[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-07-06 Thread Fangrui Song via Phabricator via cfe-commits 
This revision was automatically updated to reflect the committed changes.
Closed by commit rGa9f5119a78aa: [DebugInfo] Enable debug info emission for 
extern variables in C++ (authored by chiyuze, committed by MaskRay).

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Files:
  clang/lib/Sema/SemaDecl.cpp
  clang/test/CodeGen/debug-info-extern-basic.cpp


Index: clang/test/CodeGen/debug-info-extern-basic.cpp
===
--- /dev/null
+++ clang/test/CodeGen/debug-info-extern-basic.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -x c++ -debug-info-kind=limited -triple bpf-linux-gnu 
-emit-llvm %s -o - | FileCheck %s
+
+namespace foo {
+
+template 
+struct S {
+  T x;
+};
+
+extern S s;
+
+int test(void) {
+  return s.x;
+}
+
+}  // namespace foo
+
+// CHECK: distinct !DIGlobalVariable(name: "s", scope: 
![[NAMESPACE:[0-9]+]],{{.*}} type: ![[STRUCT_TYPE:[0-9]+]], isLocal: false, 
isDefinition: false)
+// CHECK: ![[NAMESPACE]] = !DINamespace(name: "foo", scope: null)
+// CHECK: ![[STRUCT_TYPE]] = distinct !DICompositeType(tag: 
DW_TAG_structure_type, name: "S",{{.*}}size: 8, flags: 
DIFlagTypePassByValue, elements: ![[ELEMENT_TYPE:[0-9]+]], templateParams: 
![[TEMPLATE_TYPE:[0-9]+]], identifier: "_ZTSN3foo1SIcEE")
+// CHECK: ![[ELEMENT_TYPE]] = !{![[ELEMENT_TYPE:[0-9]+]]}
+// CHECK: ![[ELEMENT_TYPE]] = !DIDerivedType(tag: DW_TAG_member, name: 
"x",{{.*}} baseType: ![[BASE_TYPE:[0-9]+]], size: 8)
+// CHECK: ![[BASE_TYPE]] = !DIBasicType(name: "char", size: 8, encoding: 
DW_ATE_signed_char)
+// CHECK: ![[TEMPLATE_TYPE]] = !{![[TEMPLATE_TYPE:[0-9]+]]}
+// CHECK: ![[TEMPLATE_TYPE]] = !DITemplateTypeParameter(name: "T", type: 
![[BASE_TYPE]])
Index: clang/lib/Sema/SemaDecl.cpp
===
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13780,7 +13780,7 @@
   }
 
   if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
-  !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
+  !Var->isInvalidDecl())
 ExternalDeclarations.push_back(Var);
 
   return;


Index: clang/test/CodeGen/debug-info-extern-basic.cpp
===
--- /dev/null
+++ clang/test/CodeGen/debug-info-extern-basic.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -x c++ -debug-info-kind=limited -triple bpf-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+namespace foo {
+
+template 
+struct S {
+  T x;
+};
+
+extern S s;
+
+int test(void) {
+  return s.x;
+}
+
+}  // namespace foo
+
+// CHECK: distinct !DIGlobalVariable(name: "s", scope: ![[NAMESPACE:[0-9]+]],{{.*}} type: ![[STRUCT_TYPE:[0-9]+]], isLocal: false, isDefinition: false)
+// CHECK: ![[NAMESPACE]] = !DINamespace(name: "foo", scope: null)
+// CHECK: ![[STRUCT_TYPE]] = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S",{{.*}}size: 8, flags: DIFlagTypePassByValue, elements: ![[ELEMENT_TYPE:[0-9]+]], templateParams: ![[TEMPLATE_TYPE:[0-9]+]], identifier: "_ZTSN3foo1SIcEE")
+// CHECK: ![[ELEMENT_TYPE]] = !{![[ELEMENT_TYPE:[0-9]+]]}
+// CHECK: ![[ELEMENT_TYPE]] = !DIDerivedType(tag: DW_TAG_member, name: "x",{{.*}} baseType: ![[BASE_TYPE:[0-9]+]], size: 8)
+// CHECK: ![[BASE_TYPE]] = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+// CHECK: ![[TEMPLATE_TYPE]] = !{![[TEMPLATE_TYPE:[0-9]+]]}
+// CHECK: ![[TEMPLATE_TYPE]] = !DITemplateTypeParameter(name: "T", type: ![[BASE_TYPE]])
Index: clang/lib/Sema/SemaDecl.cpp
===
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13780,7 +13780,7 @@
   }
 
   if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
-  !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
+  !Var->isInvalidDecl())
 ExternalDeclarations.push_back(Var);
 
   return;
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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-29 Thread Yuze Chi via Phabricator via cfe-commits 
chiyuze added a comment.

