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

[Jie Zhang <jie at codesourcery dot com>]

On 07/31/2010 01:53 AM, Jakub Jelinek wrote:
> On Fri, Jul 30, 2010 at 10:11:42AM -0700, Mark Mitchell wrote:
> > 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?"
>
> I wouldn't call it tweak to heuristics, it looks to me like an ugly hack.
> In many cases the SRA of bitfields is very desirable, especially for apps
> that use bitfields heavily like Linux kernel.  I remember Alex spending
> quite some time improving SRA to handle bitfields more efficiently,
> and this hack just disables it altogether.
My patch does not disable SRA of bit-fields. It's an adjust of the tweak 
for PR 42585. It's only disable total scalarization of small records 
containing bit-fields. As you said below, it's difficult for RTL 
combiner to combine the scalarized bit-fields again. So we'd better to 
avoid total scalarizing it. If the bit-fields are accessed explicitly in 
the IL, SRA still works as in usual way.

As my reply to Mark's email, my patch does not change the final assembly 
of linux kernel, at least for 2.6.34.1 with the default configuration of 
x86_64.


Regards,
--
Jie Zhang
CodeSourcery