Re: making aliases into the middle of a structure

[Zack Weinberg <zack at rabi dot columbia dot edu>]

On Fri, 23 Apr 1999 15:23:59 -0700, Richard Henderson wrote:
>On Thu, Apr 22, 1999 at 05:31:38PM -0400, Zack Weinberg wrote:
>> >That would probably be a good idea.  alias could accept an arbitrary
>> >expression.  If it's an lvalue within a global symbol, it will define
>> >an actual alias, otherwise it could just emit an error or ``construct
>> >a temporary and bind the reference to it'', to put it in C++ terms :-) 
>>
>> It turns out to be relatively simple to implement, too (at least if
>> you don't try to construct temporaries).  The appended patch is
>> a proof-of-concept; I don't expect it'll make it into 1.2.  I had to
>> introduce a new MD macro, ASM_OUTPUT_OFFSET_DEF...
>
>This is a very bad way to implement this.  You're hiding from the
>optimizers the fact that you've got two global symbols that alias.

Huh.  Why isn't this a problem for the existing symbol-alias
mechanism?  And what am I to do about references from another source
file?  The intended use of this feature is isolated in a file that
does nothing but export `x' and `y'.

>The only way this will ever work is for you to adjust the DECL_RTL
>of `y' such that the aliasing is visible.  You'd have something like
>
>	(const (plus (symbol_ref "x") (const_int 42)))
>
>for a global; something obvious for stack-local objects.  

Ok, if you don't mind my asking the dumb question, how do I do that?
The tree->rtl conversion isn't exactly documented.

>And of course this is only going to work without -fno-strict-aliasing
>if the programmer makes sure that the types correspond.  But that, I
>think, is not your problem.

Maybe I should verify that the types correspond inside decl_attributes.

>If you like to emit `y' as a global symbol too, that's fine. 

In this context, it's kinda necessary.

>BTW, you don't need the ifdef in varasm.c if you have a version
>in defaults.h.

The defaults.h version is only defined if SET_ASM_OP is available.

zw