Thank you for accepting this patch!

In D153898#4457432 , @MaskRay wrote:

> I wonder whether you have ready-to-use instructions to test this for folks 
> who are not familiar with Linux kernel/eBPF. (I happened to start to read 
> eBPF one week ago, but I guess it would take some time for me to be more 
> familiar with it, even if I contributed some stuff back in 2020)



In D153898#4457436 , @MaskRay wrote:

>> This patch is required so that we can still use kconfig in such BPF programs 
>> compiled from C++.
>
> Assuming that you mean 
> https://www.kernel.org/doc/html/next/kbuild/kconfig-language.html#kconfig-language
>  , how is Kconfig (a DSL used in the build system)  relevant here?

The kconfig DSL allows extern variables to be filled in by libbpf before being 
loaded into the kernel. 
https://nakryiko.com/posts/bpf-core-reference-guide/#kconfig-extern-variables 
has more detailed explanations. In prod, we use this as a mechanism to 
flag-protect BPF features. For example, if we have `test.c` with the following 
content:

  // helpers
  #define __section(x) __attribute__((section(x)))
  #define __kconfig __attribute__((section(".kconfig")))
  #define __weak __attribute__((weak))
  // end of helpers
  
  extern const int CONFIG_ENABLE_FOO __kconfig __weak;
  __section("tc")
  int test(void* ctx) {
if (CONFIG_ENABLE_FOO > 1000) {
  return 1;
}
return 0;
  }

libbpf will fill in 0 as CONFIG_ENABLE_FOO by default, and allow us to override 
it with other values using the kconfig DSL. However, without this patch, such 
BPF programs are only functional if compiled as C:

`clang -g -O2 -target bpf -c test.c -o test.c.o; sudo bpftool prog load 
test.c.o /sys/fs/bpf/test_c` returns OK.

If we compile the same source code as C++, it won't load:

`clang -g -O2 -target bpf -x c++ -c test.c -o test.cc.o; sudo bpftool prog load 
test.cc.o /sys/fs/bpf/test_cc` gives

  libbpf: failed to find BTF for extern 'CONFIG_ENABLE_FOO': -2
  Error: failed to open object file

The reason is that the generated object is missing BTF for the extern variable:

`bpftool btf dump file test.c.o` gives

  [1] PTR '(anon)' type_id=0
  [2] FUNC_PROTO '(anon)' ret_type_id=3 vlen=1
  'ctx' type_id=1
  [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
  [4] FUNC 'test' type_id=2 linkage=global
  [5] CONST '(anon)' type_id=3
  [6] VAR 'CONFIG_ENABLE_FOO' type_id=5, linkage=extern
  [7] DATASEC '.kconfig' size=0 vlen=1
  type_id=6 offset=0 size=4 (VAR 'CONFIG_ENABLE_FOO')

whereas `bpftool btf dump file test.cc.o` gives

  [1] PTR '(anon)' type_id=0
  [2] FUNC_PROTO '(anon)' ret_type_id=3 vlen=1
  'ctx' type_id=1
  [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
  [4] FUNC 'test' type_id=2 linkage=global



In D153898#4457436 , @MaskRay wrote:

> Clang BPF supports `-mcpu=v[123]`. v1 is for a very old kernel. 
> https://pchaigno.github.io/bpf/2021/10/20/ebpf-instruction-sets.html 
> I think there are some users so we cannot remove the support. Is it happy 
> with the new behavior?

Yes. This patch does not change existing behaviors, including backend 
behaviors, upon which users already depend, since it only changes debug info 
for BPF compiled in C++. BPF & C++ is a combination that was never officially 
supported (and does not fully work).

In D153898#4457810 , @dblaikie wrote:

> (though I'd still ask a bit about the issue of declaration V definition - is 
> BPF just always "standalone" debug info generally? (is there a problem with 
> the definition coming from another translation unit with debug info?)?)

