[clang] [Clang] Treat constexpr-unknown value as invalid in `EvaluateAsInitializer` (PR #128409)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

It is an alternative to https://github.com/llvm/llvm-project/pull/127525.
Close https://github.com/llvm/llvm-project/issues/127475.


---
Full diff: https://github.com/llvm/llvm-project/pull/128409.diff


3 Files Affected:

- (modified) clang/lib/AST/ExprConstant.cpp (+12-2) 
- (modified) clang/lib/CodeGen/CGExprConstant.cpp (+5-1) 
- (added) clang/test/CodeGenCXX/cxx23-p2280r4.cpp (+15) 


``````````diff
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6ccb6e23f8d2f..42801dc4e18fa 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -3628,8 +3628,6 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   if (AllowConstexprUnknown) {
     if (!Result)
       Result = &Info.CurrentCall->createConstexprUnknownAPValues(VD, Base);
-    else
-      Result->setConstexprUnknown();
   }
   return true;
 }
@@ -17000,6 +16998,18 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
 
     if (!Info.discardCleanups())
       llvm_unreachable("Unhandled cleanup; missing full expression marker?");
+
+    if (Value.allowConstexprUnknown()) {
+      assert(Value.isLValue() && "Expected an lvalue");
+      auto Base = Value.getLValueBase();
+      const auto *NewVD = Base.dyn_cast<const ValueDecl *>();
+      if (!NewVD)
+        NewVD = VD;
+      Info.FFDiag(getExprLoc(), diag::note_constexpr_var_init_non_constant, 1)
+          << NewVD;
+      NoteLValueLocation(Info, Base);
+      return false;
+    }
   }
 
   return CheckConstantExpression(Info, DeclLoc, DeclTy, Value,
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index ee5874b26f534..0295f703b9061 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1883,8 +1883,12 @@ llvm::Constant *ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &D) {
 
   // Try to emit the initializer.  Note that this can allow some things that
   // are not allowed by tryEmitPrivateForMemory alone.
-  if (APValue *value = D.evaluateValue())
+  // Bail out on constexpr-unknown values since they are invalid in CodeGen.
+  if (APValue *value = D.evaluateValue()) {
+    assert(!value->allowConstexprUnknown() &&
+           "Constexpr unknown values are not allowed in CodeGen");
     return tryEmitPrivateForMemory(*value, destType);
+  }
 
   return nullptr;
 }
diff --git a/clang/test/CodeGenCXX/cxx23-p2280r4.cpp b/clang/test/CodeGenCXX/cxx23-p2280r4.cpp
new file mode 100644
index 0000000000000..d5409be451df0
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx23-p2280r4.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++23 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++17 %s -emit-llvm -o - | FileCheck %s
+
+extern int& s;
+
+// CHECK: @_Z4testv()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[I:%.*]] = alloca ptr, align {{.*}}
+// CHECK-NEXT: [[X:%.*]] = load ptr, ptr @s, align {{.*}}
+// CHECK-NEXT: store ptr [[X]], ptr [[I]], align {{.*}}
+int& test() {
+  auto &i = s;
+  return i;
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/128409