[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)
via cfe-commits
cfe-commits at lists.llvm.org
Wed Feb 7 09:35:52 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang-codegen
Author: None (Sirraide)
<details>
<summary>Changes</summary>
This implements the C++23 `assume` attribute.
This pr currently does not include any changes to constant evaluation. Even though the standard specifies that the behaviour is undefined should an assumption evaluate to `false` at runtime [dcl.attr.assume], it also mentions that we’re not required to diagnose that [expr.const], as I understand it.
---
Full diff: https://github.com/llvm/llvm-project/pull/81014.diff
14 Files Affected:
- (modified) clang/docs/ReleaseNotes.rst (+1)
- (modified) clang/include/clang/Basic/Attr.td (+9)
- (modified) clang/include/clang/Basic/AttrDocs.td (+24)
- (modified) clang/include/clang/Basic/DiagnosticGroups.td (+3-1)
- (modified) clang/include/clang/Basic/DiagnosticParseKinds.td (+3)
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+7)
- (modified) clang/include/clang/Parse/Parser.h (+7)
- (modified) clang/lib/CodeGen/CGStmt.cpp (+9-2)
- (modified) clang/lib/Parse/ParseDeclCXX.cpp (+54-1)
- (modified) clang/lib/Parse/ParseExpr.cpp (+13)
- (modified) clang/lib/Sema/SemaStmtAttr.cpp (+37)
- (added) clang/test/CodeGenCXX/cxx23-assume.cpp (+28)
- (added) clang/test/Parser/cxx23-assume.cpp (+14)
- (added) clang/test/SemaCXX/cxx23-assume.cpp (+50)
``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 802c44b6c86080..6312e100913613 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 b2d5309e142c1a..6cd2a541da17d9 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 = [AssumeDocs];
+}
+
def NoMerge : DeclOrStmtAttr {
let Spellings = [Clang<"nomerge">];
let Documentation = [NoMergeDocs];
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 041786f37fb8a7..1b27a46ef1d901 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";
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 6765721ae7002c..192b081404a827 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 a30ab27566ec3e..9ecfdab3617e05 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 b4dc4feee8e63a..847168af288622 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 da18cf88edcc92..0f982dbb67b41c 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/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index beff0ad9da2709..66be305550b1b9 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/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 79928ddb5af599..7f2762d4fe22b9 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -4528,6 +4528,54 @@ 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();
+
+ // [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() || !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 +4644,12 @@ 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 52cebdb6f64bac..cf2a7bd026c5de 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 e6a4d3e63e4aa8..e712745a237c3e 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -303,6 +303,41 @@ 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 (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();
+
+ 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);
+}
+
static Attr *handleMustTailAttr(Sema &S, Stmt *St, const ParsedAttr &A,
SourceRange Range) {
// Validation is in Sema::ActOnAttributedStmt().
@@ -594,6 +629,8 @@ 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/CodeGenCXX/cxx23-assume.cpp b/clang/test/CodeGenCXX/cxx23-assume.cpp
new file mode 100644
index 00000000000000..8a8438e8edc057
--- /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)]];
+}
diff --git a/clang/test/Parser/cxx23-assume.cpp b/clang/test/Parser/cxx23-assume.cpp
new file mode 100644
index 00000000000000..2a9b8b6a248821
--- /dev/null
+++ b/clang/test/Parser/cxx23-assume.cpp
@@ -0,0 +1,14 @@
+// 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(x = 2)]]; // expected-error {{requires parentheses}}
+ [[assume(2, 3)]]; // expected-error {{requires parentheses}}
+}
diff --git a/clang/test/SemaCXX/cxx23-assume.cpp b/clang/test/SemaCXX/cxx23-assume.cpp
new file mode 100644
index 00000000000000..15ea831f77f10d
--- /dev/null
+++ b/clang/test/SemaCXX/cxx23-assume.cpp
@@ -0,0 +1,50 @@
+// 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}}
+ }
+};
+
+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.
+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));
``````````
</details>
https://github.com/llvm/llvm-project/pull/81014
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