Remove -Wpacked false positive for non-pod types where the layout isn't directly changed

[dblaikie]

Summary

The packed attribute can still be useful in this case if the struct is
then placed inside another packed struct - the non-pod element type's
packed attribute declares that it's OK to misalign this element inside
the packed structure. (otherwise the non-pod element is not packed/its
alignment is preserved, as per D117616/2771233)

Fixes PR62353

[dblaikie]

ping

[aaron.ballman]

Precommit CI found a valid issue (at least on Debian, the Windows failure appears to be unrelated but it's really hard to tell from that output). Also, this should have a release note, right?

clang/lib/AST/RecordLayoutBuilder.cpp

2203–2205

You probably should update the comment to explain the new changes.

[dblaikie]

Release note
Restrict change to Clang ABI 16 and above (& test that condition)
Fix AIX test

[dblaikie]

Describe condition in comment

[dblaikie]

In D149182#4312202, @aaron.ballman wrote:

Precommit CI found a valid issue (at least on Debian, the Windows failure appears to be unrelated but it's really hard to tell from that output).

Hmm, right ,yeah, valid test failure - fixed.

Also, this should have a release note, right?

Happy to add one. (done)

[aaron.ballman]

LGTM aside from a nit with the release notes. Thank you for the fix!

clang/docs/ReleaseNotes.rst

109–113 ↗

(On Diff #519247)

I think this should move down to Improvements to Clang's diagnostics instead of adding a new category for it.

[dblaikie]

clang/docs/ReleaseNotes.rst

109–113 ↗

(On Diff #519247)

Ah, hadn't spotted that category - thanks!

[dblaikie]

clang/docs/ReleaseNotes.rst

109–113 ↗

(On Diff #519247)

Failed to include the fix there in the initial commit, but got it in 793c5b1

clang/lib/AST/RecordLayoutBuilder.cpp

if (getSizeInBits() > UnpaddedSize) {
         InBits = false;
       }
       Diag(RD->getLocation(), diag::warn_padded_struct_size)
           << Context.getTypeDeclType(RD)
           << PadSize
           << (InBits ? 1 : 0); // (byte|bit)
     }
 
+    const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD);
+
     // Warn if we packed it unnecessarily, when the unpacked alignment is not
     // greater than the one after packing, the size in bits doesn't change and
     // the offset of each field is identical.
     if (Packed && UnpackedAlignment <= Alignment &&
-        UnpackedSizeInBits == getSizeInBits() && !HasPackedField)
+        UnpackedSizeInBits == getSizeInBits() && !HasPackedField &&
+        (!CXXRD || CXXRD->isPOD()))
       Diag(D->getLocation(), diag::warn_unnecessary_packed)
           << Context.getTypeDeclType(RD);
   }
 }
 
 void ItaniumRecordLayoutBuilder::UpdateAlignment(
     CharUnits NewAlignment, CharUnits UnpackedNewAlignment,
     CharUnits PreferredNewAlignment) {

clang/test/CodeGenCXX/warn-padded-packed.cpp

protected:
   char c;
 };
 struct S29 {
   S29_non_pod_align_1 p1;
   int i;
 } __attribute__((packed)); // no warning
 static_assert(alignof(S29) == 1, "");
 
+struct S30 {
+protected:
+ short s;
+} __attribute__((packed)); // no warning
+struct S30_use {
+  char c;
+  S30 u;
+} __attribute__((packed));
+static_assert(sizeof(S30_use) == 3, "");
+
 // The warnings are emitted when the layout of the structs is computed, so we have to use them.
 void f(S1*, S2*, S3*, S4*, S5*, S6*, S7*, S8*, S9*, S10*, S11*, S12*, S13*,
        S14*, S15*, S16*, S17*, S18*, S19*, S20*, S21*, S22*, S23*, S24*, S25*,
        S26*, S27*, S28*, S29*){}