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

[David Blaikie via Phabricator via cfe-commits]

dblaikie added inline comments.


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Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1891
+  llvm::Triple Target = Context.getTargetInfo().getTriple();
+  bool FieldPacked = (Packed && (!FieldClass || FieldClass->isPOD() ||
+                                 Context.getLangOpts().getClangABICompat() <=
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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... 


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Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1895
+                                 Target.isPS4() || Target.isOSDarwin())) ||
+                     D->hasAttr<PackedAttr>();
 
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rsmith wrote:
> Would it make sense to warn here if `Packed && !FieldPacked`, like GCC does?
Probably? Happy to implement that in a follow-up.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D117616