I'm not aware of such problems, but my experience may be limited to the use 
cases owned by our team. AFAIK, clang always compiles BPF as individual 
translation units. "Linking" is implemented by libbpf 
(https://lwn.net/Articles/848997/), which is not used in BPF programs owned by 
our team today (those BPF programs predate libbpf's linking support). Our plan 
is to migrate existing use cases of BPF to C++ first, before trying to support 
more general use cases (e.g. libbpf linking). This patch is our first clang 
patch towards compiling BPF from C++; we'll send more if we found them 
necessary in the future :)

In D153898#4460475 , @yonghong-song 
wrote:

> I am okay with this change. Once you used this patch and implemented the 
> mechanism inside Google. Could you send a follow-up summary on what the 
> implementation looks like in Google for the thread:
>
>   
> https://lore.kernel.org/bpf/cakh8qbt4xqbupxefqpk5ayu1rr0-h-vcjzs_0bu-987gl4w...@mail.gmail.com/
>
> This will give people a sense why this is useful/better than other 
> alternatives (like macro based approach).

Sure thing, will do!


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-29 Thread Yonghong Song via Phabricator via cfe-commits 
yonghong-song accepted this revision.
yonghong-song added a comment.

I am okay with this change. Once you used this patch and implemented the 
mechanism inside Google. Could you send a follow-up summary on what the 
implementation looks like in Google? This will give people a sense why this is 
useful/better than other alternatives (like macro based approach).


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-28 Thread David Blaikie via Phabricator via cfe-commits 
dblaikie accepted this revision.
dblaikie added a comment.

In D153898#4457263 , @chiyuze wrote:

> Regarding cost, only BPF target triggers this debug info generation for 
> extern variables.

Ah, OK - if that's what you need for BPF and it doesn't affect anything else - 
have at.

(though I'd still ask a bit about the issue of declaration V definition - is 
BPF just always "standalone" debug info generally? (is there a problem with the 
definition coming from another translation unit with debug info?)?)

> There is no impact on x86 builds of clang:

Oh, OK - no worries then.


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-28 Thread Fangrui Song via Phabricator via cfe-commits 
MaskRay added a comment.

> This patch is required so that we can still use kconfig in such BPF programs 
> compiled from C++.

Assuming that you mean 
https://www.kernel.org/doc/html/next/kbuild/kconfig-language.html#kconfig-language
 , how is Kconfig relevant here?

Clang BPF supports `-mcpu=v[123]`. v1 is for a very old kernel. 
https://pchaigno.github.io/bpf/2021/10/20/ebpf-instruction-sets.html 
I think there are some users so we cannot remove the support. Is it happy with 
the new behavior?


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-28 Thread Fangrui Song via Phabricator via cfe-commits 
MaskRay added a comment.

I wonder whether you have ready-to-use instructions to test this for folks who 
are not familiar with Linux kernel/eBPF. (I happened to start to read eBPF one 
week ago, but I guess it would take some time for me to be more familiar with 
it, even if I contributed some stuff back in 2020)

  # /tmp/Rel is a llvm-project build directory.
  ninja -C /tmp/Rel clang lld llvm-{ar,nm,strings,objdump,objcopy,readelf,strip}
  
  # In a Linux kernel tree,
  PATH=/tmp/Rel/bin:$PATH make O=/tmp/linux/x86_64 LLVM=1 defconfig all -j 50
  
  qemu-system-x86_64 ...
  
  clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj -o bpf.o
  
  Run XXX command to load bpf.o into some tools, say, tc
  Run XXX command to inspect that the tool is happy with new Clang behavior


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-28 Thread Yuze Chi via Phabricator via cfe-commits 
chiyuze added a comment.

Regarding cost, only BPF target triggers this debug info generation for extern 
variables. There is no impact on x86 builds of clang:

Without patch:

  bloaty build/RelWithDebInfo/83d47ba15a1229a21aaca8a8d6a33e0e90aabfd4/bin/clang
  FILE SIZEVM SIZE
   --  -- 
74.7%  2.98Gi   0.0%   0.debug_info
 9.2%   377Mi   0.0%   0.debug_loclists
 5.2%   210Mi   0.0%   0.debug_str
 4.3%   177Mi   0.0%   0.debug_line
 2.0%  81.5Mi  53.7%  81.5Mi.text
 1.7%  68.0Mi   0.0%   0.debug_rnglists
 1.1%  43.2Mi  28.5%  43.2Mi.rodata
 0.5%  21.4Mi   0.0%   0.debug_abbrev
 0.5%  20.3Mi   0.0%   0.strtab
 0.2%  8.79Mi   5.8%  8.79Mi.eh_frame
 0.2%  6.73Mi   4.4%  6.73Mi.rela.dyn
 0.1%  4.86Mi   0.0%   0.symtab
 0.1%  4.09Mi   2.7%  4.09Mi.dynstr
 0.1%  3.60Mi   2.4%  3.60Mi.data.rel.ro
 0.1%  3.30Mi   0.0%   0.debug_aranges
 0.0%  1.19Mi   0.8%  1.19Mi.dynsym
 0.0%  1.09Mi   0.7%  1.09Mi.eh_frame_hdr
 0.0%   0   0.4%   611Ki.bss
 0.0%   449Ki   0.1%   127Ki[25 Others]
 0.0%   393Ki   0.3%   393Ki.gnu.hash
 0.0%   332Ki   0.2%   332Ki.data
   100.0%  3.99Gi 100.0%   151MiTOTAL

