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Re: ipa-visibility TLC 2/n


On Fri, Jun 6, 2014 at 3:09 AM, Jan Hubicka <hubicka@ucw.cz> wrote:

> sorry for taking time to get back to it.  I went through the code and see no obvious flaws
> except that I am somewhat concerned what happens with local aliases into the anchors.
> The problem however does not seem to be caused by it.  I tracked it down to the following
> difference:
>
> --- d2.s        2014-06-06 08:59:01.101401622 +0200
> +++ d3.s        2014-06-06 08:59:01.491377632 +0200
> @@ -499,7 +499,7 @@
>         .long   4
>         .long   4
>         .long   _ZTI1B
> -       .long   _ZTv0_n12_N1B1fEv
> +       .long   _ZTv0_n12_N1B1fEv.localalias.5
>         .weak   _ZTV1A
>         .align 2
>  _ZTV1A:
>
> Now _ZTv0_n12_N1B1fEv is thunk declared as:
>
>         .lglobl .LTHUNK..0
>         .lglobl LTHUNK..0
>         .set    .LTHUNK..0,._ZN1B1fEv
>         .set LTHUNK..0,_ZN1B1fEv
>         .lglobl ._ZTv0_n12_N1B1fEv.localalias.5
>         .lglobl _ZTv0_n12_N1B1fEv.localalias.5
>         .set    ._ZTv0_n12_N1B1fEv.localalias.5,._ZTv0_n12_N1B1fEv
>         .set _ZTv0_n12_N1B1fEv.localalias.5,_ZTv0_n12_N1B1fEv
>         .align 2
>         .weak   _ZTv0_n12_N1B1fEv[DS]
>         .weak   ._ZTv0_n12_N1B1fEv
>         .csect _ZTv0_n12_N1B1fEv[DS]
> _ZTv0_n12_N1B1fEv:
>         .long ._ZTv0_n12_N1B1fEv, TOC[tc0], 0
>         .csect .text[PR]
> ._ZTv0_n12_N1B1fEv:
> LFB..16:
>         lwz 12,0(3)
>         lwz 12,-12(12)
>         add 3,3,12
>         b .LTHUNK..0
>
> Now when I move the declaration of localalias after the thunk:
>
> --- d3.s        2014-06-06 08:59:01.491377632 +0200
> +++ d4.s        2014-06-06 09:03:58.129810951 +0200
> @@ -212,10 +212,6 @@
>         .lglobl LTHUNK..0
>         .set    .LTHUNK..0,._ZN1B1fEv
>         .set LTHUNK..0,_ZN1B1fEv
> -       .lglobl ._ZTv0_n12_N1B1fEv.localalias.5
> -       .lglobl _ZTv0_n12_N1B1fEv.localalias.5
> -       .set    ._ZTv0_n12_N1B1fEv.localalias.5,._ZTv0_n12_N1B1fEv
> -       .set _ZTv0_n12_N1B1fEv.localalias.5,_ZTv0_n12_N1B1fEv
>         .align 2
>         .weak   _ZTv0_n12_N1B1fEv[DS]
>         .weak   ._ZTv0_n12_N1B1fEv
> @@ -229,6 +225,10 @@
>         lwz 12,-12(12)
>         add 3,3,12
>         b .LTHUNK..0
> +       .lglobl ._ZTv0_n12_N1B1fEv.localalias.5
> +       .lglobl _ZTv0_n12_N1B1fEv.localalias.5
> +       .set    ._ZTv0_n12_N1B1fEv.localalias.5,._ZTv0_n12_N1B1fEv
> +       .set _ZTv0_n12_N1B1fEv.localalias.5,_ZTv0_n12_N1B1fEv
>  LFE..16:
>         .align 2
>         .weak   _ZN1D1fEv[DS]
>
> The code starts to work.  To me both sources should make same binary, but they don't.
> Can you make better sense of this than claiming it is an assembler bug?
>
> We output aliases after definitions for functions, but before definitions for thunks.
> I am testing patch to change the second order.  I remeber tweaking this order once before
> because of problems on solaris.  Lets see if we find order that works for everyone.

Honza,

The AIX assembler is a very simple, single-pass assembler. The order
has a lot of importance because the assembler makes assumptions about
symbols, such as section placement and annotations, if they are
referenced before defined. I notice that your change places the
.lglobl declaration after the .weak declaration.  That may be more
important than placing it after the thunk.

Thanks, David


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