[clang] [Clang][CodeGen] Bail out on constexpr unknown values in ConstantEmitter (PR #127525)

[Yingwei Zheng via cfe-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/127525

Close https://github.com/llvm/llvm-project/issues/127475

>From d931667e3b7e18f8ba6f054405ffc5fe2fac36b4 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Tue, 18 Feb 2025 01:26:26 +0800
Subject: [PATCH] [Clang][CodeGen] Bail out on constexpr unknown values in
 ConstantEmitter

---
 clang/lib/CodeGen/CGExprConstant.cpp    |  6 ++++--
 clang/test/CodeGenCXX/cxx23-p2280r4.cpp | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CodeGenCXX/cxx23-p2280r4.cpp

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index ee5874b26f534..075f3f435ad74 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1883,8 +1883,10 @@ 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())
-    return tryEmitPrivateForMemory(*value, destType);
+  if (APValue *value = D.evaluateValue()) {
+    if (!value->allowConstexprUnknown())
+      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..bb9a90ba18ece
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx23-p2280r4.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++23 %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;
+}