With patch:

  bloaty build/RelWithDebInfo/23bc4427ab49716ce2c24c81529d9c90953b3c54/bin/clang
  FILE SIZEVM SIZE
   --  -- 
74.7%  2.98Gi   0.0%   0.debug_info
 9.2%   377Mi   0.0%   0.debug_loclists
 5.2%   210Mi   0.0%   0.debug_str
 4.3%   177Mi   0.0%   0.debug_line
 2.0%  81.5Mi  53.7%  81.5Mi.text
 1.7%  68.0Mi   0.0%   0.debug_rnglists
 1.1%  43.2Mi  28.5%  43.2Mi.rodata
 0.5%  21.4Mi   0.0%   0.debug_abbrev
 0.5%  20.3Mi   0.0%   0.strtab
 0.2%  8.79Mi   5.8%  8.79Mi.eh_frame
 0.2%  6.73Mi   4.4%  6.73Mi.rela.dyn
 0.1%  4.86Mi   0.0%   0.symtab
 0.1%  4.09Mi   2.7%  4.09Mi.dynstr
 0.1%  3.60Mi   2.4%  3.60Mi.data.rel.ro
 0.1%  3.30Mi   0.0%   0.debug_aranges
 0.0%  1.19Mi   0.8%  1.19Mi.dynsym
 0.0%  1.09Mi   0.7%  1.09Mi.eh_frame_hdr
 0.0%   0   0.4%   611Ki.bss
 0.0%   449Ki   0.1%   127Ki[25 Others]
 0.0%   393Ki   0.3%   393Ki.gnu.hash
 0.0%   332Ki   0.2%   332Ki.data
   100.0%  3.99Gi 100.0%   151MiTOTAL

For BPF, this debug info is needed to generate proper BTF and use kconfig 
.
 This patch just enables it for C++, which makes it possible for us to leverage 
templates to deduplicate almost identical BPF code for IPv4 and IPv6.

https://reviews.llvm.org/D70696 has more details regarding why it is needed for 
BPF in general and 
https://lore.kernel.org/bpf/cakh8qbt4xqbupxefqpk5ayu1rr0-h-vcjzs_0bu-987gl4w...@mail.gmail.com/
 has more details regarding why we are trying to compile BPF from C++.


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-28 Thread David Blaikie via Phabricator via cfe-commits 
dblaikie added a comment.

Got some details on how much this costs? (eg: `bloaty` comparison for a clang 
bootstrap with/without this patch applied?)
Could you provide a quick summary of why this debug info is required? The 
definition should be provided in whatever translation unit defines the variable 
and you should be able to get debug info from there - if that translation unit 
is built without -g, then maybe this feature should be gated on 
`-fstandalone-debug` which is intended to express the intent to get debug info 
coverage when a translation unit may be the only one built with `-g`?


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-27 Thread Reid Kleckner via Phabricator via cfe-commits 
rnk accepted this revision.
rnk added subscribers: MaskRay, dblaikie.
rnk added a comment.
This revision is now accepted and ready to land.

This seems reasonable to me, but I want to get a second opinion from @dblaikie 
and @maskray for some debug info and BPF perspective.


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[PATCH] D153898: [DebugInfo] Enable debug info emission for extern variables in C++

2023-06-27 Thread Yuze Chi via Phabricator via cfe-commits 
chiyuze created this revision.
chiyuze added a reviewer: rnk.
Herald added a project: All.
chiyuze requested review of this revision.
Herald added a project: clang.

Debug info emission for extern variables in C++ was previously disabled
when the functionality was added in https://reviews.llvm.org/D71818 and
originally in https://reviews.llvm.org/D70696, because there was no use
case. We are enabling it now, as we start to deploy BPF programs
compiled from C++, leveraging C++ features like templates to reduce code
complexity. This patch is required so that we can still use kconfig in
such BPF programs compiled from C++.


