[clang][Interp] Implement C++23 [[assume]] support

Author: tbaederr

clang/lib/AST/Interp/ByteCodeStmtGen.cpp

@@ -675,7 +675,30 @@ bool ByteCodeStmtGen<Emitter>::visitDefaultStmt(const DefaultStmt *S) {
 
 template <class Emitter>
 bool ByteCodeStmtGen<Emitter>::visitAttributedStmt(const AttributedStmt *S) {
-  // Ignore all attributes.
+
+  for (const Attr *A : S->getAttrs()) {
+    auto *AA = dyn_cast<CXXAssumeAttr>(A);
+    if (!AA)
+      continue;
+
+    assert(isa<NullStmt>(S->getSubStmt()));
+
+    const Expr *Assumption = AA->getAssumption();
+    if (Assumption->isValueDependent())
+      return false;
+
+    if (Assumption->HasSideEffects(this->Ctx.getASTContext()))
+      continue;
+
+    // Evaluate assumption.
+    if (!this->visitBool(Assumption))
+      return false;
+
+    if (!this->emitAssume(Assumption))
+      return false;
+  }
+
+  // Ignore other attributes.
   return this->visitStmt(S->getSubStmt());
 }
 

clang/lib/AST/Interp/Interp.h

@@ -2327,6 +2327,18 @@ inline bool InvalidDeclRef(InterpState &S, CodePtr OpPC,
   return CheckDeclRef(S, OpPC, DR);
 }
 
+inline bool Assume(InterpState &S, CodePtr OpPC) {
+  const auto Val = S.Stk.pop<Boolean>();
+
+  if (Val)
+    return true;
+
+  // Else, diagnose.
+  const SourceLocation &Loc = S.Current->getLocation(OpPC);
+  S.CCEDiag(Loc, diag::note_constexpr_assumption_failed);
+  return false;
+}
+
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 inline bool OffsetOf(InterpState &S, CodePtr OpPC, const OffsetOfExpr *E) {
   llvm::SmallVector<int64_t> ArrayIndices;

clang/lib/AST/Interp/Opcodes.td

@@ -736,6 +736,8 @@ def InvalidDeclRef : Opcode {
   let Args = [ArgDeclRef];
 }
 
+def Assume : Opcode;
+
 def ArrayDecay : Opcode;
 
 def CheckNonNullArg : Opcode {

clang/test/SemaCXX/cxx23-assume.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -std=c++23  -x c++ %s -verify
 // RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext,expected
+// RUN: %clang_cc1 -std=c++23  -x c++ %s -verify -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++20 -pedantic -x c++ %s -verify=ext,expected -fexperimental-new-constant-interpreter
 
 struct A{};
 struct B{ explicit operator bool() { return true; } };