[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/RecordLayoutBuilder.cpp:1254
+  // space or zero-extent array.
+  if (DefaultsToAIXPowerAlignment && !getDataSize().isZero()) {
+    PreferredBaseAlign = BaseAlign;
----------------
This needs to check `HandledFirstNonOverlappingEmptyField`:
```
struct A {
  char x[0];
};
struct B {
  double d;
};
struct C : A, B { char x; } c;
```


================
Comment at: clang/lib/AST/RecordLayoutBuilder.cpp:1255
+  if (DefaultsToAIXPowerAlignment && !getDataSize().isZero()) {
+    PreferredBaseAlign = BaseAlign;
+  }
----------------
Note that `PreferredBaseAlign` is only truly meaningful after this line, and related fields, such as `UnpackedPreferredBaseAlign`, require similar treatment. With `UnpackedAlignTo` being dependent on a meaningful value of `UnpackedPreferredBaseAlign`, it needs an update here as well.

Consider the following source. Noting that `C` and `D` are identical except for the packed attribute and yield identical layout properties despite that difference, we should be getting a `-Wpacked` warning with `-Wpacked` (but it is missing).
```
struct A {
  double d;
};

struct B {
  char x[8];
};

struct [[gnu::packed]] C : B, A { // expected-warning {{packed attribute is unnecessary}}
  char x alignas(4)[8];
} c;

struct D : B, A {
  char x alignas(4)[8];
} d;
```
```
*** Dumping AST Record Layout
         0 | struct C
         0 |   struct B (base)
         0 |     char [8] x
         8 |   struct A (base)
         8 |     double d
        16 |   char [8] x
           | [sizeof=24, dsize=24, align=4, preferredalign=4,
           |  nvsize=24, nvalign=4, preferrednvalign=4]
```
```
*** Dumping AST Record Layout
         0 | struct D
         0 |   struct B (base)
         0 |     char [8] x
         8 |   struct A (base)
         8 |     double d
        16 |   char [8] x
           | [sizeof=24, dsize=24, align=4, preferredalign=4,
           |  nvsize=24, nvalign=4, preferrednvalign=4]
```



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79719/new/

https://reviews.llvm.org/D79719