[clang] [C++20] [Modules] Don't mark variables from other modules as constant if its initializer is not constant (PR #93530)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

<details>
<summary>Changes</summary>

Close https://github.com/llvm/llvm-project/issues/93497

The root cause of the problem is, we mark the variable from other modules as constnant in LLVM incorrectly. This patch fixes this problem and only mark the variable from other modules as constant if its initializer is const.

---
Full diff: https://github.com/llvm/llvm-project/pull/93530.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenModule.cpp (+2) 
- (added) clang/test/Modules/pr93497.cppm (+81) 


``````````diff
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index e4774a587707a..5596a6708e4a1 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -5492,6 +5492,8 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D,
 
   // If it is safe to mark the global 'constant', do so now.
   GV->setConstant(!NeedsGlobalCtor && !NeedsGlobalDtor &&
+                  (!IsDefinitionAvailableExternally ||
+                   D->hasConstantInitialization()) &&
                   D->getType().isConstantStorage(getContext(), true, true));
 
   // If it is in a read-only section, mark it 'constant'.
diff --git a/clang/test/Modules/pr93497.cppm b/clang/test/Modules/pr93497.cppm
new file mode 100644
index 0000000000000..dc0d8eb462e27
--- /dev/null
+++ b/clang/test/Modules/pr93497.cppm
@@ -0,0 +1,81 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 %t/mod.cppm \
+// RUN:     -emit-module-interface -o %t/mod.pcm
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 %t/use.cpp \
+// RUN:     -fmodule-file=mod=%t/mod.pcm -emit-llvm \
+// RUN:     -o - | FileCheck %t/use.cpp
+
+//--- mod.cppm
+export module mod;
+
+export struct Thing {
+    static const Thing One;
+    explicit Thing(int raw) :raw(raw) { }
+    int raw;
+};
+
+const Thing Thing::One = Thing(1);
+
+export struct C {
+    int value;
+};
+export const C ConstantValue = {1};
+
+export const C *ConstantPtr = &ConstantValue;
+
+C NonConstantValue = {1};
+export const C &ConstantRef = NonConstantValue;
+
+//--- use.cpp
+import mod;
+
+int consume(int);
+int consumeC(C);
+
+extern "C" __attribute__((noinline)) inline int unneeded() {
+    return consume(43);
+}
+
+extern "C" __attribute__((noinline)) inline int needed() {
+    return consume(43);
+}
+
+int use() {
+    Thing t1 = Thing::One;
+    return consume(t1.raw);
+}
+
+int use2() {
+    if (ConstantValue.value)
+        return consumeC(ConstantValue);
+    return unneeded();
+}
+
+int use3() {
+    if (ConstantPtr->value)
+        return consumeC(*ConstantPtr);
+    return needed();
+}
+
+int use4() {
+    if (ConstantRef.value)
+        return consumeC(ConstantRef);
+    return needed();
+}
+
+// CHECK-NOT: @_ZNW3mod5Thing3OneE = {{.*}}constant
+// CHECK: @_ZW3mod13ConstantValue ={{.*}}available_externally{{.*}} constant 
+
+// Check that the middle end can optimize the program by the constant information.
+// CHECK-NOT: @unneeded(
+
+// Check that the use of ConstantPtr won't get optimized incorrectly.
+// CHECK-LABEL: @_Z4use3v(
+// CHECK: @needed(
+
+// Check that the use of ConstantRef won't get optimized incorrectly.
+// CHECK-LABEL: @_Z4use4v(
+// CHECK: @needed(

``````````

</details>


https://github.com/llvm/llvm-project/pull/93530