[PATCH] D79719: [AIX] Implement AIX special alignment rule about double/long double

[Hubert Tong via Phabricator via cfe-commits]

hubert.reinterpretcast added inline comments.


================
Comment at: clang/lib/AST/ASTContext.cpp:2424
+      (T->isSpecificBuiltinType(BuiltinType::LongDouble) &&
+       Target->supportsAIXPowerAlignment()))
     // Don't increase the alignment if an alignment attribute was specified on a
----------------
hubert.reinterpretcast wrote:
> Xiangling_L wrote:
> > hubert.reinterpretcast wrote:
> > > Does `supportsAIXPowerAlignment` express the condition we want to check here? That might be true for an implementation operating with `mac68k` alignment rules.
> > Yeah, `supportsAIXPowerAlignment` cannot separate the preferred alignment of double, long double between `power/natural` and `mac68k` alignment rules. But I noticed that currently, AIX target on wyvern or XL don't support `mac68k` , so maybe we should leave further changes to the patch which is gonna implement `mac68k` alignment rules? The possible solution I am thinking is we can add checking if the decl has `AlignMac68kAttr` into query to separate things out.
> > 
> > Another thing is that once we start supporting mac68k alignment rule(if we will), should we also change the ABI align values as well? (e.g. for double, it should be 2 instead)
> If the "base state" is AIX `power` alignment for a platform, I suggest that the name be `defaultsToAIXPowerAlignment`.
This last question about the ABI align values is relevant to considerations for `natural` alignment support as well. More generally, the question is whether the "minimum alignment" of the type in a context subject to alternative alignment rules is altered to match said alignment rule. This is observable via the diagnostic associated with C++11 alignment specifiers.

The existing behaviour of `mac68k` alignment suggests that the "minimum alignment" is context-free.

```
#pragma options align=mac68k
struct Q {
  double x alignas(2);  // expected-error {{less than minimum alignment}}
};
#pragma options align=reset
```

Compiler Explorer link: https://godbolt.org/z/9NM5_-


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D79719/new/

https://reviews.llvm.org/D79719