[RFC] Don't completely scalarize a record if it contains bit-field (PR tree-optimization/45144)

[Jie Zhang]

On 07/31/2010 01:11 AM, Mark Mitchell wrote:
> Richard Guenther wrote:
>
>>> Until the aggregates copy propagation is implemented, I think it would
>>> better to disable full scalarization for such records. The patch is
>>> attached. It's bootstrapped on x86_64-linux-gnu and regression tested.
>
>> Without looking at the patch I have two comments.
>
>  From reading your comments, I'm not sure if you're saying that you don't
> like Jie's idea, or if you think it's an OK idea, but there are some
> other things that you would like to see done as well or instead.  It
> would be helpful if you could make that clear.
>
> To me, Jie's change seems like a plausible heuristic within the current
> infrastructure.  I'm all for building new infrastructure when
> possible/necessary, but I don't think we should prevent these kinds of
> tweaks to heuristics just because we can think of another way of doing
> things.  To me, the question ought to be "does this make the compiler
> better for users?"
>
> To answer that question, what I guess I'd like to know is what the
> impact is on some benchmark that matters.  Here, I don't think SPEC,
> EEMBC, etc. are probably the right places to look; they probably don't
> have much of this kind of code.  Perhaps it would be interesting to know
> how many SRA opportunities we lose in the Linux kernel because of this
> change -- and then spot-check some of them to see whether those are
> cases where we really lose by not doing SRA, or really win because we're
> not doing the kind of ugly stuff that inspired this change.
>
My patch causes no changes on EEMBC for arm-none-eabi target.

My patch prevents several full scalarizations of records with bit-field 
when compiling Linux kernel for x86_64, but none of these causes 
differences in final assemblies. I use 2.6.34.1 and the default config 
for x86_64. I checked -O2 and -Os.

I also checked the effect of my patch on GCC itself. My patch prevents 
one full scalarization of records with bit-field when compiling files 
under gcc/ with -O2. But there is no difference in the final assemblies.


Regards,
-- 
Jie Zhang
CodeSourcery