DmitryPolukhin added a comment.
This CL doesn't changes anything for ms_struct cases and ms_struct seems to be
broken for bit-fields even for very simple cases so filed separate bug
https://llvm.org/bugs/show_bug.cgi?id=25707
PTAL
================
Comment at: lib/AST/RecordLayoutBuilder.cpp:1606
@@ -1605,1 +1605,3 @@
+ } else if (ExplicitFieldAlign)
+ FieldOffset = llvm::RoundUpToAlignment(FieldOffset, ExplicitFieldAlign);
----------------
rjmccall wrote:
> It does still seem to be limited by platforms that ignore natural alignment,
> though, and by #pragma pack (2) and so on, so at least some of the
> modifications made to FieldAlign need to be reflected in ExplicitFieldAlign.
>
> I had to spend some time trying to work out counter-examples, but I did
> convince myself that the effect of the fallback here is correct. My concern
> was that there was a way we might round up to FieldAlign when we only needed
> to round up to ExplicitFieldAlign, which could matter if there were another
> properly-aligned position within the current storage unit. However, the
> second clause of the first condition will only trigger at the very end of a
> storage unit, which means that there cannot be any other properly-aligned
> positions within it.
>
> It might be wrong for zero-width bit-fields, though, since we'll always round
> up to FieldAlign instead of ExplicitFieldAlign.
I think compiler can ignore natural type alignment here because it is bit-field
and therefore compiler has to generate shift/mask sequence for accessing
unaligned field anyway. I tried examples like:
struct __attribute__((packed)) B {
char AField;
__attribute__((aligned(1))) __int128 i : 124;
char BField;
};
Both GCC and Clang with my patch generates the same result on x86 and ARM:
sizeof(struct B) = 18
offsetof(struct B, BField) = 17
__alignof(struct B) = 1
Also tried zero-width bit-filed, my patch works fine. So I was not able to find
arch that requires round up for ExplicitFieldAlign and what is required. All
examples that I can think of work identical on GCC and Clang.
http://reviews.llvm.org/D14980
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