[PATCH] D117616: GCC ABI Compatibility: Preserve alignment of non-pod members in packed structs

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1891
+  llvm::Triple Target = Context.getTargetInfo().getTriple();
+  bool FieldPacked = (Packed && (!FieldClass || FieldClass->isPOD() ||
+                                 Context.getLangOpts().getClangABICompat() <=
----------------
dblaikie wrote:
> rsmith wrote:
> > dblaikie wrote:
> > > rsmith wrote:
> > > > `isPOD` is C++ standard specific, and our ABI rule really shouldn't be. Does GCC use the C++98 rules here, the C++11 rules, or something else? (Hopefully the GCC behavior doesn't change between `-std=c++98` and `-std=c++11`!)
> > > > 
> > > > From a quick test, it looks like GCC doesn't pack fields whose types are classes with base classes, even if they're trivially-copyable and standard-layout, suggesting that it's probably using the C++98 POD rules.
> > > I /think/ `CXXRecordDecl::isPOD` doesn't use a language-varying definition, according to its documentation at least:
> > > https://github.com/llvm/llvm-project/blob/1e3a02162db20264e9615b1346420c8d199cb347/clang/include/clang/AST/DeclCXX.h#L1124
> > > & the code that sets the field that the accessor returns looks, to me at least, consistent with that claim: https://github.com/llvm/llvm-project/blob/1e3a02162db20264e9615b1346420c8d199cb347/clang/lib/AST/DeclCXX.cpp#L983
> > > 
> > > But if there's another way I should spell this to make it more clear/correct, more test cases to add to show the difference between these definitions - I'm open to that... 
> > Ah, right you are, I was thinking of `QualType::isPODType`, which does depend on the language mode: https://github.com/llvm/llvm-project/blob/1e3a02162db20264e9615b1346420c8d199cb347/clang/lib/AST/Type.cpp#L2350
> > 
> > No action necessary here, I think.
> Cool - reckon it's worth renaming it to be more clear about which POD it represents? `IsClassicPOD`, `isRetroPOD`, `isCXX03POD`?
If anything I think it might make more sense to remove the language-sensitive version and always use POD to mean the C++03 thing. I think the only thing that should want standard-layout + trivially-copyable should be the `__is_pod` trait (and maybe some target-specific ABI rules?). The very notion of "POD" as a shorthand for "standard-layout and trivially-copyable" was deprecated in C++20.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D117616/new/

https://reviews.llvm.org/D117616