[RecordLayout] Don't align to non-power-of-2 sizes when using -mms-bitfields

[mstorsjo]

Summary

When targeting GNU/MinGW for i386, the size of the "long double" data type is 12 bytes (while it is 8 bytes in MSVC). When building
with -mms-bitfields to have struct layouts match MSVC, data types are laid out in a struct with alignment according to their size. However, this doesn't make sense for the long double type, since it doesn't match MSVC at all, and aligning to a non-power-of-2 size triggers other asserts later.

This matches what GCC does, aligning a long double to 4 bytes in structs on i386 even when -mms-bitfields is specified.

This fixes asserts when using the max_align_t data type when building for MinGW/i386 with the -mms-bitfields flag.

[compnerd]

lib/AST/RecordLayoutBuilder.cpp

1755–1760

Can you add an assertion that the size is a power of two unless it is the GNU environment?

[mstorsjo]

Asserting that non power of 2 only occurs in gnu mode.

lib/AST/RecordLayoutBuilder.cpp

1755–1760

Sure, done.

[compnerd]

If its easy enough to wire that through to the frontend as a proper diagnostic, that would be better with a test. Otherwise, this is good to continue to make progress.

[rnk]

Thanks!

[efriedma]

cfe/trunk/lib/AST/RecordLayoutBuilder.cpp

1758 ↗

(On Diff #136037)

This assertion seems weird. sizeof(long double) is 12 for other targets, including x86-32 Linux.

[mstorsjo]

cfe/trunk/lib/AST/RecordLayoutBuilder.cpp

1758 ↗

(On Diff #136037)

Perhaps it'd make more sense to flip it around, assert(isPowerOf2() || !Triple.isWindowsMSVCEnvironment())?

[efriedma]

cfe/trunk/lib/AST/RecordLayoutBuilder.cpp

1758 ↗

(On Diff #136037)

Yes, that makes sense.

lib/AST/RecordLayoutBuilder.cpp

       // If MS bitfield layout is required, figure out what type is being
       // laid out and align the field to the width of that type.
       
       // 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);
-        if (TypeSize > FieldAlign)
+        assert(
+            (llvm::isPowerOf2_64(TypeSize.getQuantity()) ||
+             Context.getTargetInfo().getTriple().isWindowsGNUEnvironment()) &&
+            "Non PowerOf2 size outside of GNU 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
   // was unnecessary (-Wpacked).
   CharUnits UnpackedFieldAlign = FieldAlign;

test/CodeGen/mingw-long-double.c

 // RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -o - %s \
 // RUN:    | FileCheck %s --check-prefix=GNU32
+// RUN: %clang_cc1 -triple i686-windows-gnu -emit-llvm -o - %s -mms-bitfields \
+// RUN:    | FileCheck %s --check-prefix=GNU32
 // RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -o - %s \
 // RUN:    | FileCheck %s --check-prefix=GNU64
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -o - %s \
 // RUN:    | FileCheck %s --check-prefix=MSC64
 
 struct {
   char c;
   long double ldb;