rnk 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;
----------------
dblaikie wrote:
> 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.
If you use the MSVC ABI, we will assign unique identifiers to these two structs:
```
!9 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", scope:
!10, file: !7, line: 1, size: 64, flags: DIFlagTypePassByValue, elements: !11,
identifier: ".?AUt1@?A0xF964240C@@")
```
The `A0xF964240C` is set up here, and it is based on the source file path (the
hash will only be unique when source file paths are unique across the build):
https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/MicrosoftMangle.cpp#L464
```
// It's important to make the mangled names unique because, when CodeView
// debug info is in use, the debugger uses mangled type names to distinguish
// between otherwise identically named types in anonymous namespaces.
```
Maybe this should use a "is MSVC ABI" check instead, since that is what
controls whether the identifier will be unique, and the unique-ness is what
matters for LTO and PDBs.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D45438/new/
https://reviews.llvm.org/D45438
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