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Files:
  clang/lib/Sema/SemaDecl.cpp
  clang/test/CodeGen/debug-info-extern-basic.cpp


Index: clang/test/CodeGen/debug-info-extern-basic.cpp
===
--- /dev/null
+++ clang/test/CodeGen/debug-info-extern-basic.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -x c++ -debug-info-kind=limited -triple bpf-linux-gnu 
-emit-llvm %s -o - | FileCheck %s
+
+namespace foo {
+
+template 
+struct S {
+  T x;
+};
+
+extern S s;
+
+int test(void) {
+  return s.x;
+}
+
+}  // namespace foo
+
+// CHECK: distinct !DIGlobalVariable(name: "s", scope: 
![[NAMESPACE:[0-9]+]],{{.*}} type: ![[STRUCT_TYPE:[0-9]+]], isLocal: false, 
isDefinition: false)
+// CHECK: ![[NAMESPACE]] = !DINamespace(name: "foo", scope: null)
+// CHECK: ![[STRUCT_TYPE]] = distinct !DICompositeType(tag: 
DW_TAG_structure_type, name: "S",{{.*}}size: 8, flags: 
DIFlagTypePassByValue, elements: ![[ELEMENT_TYPE:[0-9]+]], templateParams: 
![[TEMPLATE_TYPE:[0-9]+]], identifier: "_ZTSN3foo1SIcEE")
+// CHECK: ![[ELEMENT_TYPE]] = !{![[ELEMENT_TYPE:[0-9]+]]}
+// CHECK: ![[ELEMENT_TYPE]] = !DIDerivedType(tag: DW_TAG_member, name: 
"x",{{.*}} baseType: ![[BASE_TYPE:[0-9]+]], size: 8)
+// CHECK: ![[BASE_TYPE]] = !DIBasicType(name: "char", size: 8, encoding: 
DW_ATE_signed_char)
+// CHECK: ![[TEMPLATE_TYPE]] = !{![[TEMPLATE_TYPE:[0-9]+]]}
+// CHECK: ![[TEMPLATE_TYPE]] = !DITemplateTypeParameter(name: "T", type: 
![[BASE_TYPE]])
Index: clang/lib/Sema/SemaDecl.cpp
===
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13773,7 +13773,7 @@
   }
 
   if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
-  !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
+  !Var->isInvalidDecl())
 ExternalDeclarations.push_back(Var);
 
   return;


Index: clang/test/CodeGen/debug-info-extern-basic.cpp
===
--- /dev/null
+++ clang/test/CodeGen/debug-info-extern-basic.cpp
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -x c++ -debug-info-kind=limited -triple bpf-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+namespace foo {
+
+template 
+struct S {
+  T x;
+};
+
+extern S s;
+
+int test(void) {
+  return s.x;
+}
+
+}  // namespace foo
+
+// CHECK: distinct !DIGlobalVariable(name: "s", scope: ![[NAMESPACE:[0-9]+]],{{.*}} type: ![[STRUCT_TYPE:[0-9]+]], isLocal: false, isDefinition: false)
+// CHECK: ![[NAMESPACE]] = !DINamespace(name: "foo", scope: null)
+// CHECK: ![[STRUCT_TYPE]] = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S",{{.*}}size: 8, flags: DIFlagTypePassByValue, elements: ![[ELEMENT_TYPE:[0-9]+]], templateParams: ![[TEMPLATE_TYPE:[0-9]+]], identifier: "_ZTSN3foo1SIcEE")
+// CHECK: ![[ELEMENT_TYPE]] = !{![[ELEMENT_TYPE:[0-9]+]]}
+// CHECK: ![[ELEMENT_TYPE]] = !DIDerivedType(tag: DW_TAG_member, name: "x",{{.*}} baseType: ![[BASE_TYPE:[0-9]+]], size: 8)
+// CHECK: ![[BASE_TYPE]] = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+// CHECK: ![[TEMPLATE_TYPE]] = !{![[TEMPLATE_TYPE:[0-9]+]]}
+// CHECK: ![[TEMPLATE_TYPE]] = !DITemplateTypeParameter(name: "T", type: ![[BASE_TYPE]])
Index: clang/lib/Sema/SemaDecl.cpp
===
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13773,7 +13773,7 @@
   }
 
   if (Context.getTargetInfo().allowDebugInfoForExternalRef() &&
-  !Var->isInvalidDecl() && !getLangOpts().CPlusPlus)
+  !Var->isInvalidDecl())
 ExternalDeclarations.push_back(Var);
 
   return;
___
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