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

[Xiangling Liao via Phabricator via cfe-commits]

Xiangling_L marked 27 inline comments as done.
Xiangling_L added inline comments.


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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
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hubert.reinterpretcast wrote:
> 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_-
Thank you for your explanation.


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Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1885
+              Context.getBaseElementType(CTy->getElementType())
+                  ->getAs<BuiltinType>())
+        if (BTy->getKind() == BuiltinType::Double ||
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hubert.reinterpretcast wrote:
> I believe `castAs` should be expected to succeed here.
`castAs` is not declared in current context, do we really want to use it by introducing one more header?


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Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1898
+    } else if (const RecordType *RT = D->getType()
+                                          ->getBaseElementTypeUnsafe()
+                                          ->getAs<RecordType>()) {
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hubert.reinterpretcast wrote:
> Is there a reason to use `getBaseElementTypeUnsafe` for this case and `Context.getBaseElementType` for the other ones? Also, it would make sense to factor out the array-type considerations once at the top of the if-else chain instead of doing so in each alternative.
Sorry, I didn't pay attention to the different versions of `getBaseElementType` functions here and I believe this part of code came from our old compiler's changesets. My understanding would be since type qualifiers are not very meaningful in our case and `getBaseElementTypeUnsafe()` is more efficient than `getBaseElementType()`, we can use `getBaseElementTypeUnsafe()` all over the place instead.


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

https://reviews.llvm.org/D79719