx23-assume.cpp
new file mode 100644
index 0000000000000..8a8438e8edc05
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx23-assume.cpp
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++23 %s -emit-llvm -o - | FileCheck %s
+
+bool f();
+
+// 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))]];
+
+  // [[X1:%.*]] = load i32, ptr [[X_ADDR]]
+  // [[CMP1:%.*]] = icmp ne i32 [[X1]], 27
+  // call void @llvm.assume(i1 [[CMP1]])
+  [[assume(x != 27)]];
+
+  // [[X2:%.*]] = load i32, ptr [[X_ADDR]]
+  // [[Y2:%.*]] = load i32, ptr [[Y_ADDR]]
+  // [[CMP2:%.*]] = icmp eq i32 [[X2]], [[Y2]]
+  // call void @llvm.assume(i1 [[CMP2]])
+  [[assume(x == y)]];
+}

>From dd8febdf4858c72194da2dea6460256e40816b1f Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:14:21 +0100
Subject: [PATCH 5/9] [Clang] Check that __has_cpp_attribute returns 1 for
 assume

---
 clang/test/SemaCXX/cxx23-assume.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/clang/test/SemaCXX/cxx23-assume.cpp b/clang/test/SemaCXX/cxx23-assume.cpp
index d3132c55f4b47..15ea831f77f10 100644
--- a/clang/test/SemaCXX/cxx23-assume.cpp
+++ b/clang/test/SemaCXX/cxx23-assume.cpp
@@ -46,3 +46,5 @@ void g(int x) {
 // 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));

>From b498be8971e945bff3592f76f162c1ed8d0abb4d Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:18:47 +0100
Subject: [PATCH 6/9] [NFC] Add newline at end of file

---
 clang/test/Parser/cxx23-assume.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/test/Parser/cxx23-assume.cpp b/clang/test/Parser/cxx23-assume.cpp
index 968caae7dab9a..2a9b8b6a24882 100644
--- a/clang/test/Parser/cxx23-assume.cpp
+++ b/clang/test/Parser/cxx23-assume.cpp
@@ -11,4 +11,4 @@ void f(int x, int y) {
 
   [[assume(x = 2)]]; // expected-error {{requires parentheses}}
   [[assume(2, 3)]]; // expected-error {{requires parentheses}}
-}
\ No newline at end of file
+}

>From 36fc751a4b3d55798f0e215bdb92f10e0d82afca Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:31:04 +0100
Subject: [PATCH 7/9] [Clang] Update documentation

---
 clang/docs/ReleaseNotes.rst           |  1 +
 clang/include/clang/Basic/Attr.td     |  2 +-
 clang/include/clang/Basic/AttrDocs.td | 24 ++++++++++++++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 802c44b6c8608..6312e10091361 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -95,6 +95,7 @@ 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>`_.
 
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 2804ff1aea005..6cd2a541da17d 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1570,7 +1570,7 @@ def Assume : StmtAttr {
   // The standard only allows a conditional-expression here, but we ought
   // to get better results by handling that in Sema.
   let Args = [ExprArgument<"Assumption">];
-  let Documentation = [AssumptionDocs];
+  let Documentation = [AssumeDocs];
 }
 
 def NoMerge : DeclOrStmtAttr {
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 041786f37fb8a..1b27a46ef1d90 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -1996,6 +1996,30 @@ Here is an example:
   }];
 }
 
+def AssumeDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "assume";
+  let Content = [{
+The ``assume`` attribute is used to indicate to the optimizer that a
+certain condition can be 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.
+
+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; // Will be optimised to `return 28`.
+  }
+  }];
+}
+
 def LikelihoodDocs : Documentation {
   let Category = DocCatStmt;
   let Heading = "likely and unlikely";

>From fc720294e740581c38fc532560c9ad6aece626a4 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:37:43 +0100
Subject: [PATCH 8/9] [Clang][NFC] Remove outdated comment

---
 clang/include/clang/Basic/Attr.td | 2 --
 1 file changed, 2 deletions(-)

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 6cd2a541da17d..d5810221b633b 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1567,8 +1567,6 @@ def : MutualExclusions<[Likely, Unlikely]>;
 def Assume : StmtAttr {
   let Spellings = [CXX11<"", "assume", 202302>];
   let Subjects = SubjectList<[NullStmt], ErrorDiag, "empty statements">;
-  // The standard only allows a conditional-expression here, but we ought
-  // to get better results by handling that in Sema.
   let Args = [ExprArgument<"Assumption">];
   let Documentation = [AssumeDocs];
 }

>From 0d869a5478b06fb7ae2eadc71034ed3530c9ebc5 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Wed, 7 Feb 2024 18:40:40 +0100
Subject: [PATCH 9/9] [Clang][NFC] Fix formatting

---
 clang/lib/Sema/SemaStmtAttr.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaStmtAttr.cpp b/clang/lib/Sema/SemaStmtAttr.cpp
index e712745a237c3..0812bc8761c9a 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -329,7 +329,8 @@ static Attr *handleAssumeAttr(Sema &S, Stmt *St, const ParsedAttr &A,
     Assumption = Res.get();
 
     if (Assumption->HasSideEffects(S.Context, /*IncludePossibleEffects=*/true))
-      S.Diag(A.getLoc(), diag::warn_assume_side_effects) << A.getAttrName() << Range;
+      S.Diag(A.getLoc(), diag::warn_assume_side_effects)
+          << A.getAttrName() << Range;
   }
 
   if (!S.getLangOpts().CPlusPlus23)



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