llvm dev - Sep 2010 - [LLVMdev] Union type, is it really used or necessary?

Eli Friedman wrote:
> On Tue, Sep 7, 2010 at 8:16 AM, Talin <viridia at gmail.com> wrote:
> > Here's a suggestion - can we make the "union patch" (the
inverse of the
> > patch that removed unions) as a downloadable file so that people who
are
> > interested in finishing the work can do so?
> 
> Anyone who's really interested in working on it can just use "svn
diff
> -c 112356", and apply it with "patch -R".
Well I tried that, the patch fails to reverse apply. Out of the
34 files touched by the patch, not a single hunk actually manages
to reverse apply.

Assuming I was to decide to embark on the effort of getting unions
back into LLVM:

  a) What is required for them to be accepted back in?

  b) What are the chances of getting them in the 2.8 release?

Erik
-- 
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Erik de Castro Lopo
http://www.mega-nerd.com/

On Sep 7, 2010, at 2:24 PM, Erik de Castro Lopo wrote:
> Eli Friedman wrote:
> 
>> On Tue, Sep 7, 2010 at 8:16 AM, Talin <viridia at gmail.com>
wrote:
>>> Here's a suggestion - can we make the "union patch"
(the inverse of the
>>> patch that removed unions) as a downloadable file so that people
who are
>>> interested in finishing the work can do so?
>> 
>> Anyone who's really interested in working on it can just use
"svn diff
>> -c 112356", and apply it with "patch -R".
> 
> Well I tried that, the patch fails to reverse apply. Out of the
> 34 files touched by the patch, not a single hunk actually manages
> to reverse apply.
> 
> Assuming I was to decide to embark on the effort of getting unions
> back into LLVM:
> 
>  a) What is required for them to be accepted back in?
It needs to work.  When reverted, it was broken in almost all cases.
>  b) What are the chances of getting them in the 2.8 release?
Zero.

-Chris

Chris Lattner wrote:
> >  a) What is required for them to be accepted back in?
> 
> It needs to work.  When reverted, it was broken in almost all cases.
'It needs work' and 'it was broken' doesn't really give me
an
idea of what specifically is required.

Specifically, what I am interested in is using unions within
packed structs to force alignment. Using unions like this was
the easiest and most reliable way of forcing specific alignment.
It made it really easy to calculate offsets in high level code
allowing me to completely ignore whether I was generating code
for 32 or 64 bits.
> >  b) What are the chances of getting them in the 2.8 release?
> 
> Zero.
So a feature, of which a subset was actually working (I know
this because I am using unions successfully in the compiler
I'm working on) in the 2.7 release and was documented on the
web site

    http://llvm.org/releases/2.7/docs/LangRef.html#t_union

just gets yanked?

Are you really trying to tell me that anyone using LLVM in anger
needs to be running SVN HEAD and keep an eye on the mailing list
to make sure that features they use aren't going to get arbitrarily
yanked?

Erik
-- 
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/