[clang codegen] Fix alignment of "Address" for incomplete array pointer.

[efriedma]

Summary

The code was assuming all incomplete types don't have meaningful alignment, but that clearly isn't true.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45710

[rsmith]

clang/lib/CodeGen/CodeGenFunction.cpp

176

I don't know if it matters in practice, but this is still wrong. An incomplete type can have a known alignment, for a case like struct alignas(32) S;. Perhaps we should remove this test entirely and call getTypeAlignIfKnown instead of getTypeAlign[InChars] below.

[efriedma]

clang/lib/CodeGen/CodeGenFunction.cpp

176

I can't think of any way to observe the alignment computed by getNaturalTypeAlignment for an incomplete class. We usually only use the alignment computed by getNaturalTypeAlignment() to set the alignment of memory operations, and you can't do any memory operations with an incomplete class.

But the result might be easier to read, in any case.

[rjmccall]

clang/lib/CodeGen/CodeGenFunction.cpp

176

If there's a getTypeAlignIfKnown(), it would be better to use it.

[efriedma]

"Address" the review comments. Not really happy with this, but not sure what else to do.

[efriedma]

Hang on, I submitted this too early. Need to look a bit more.

[efriedma]

This should work correctly, now, I think?

[rjmccall]

clang/lib/CodeGen/CodeGenFunction.cpp

if (auto Align = TT->getDecl()->getMaxAlignment()) {
       return getContext().toCharUnitsFromBits(Align);
     }
   }
 
   if (BaseInfo)
     *BaseInfo = LValueBaseInfo(AlignmentSource::Type);
 
   CharUnits Alignment;
-  if (T->isIncompleteType()) {
+  if (T->getBaseElementTypeUnsafe()->isIncompleteType()) {
     Alignment = CharUnits::One(); // Shouldn't be used, but pessimistic is best.
   } else {
     // For C++ class pointees, we don't know whether we're pointing at a
     // base or a complete object, so we generally need to use the
     // non-virtual alignment.
     const CXXRecordDecl *RD;
     if (forPointeeType && (RD = T->getAsCXXRecordDecl())) {
       Alignment = CGM.getClassPointerAlignment(RD);

clang/test/CodeGenCXX/alignment.cpp

void g() {
     // CHECK: [[ARRAY_P:%.*]] = getelementptr inbounds [[B]], [[B]]* [[B_P]], i32 0, i32 2
     // CHECK: [[T0:%.*]] = bitcast [[ARRAY]]* [[RESULT]] to i8*
     // CHECK: [[T1:%.*]] = bitcast [[ARRAY]]* [[ARRAY_P]] to i8*
     // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 64 [[T0]], i8* align 8 [[T1]], i64 16, i1 false)
     D d;
     AlignedArray result = d.bArray;
   }
 }
+
+// CHECK-LABEL: @_Z22incomplete_array_derefPA_i
+// CHECK: load i32, i32* {{%.*}}, align 4
+int incomplete_array_deref(int (*p)[]) { return (*p)[2]; }