[PATCH] D45438: [CodeView] Enable debugging of captured variables within C++ lambdas

[David Blaikie via Phabricator via cfe-commits]

dblaikie added inline comments.


================
Comment at: lib/CodeGen/CGDebugInfo.cpp:812-814
+  // CodeView types with C++ mangling need a type identifier.
+  if (CGM.getCodeGenOpts().EmitCodeView)
+    return true;
----------------
Just came across this code today - it's /probably/ a problem for LTO. LLVM IR linking depends on the identifier to determine if two structs are the same for linkage purposes - so if an identifier is added for a non-linkage (local/not externally visible) type, LLVM will consider them to be the same even though they're unrelated:
```
namespace { struct t1 { int i; int j; }; }
t1 v1;
void *v3 = &v1;
```
```
namespace { struct t1 { int i; }; }
t1 v1;
void *v2 = &v1;
```
```
$ clang++-tot -emit-llvm -S a.cpp b.cpp -g -gcodeview  && ~/dev/llvm/build/default/bin/llvm-link -o ab.bc a.ll b.ll && ~/dev/llvm/build/default/bin/llvm-dis ab.bc && cat ab.ll | grep "\"t1\""
!8 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", scope: !9, file: !3, line: 1, size: 64, flags: DIFlagTypePassByValue, elements: !10, identifier: "_ZTSN12_GLOBAL__N_12t1E")
$ clang++-tot -emit-llvm -S a.cpp b.cpp -g && ~/dev/llvm/build/default/bin/llvm-link -o ab.bc a.ll b.ll && ~/dev/llvm/build/default/bin/llvm-dis ab.bc && cat ab.ll | grep "\"t1\""
!8 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", scope: !9, file: !3, line: 1, size: 64, flags: DIFlagTypePassByValue, elements: !10)
!21 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", scope: !9, file: !17, line: 1, size: 32, flags: DIFlagTypePassByValue, elements: !22)
```

So in linking, now both `a.cpp` and `b.cpp` refer to a single `t1` type (in this case, it looks like the first one - the 64 bit wide one).

If CodeView actually can't represent these two distinct types with the same name in the same object file, so be it? But this looks like it's likely to cause problems for consumers/users.


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D45438/new/

https://reviews.llvm.org/D45438