r186370 - Fix alignment of class derived from empty class.

[Richard Smith]

It looks like this fixes PR14564. Does it also fix the case in PR14564
comment#1?

On Mon, Jul 15, 2013 at 5:21 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> Author: efriedma
> Date: Mon Jul 15 19:21:28 2013
> New Revision: 186370
>
> URL: http://llvm.org/viewvc/llvm-project?rev=186370&view=rev
> Log:
> Fix alignment of class derived from empty class.
>
> The record layout code didn't properly take into account that
> an empty class at offset 0 can have an alignment greater than 1.
>
> Patch by Andrea Di Biagio.
>
> Added:
>     cfe/trunk/test/SemaCXX/alignment-of-derived-class.cpp
> Modified:
>     cfe/trunk/lib/AST/RecordLayoutBuilder.cpp
>
> Modified: cfe/trunk/lib/AST/RecordLayoutBuilder.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/RecordLayoutBuilder.cpp?rev=186370&r1=186369&r2=186370&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/AST/RecordLayoutBuilder.cpp (original)
> +++ cfe/trunk/lib/AST/RecordLayoutBuilder.cpp Mon Jul 15 19:21:28 2013
> @@ -1532,18 +1532,19 @@ CharUnits RecordLayoutBuilder::LayoutBas
>      }
>    }
>
> +  CharUnits UnpackedBaseAlign = Layout.getNonVirtualAlign();
> +  CharUnits BaseAlign = (Packed) ? CharUnits::One() : UnpackedBaseAlign;
> +
>    // If we have an empty base class, try to place it at offset 0.
>    if (Base->Class->isEmpty() &&
>        (!HasExternalLayout || Offset == CharUnits::Zero()) &&
>        EmptySubobjects->CanPlaceBaseAtOffset(Base, CharUnits::Zero())) {
>      setSize(std::max(getSize(), Layout.getSize()));
> +    UpdateAlignment(BaseAlign, UnpackedBaseAlign);
>
>      return CharUnits::Zero();
>    }
>
> -  CharUnits UnpackedBaseAlign = Layout.getNonVirtualAlign();
> -  CharUnits BaseAlign = (Packed) ? CharUnits::One() : UnpackedBaseAlign;
> -
>    // The maximum field alignment overrides base align.
>    if (!MaxFieldAlignment.isZero()) {
>      BaseAlign = std::min(BaseAlign, MaxFieldAlignment);
>
> Added: cfe/trunk/test/SemaCXX/alignment-of-derived-class.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/alignment-of-derived-class.cpp?rev=186370&view=auto
>
> ==============================================================================
> --- cfe/trunk/test/SemaCXX/alignment-of-derived-class.cpp (added)
> +++ cfe/trunk/test/SemaCXX/alignment-of-derived-class.cpp Mon Jul 15
> 19:21:28 2013
> @@ -0,0 +1,41 @@
> +// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
> +// expected-no-diagnostics
> +
> +// Test that the alignment of a empty direct base class is correctly
> +// inherited by the derived class.
> +
> +struct A {
> +} __attribute__ ((aligned(16)));
> +
> +static_assert(__alignof(A) == 16, "A should be aligned to 16 bytes");
> +
> +struct B1 : public A {
> +};
> +
> +static_assert(__alignof(B1) == 16, "B1 should be aligned to 16 bytes");
> +
> +struct B2 : public A {
> +} __attribute__ ((aligned(2)));
> +
> +static_assert(__alignof(B2) == 16, "B2 should be aligned to 16 bytes");
> +
> +struct B3 : public A {
> +} __attribute__ ((aligned(4)));
> +
> +static_assert(__alignof(B3) == 16, "B3 should be aligned to 16 bytes");
> +
> +struct B4 : public A {
> +} __attribute__ ((aligned(8)));
> +
> +static_assert(__alignof(B4) == 16, "B4 should be aligned to 16 bytes");
> +
> +struct B5 : public A {
> +} __attribute__ ((aligned(16)));
> +
> +static_assert(__alignof(B5) == 16, "B5 should be aligned to 16 bytes");
> +
> +struct B6 : public A {
> +} __attribute__ ((aligned(32)));
> +
> +static_assert(__alignof(B6) == 32, "B6 should be aligned to 32 bytes");
> +
>
>
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