[RecordLayout] Only assert that fundamental type sizes are power of two on MSVC

[mstorsjo]

Summary

Make types with sizes that aren't a power of two an error (that can be disabled) in structs with ms_struct layout, except on mingw where the situation is quite likely to occur and GCC handles it silently.

[efriedma]

LGTM

[compnerd]

Ugh, really not a fan of this change.

[mstorsjo]

Added an error-by-default diagnostic (just like the existing warn_cxx_ms_struct) for this case, trigger it on all targets other than mingw (where the situation is quite likely to occur, and GCC handles it silently).

@compnerd, do you like this better?

[compnerd]

Awesome, thanks, this makes me feel much more comfortable.

lib/AST/RecordLayoutBuilder.cpp

       
       // Resolve all typedefs down to their base type and round up the field
       // alignment if necessary.
       QualType T = Context.getBaseElementType(D->getType());
       if (const BuiltinType *BTy = T->getAs<BuiltinType>()) {
         CharUnits TypeSize = Context.getTypeSizeInChars(BTy);
         assert(
             (llvm::isPowerOf2_64(TypeSize.getQuantity()) ||
-             Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) &&
-            "Non PowerOf2 size outside of GNU mode");
+             !Context.getTargetInfo().getTriple().isWindowsMSVCEnvironment()) &&
+            "Non PowerOf2 size in MSVC mode");
         if (TypeSize > FieldAlign &&
             llvm::isPowerOf2_64(TypeSize.getQuantity()))
           FieldAlign = TypeSize;
       }
     }
   }
 
   // The align if the field is not packed. This is to check if the attribute

test/CodeGen/ms_struct-long-double.c

+// RUN: %clang_cc1 -emit-llvm-only -triple i686-windows-gnu -fdump-record-layouts %s | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm-only -triple i686-linux -fdump-record-layouts %s | FileCheck %s
+
+struct ldb_struct {
+  char c;
+  long double ldb;
+} __attribute__((__ms_struct__));
+
+struct ldb_struct a;
+
+// CHECK:             0 | struct ldb_struct
+// CHECK-NEXT:        0 |   char c
+// CHECK-NEXT:        4 |   long double ldb
+// CHECK-NEXT:          | [sizeof=16, align=4]