r217495 - CodeGen: Use a fixed alignment for vtables.

[David Blaikie]

On Wed, Sep 10, 2014 at 5:50 AM, Benjamin Kramer <benny.kra at googlemail.com>
wrote:

> Author: d0k
> Date: Wed Sep 10 07:50:59 2014
> New Revision: 217495
>
> URL: http://llvm.org/viewvc/llvm-project?rev=217495&view=rev
> Log:
> CodeGen: Use a fixed alignment for vtables.
>

Any chance this change is to blame for
http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/17224 ?
Anything I can do to help debug this?


>
> Pointer-sized alignment is sufficient as we only ever read single values
> from the table. Otherwise we'd bump the alignment to 16 bytes in the
> backend if the vtable is larger than 16 bytes. This is great for
> structures that are accessed with vector instructions or copied around, but
> that's simply not the case for vtables.
>
> Shrinks the data segment of a Release x86_64 clang by 0.3%. The wins are
> larger for i386 and code bases that use vtables more often than we do.
>
> This matches the behavior of GCC 5.
>
> Added:
>     cfe/trunk/test/CodeGenCXX/vtable-align.cpp
> Modified:
>     cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
>
> Modified: cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp?rev=217495&r1=217494&r2=217495&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp (original)
> +++ cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp Wed Sep 10 07:50:59 2014
> @@ -1207,6 +1207,12 @@ void ItaniumCXXABI::emitVTableDefinition
>    // Set the right visibility.
>    CGM.setGlobalVisibility(VTable, RD);
>
> +  // Use pointer alignment for the vtable. Otherwise we would align them
> based
> +  // on the size of the initializer which doesn't make sense as only
> single
> +  // values are read.
> +  unsigned PAlign = CGM.getTarget().getPointerAlign(0);
> +
> VTable->setAlignment(getContext().toCharUnitsFromBits(PAlign).getQuantity());
> +
>    // If this is the magic class __cxxabiv1::__fundamental_type_info,
>    // we will emit the typeinfo for the fundamental types. This is the
>    // same behaviour as GCC.
>
> Added: cfe/trunk/test/CodeGenCXX/vtable-align.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/vtable-align.cpp?rev=217495&view=auto
>
> ==============================================================================
> --- cfe/trunk/test/CodeGenCXX/vtable-align.cpp (added)
> +++ cfe/trunk/test/CodeGenCXX/vtable-align.cpp Wed Sep 10 07:50:59 2014
> @@ -0,0 +1,14 @@
> +// RUN: %clang_cc1 %s -triple=i386-apple-darwin10 -emit-llvm -o - |
> FileCheck %s -check-prefix=CHECK-32
> +// RUN: %clang_cc1 %s -triple=x86_64-apple-darwin10 -emit-llvm -o - |
> FileCheck %s -check-prefix=CHECK-64
> +
> +struct A {
> +  virtual void f();
> +  virtual void g();
> +  virtual void h();
> +};
> +
> +void A::f() {}
> +
> +// CHECK-32: @_ZTV1A = unnamed_addr constant [5 x i8*] [i8* null, i8*
> bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast (void (%struct.A*)*
> @_ZN1A1fEv to i8*), i8* bitcast (void (%struct.A*)* @_ZN1A1gEv to i8*), i8*
> bitcast (void (%struct.A*)* @_ZN1A1hEv to i8*)], align 4
> +
> +// CHECK-64: @_ZTV1A = unnamed_addr constant [5 x i8*] [i8* null, i8*
> bitcast ({ i8*, i8* }* @_ZTI1A to i8*), i8* bitcast (void (%struct.A*)*
> @_ZN1A1fEv to i8*), i8* bitcast (void (%struct.A*)* @_ZN1A1gEv to i8*), i8*
> bitcast (void (%struct.A*)* @_ZN1A1hEv to i8*)], align 8
>
>
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