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Microsoft allows the support of ‘constexpr’ with ‘__declspec(dllimport) starting
from VS2017 15.u update 4. See Constexpr doesn't support __declspec(dllimport)
in VS2017 15.8 - Visual Studio Feedback.
Let’s consider this example.
lib.cpp:
__declspec(dllexport) int val=12;
__declspec(dllexport)
int next(int n)
{
return n + 1;
}
app1.cpp:
#include <iostream>
extern int __declspec(dllimport) next(int n);
int main () {
extern int __declspec(dllimport) val;
constexpr int& val_ref = val;
int i = next(val_ref);
std::cout << "i: " << i << std::endl;
return i;
}
Compiling this will give the expected output.
$ cl /LD lib.cpp
$ cl /EHsc app1.cpp /link lib.lib
$ ./app1.exe
i: 13
The Intel compiler has the same behavior than MSVC.
Clang compiles this test case with this error:
error: constexpr variable 'val_ref' must be initialized by a constant
expression
constexpr int& val_ref = val;
^ ~~~
1 error generated.
I think this should be fixed. This patch is doing that.
Now let’s look now at this example (dllimport at TU scope level):
app2.cpp:
#include <iostream>
extern int __declspec(dllimport) next(int n);
extern int __declspec(dllimport) val;
constexpr int& val_ref = val;
int main () {
int i = next(val_ref);
std::cout << "i: " << i << std::endl;
return i;
}
Compiling this will result into an unresolved symbol:
$ cl /EHsc app2.cpp /link lib.lib
app2.obj : error LNK2001: unresolved external symbol "int val" (?val@@3HA)
app2.exe : fatal error LNK1120: 1 unresolved externals
ICL and clang generate the same error.
These are the symols generated for all 3 compilers:
ICL:
003 00000000 UNDEF notype External | __imp_?next@@YAHH@Z
(__declspec(dllimport) int __cdecl next(int))
004 00000000 UNDEF notype External | __imp_?val@@3HA
(__declspec(dllimport) int val)
00C 00000000 SECT4 notype External | ?val_ref@@3AEAHEA (int &
val_ref)
00D 00000000 UNDEF notype External | ?val@@3HA (int val)
MSVC:
00A 00000000 UNDEF notype External | __imp_?next@@YAHH@Z
(__declspec(dllimport) int __cdecl next(int))
015 00000000 SECT6 notype Static | ?val_ref@@3AEAHEA (int &
val_ref)
The symbols generated by ICL seem to be what's expected. MSVC should be
generating an "__imp_?val" symbol.
Clang with the change in this patch is generating the expected symbols:
010 00000000 UNDEF notype External | __imp_?val@@3HA
(__declspec(dllimport) int val)
011 00000000 UNDEF notype External | __imp_?next@@YAHH@Z
(__declspec(dllimport) int __cdecl next(int))
012 00000000 SECT5 notype External | ?val_ref@@3AEAHEA (int &
val_ref)
013 00000000 UNDEF notype External | ?val@@3HA (int val)
Fixes issue https://github.com/llvm/llvm-project/issues/53182
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D137107
Files:
clang/lib/AST/ExprConstant.cpp
clang/test/CodeGenCXX/PR19955.cpp
clang/test/CodeGenCXX/dllimport.cpp
clang/test/SemaCXX/PR19955.cpp
clang/test/SemaCXX/dllimport-constexpr.cpp
Index: clang/test/SemaCXX/dllimport-constexpr.cpp
===================================================================
--- clang/test/SemaCXX/dllimport-constexpr.cpp
+++ clang/test/SemaCXX/dllimport-constexpr.cpp
@@ -40,7 +40,6 @@
// constexpr initialization doesn't work for dllimport things.
// expected-error@+1{{must be initialized by a constant expression}}
constexpr void (*constexpr_import_func)() = &imported_func;
-// expected-error@+1{{must be initialized by a constant expression}}
constexpr int *constexpr_import_int = &imported_int;
// expected-error@+1{{must be initialized by a constant expression}}
constexpr void (Foo::*constexpr_memptr)() = &Foo::imported_method;
Index: clang/test/SemaCXX/PR19955.cpp
===================================================================
--- clang/test/SemaCXX/PR19955.cpp
+++ clang/test/SemaCXX/PR19955.cpp
@@ -2,7 +2,7 @@
// RUN: %clang_cc1 -triple i686-mingw32 -verify -std=c++11 %s
extern int __attribute__((dllimport)) var;
-constexpr int *varp = &var; // expected-error {{must be initialized by a
constant expression}}
+constexpr int *varp = &var;
extern __attribute__((dllimport)) void fun();
constexpr void (*funp)(void) = &fun; // expected-error {{must be initialized
by a constant expression}}
Index: clang/test/CodeGenCXX/dllimport.cpp
===================================================================
--- clang/test/CodeGenCXX/dllimport.cpp
+++ clang/test/CodeGenCXX/dllimport.cpp
@@ -97,9 +97,9 @@
};
USE(inlineStaticLocalsFunc);
-// The address of a dllimport global cannot be used in constant initialization.
-// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal global
[1 x ptr] zeroinitializer
-// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal global [1 x ptr]
zeroinitializer
+// The address of a dllimport global can be used in constant initialization.
+// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal
constant [1 x ptr] [ptr @"?ExternGlobalDecl@@3HA"]
+// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal constant [1 x ptr] [ptr
@ExternGlobalDecl]
int *initializationFunc() {
static int *const arr[] = {&ExternGlobalDecl};
return arr[0];
Index: clang/test/CodeGenCXX/PR19955.cpp
===================================================================
--- clang/test/CodeGenCXX/PR19955.cpp
+++ clang/test/CodeGenCXX/PR19955.cpp
@@ -6,20 +6,15 @@
extern int *varp;
int *varp = &var;
-// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr null
-// X64-DAG: @"?varp@@3PEAHEA" = dso_local global ptr null
+// CHECK-DAG: @"?var@@3HA" = external dllimport global i32
+// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr @"?var@@3HA"
+
extern void (*funp)();
void (*funp)() = &fun;
// CHECK-DAG: @"?funp@@3P6AXXZA" = dso_local global ptr null
// X64-DAG: @"?funp@@3P6AXXZEA" = dso_local global ptr null
-// CHECK-LABEL: @"??__Evarp@@YAXXZ"
-// CHECK-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PAHA"
-
-// X64-LABEL: @"??__Evarp@@YAXXZ"
-// X64-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PEAHEA"
-
// CHECK-LABEL: @"??__Efunp@@YAXXZ"()
// CHECK-DAG: store ptr @"?fun@@YAXXZ", ptr @"?funp@@3P6AXXZA"
Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -2228,12 +2228,16 @@
// FIXME: Diagnostic!
return false;
- // A dllimport variable never acts like a constant, unless we're
- // evaluating a value for use only in name mangling.
- if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>())
+ if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>()) {
// FIXME: Diagnostic!
- return false;
-
+ if (Info.getLangOpts().C99)
+ // Address of dllimport variables can't be used for initialization in
+ // C language modes.
+ return false;
+ // A dllimport variable can be a constant address. This is a MS
+ // extension.
+ return true;
+ }
// In CUDA/HIP device compilation, only device side variables have
// constant addresses.
if (Info.getCtx().getLangOpts().CUDA &&
Index: clang/test/SemaCXX/dllimport-constexpr.cpp
===================================================================
--- clang/test/SemaCXX/dllimport-constexpr.cpp
+++ clang/test/SemaCXX/dllimport-constexpr.cpp
@@ -40,7 +40,6 @@
// constexpr initialization doesn't work for dllimport things.
// expected-error@+1{{must be initialized by a constant expression}}
constexpr void (*constexpr_import_func)() = &imported_func;
-// expected-error@+1{{must be initialized by a constant expression}}
constexpr int *constexpr_import_int = &imported_int;
// expected-error@+1{{must be initialized by a constant expression}}
constexpr void (Foo::*constexpr_memptr)() = &Foo::imported_method;
Index: clang/test/SemaCXX/PR19955.cpp
===================================================================
--- clang/test/SemaCXX/PR19955.cpp
+++ clang/test/SemaCXX/PR19955.cpp
@@ -2,7 +2,7 @@
// RUN: %clang_cc1 -triple i686-mingw32 -verify -std=c++11 %s
extern int __attribute__((dllimport)) var;
-constexpr int *varp = &var; // expected-error {{must be initialized by a constant expression}}
+constexpr int *varp = &var;
extern __attribute__((dllimport)) void fun();
constexpr void (*funp)(void) = &fun; // expected-error {{must be initialized by a constant expression}}
Index: clang/test/CodeGenCXX/dllimport.cpp
===================================================================
--- clang/test/CodeGenCXX/dllimport.cpp
+++ clang/test/CodeGenCXX/dllimport.cpp
@@ -97,9 +97,9 @@
};
USE(inlineStaticLocalsFunc);
-// The address of a dllimport global cannot be used in constant initialization.
-// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal global [1 x ptr] zeroinitializer
-// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal global [1 x ptr] zeroinitializer
+// The address of a dllimport global can be used in constant initialization.
+// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal constant [1 x ptr] [ptr @"?ExternGlobalDecl@@3HA"]
+// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal constant [1 x ptr] [ptr @ExternGlobalDecl]
int *initializationFunc() {
static int *const arr[] = {&ExternGlobalDecl};
return arr[0];
Index: clang/test/CodeGenCXX/PR19955.cpp
===================================================================
--- clang/test/CodeGenCXX/PR19955.cpp
+++ clang/test/CodeGenCXX/PR19955.cpp
@@ -6,20 +6,15 @@
extern int *varp;
int *varp = &var;
-// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr null
-// X64-DAG: @"?varp@@3PEAHEA" = dso_local global ptr null
+// CHECK-DAG: @"?var@@3HA" = external dllimport global i32
+// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr @"?var@@3HA"
+
extern void (*funp)();
void (*funp)() = &fun;
// CHECK-DAG: @"?funp@@3P6AXXZA" = dso_local global ptr null
// X64-DAG: @"?funp@@3P6AXXZEA" = dso_local global ptr null
-// CHECK-LABEL: @"??__Evarp@@YAXXZ"
-// CHECK-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PAHA"
-
-// X64-LABEL: @"??__Evarp@@YAXXZ"
-// X64-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PEAHEA"
-
// CHECK-LABEL: @"??__Efunp@@YAXXZ"()
// CHECK-DAG: store ptr @"?fun@@YAXXZ", ptr @"?funp@@3P6AXXZA"
Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -2228,12 +2228,16 @@
// FIXME: Diagnostic!
return false;
- // A dllimport variable never acts like a constant, unless we're
- // evaluating a value for use only in name mangling.
- if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>())
+ if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>()) {
// FIXME: Diagnostic!
- return false;
-
+ if (Info.getLangOpts().C99)
+ // Address of dllimport variables can't be used for initialization in
+ // C language modes.
+ return false;
+ // A dllimport variable can be a constant address. This is a MS
+ // extension.
+ return true;
+ }
// In CUDA/HIP device compilation, only device side variables have
// constant addresses.
if (Info.getCtx().getLangOpts().CUDA &&
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