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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

After https://github.com/llvm/llvm-project/pull/95474, unknown references are valid in constant expressions. However, constexpr-unknown values are not allowed in CodeGen. This patch bails out on constexpr-unknown values to ensure the load of reference is emitted.

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


---
Full diff: https://github.com/llvm/llvm-project/pull/127525.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGExprConstant.cpp (+4-2) 
- (added) clang/test/CodeGenCXX/cxx23-p2280r4.cpp (+15) 


``````````diff
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..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/127525