Re: [PATCH] no lowering of &a.b (and &a->b) in the C front-end

[Andrew Pinski <pinskia at physics dot uc dot edu>]

On Aug 22, 2004, at 5:07 PM, Zack Weinberg wrote:

> Andrew Pinski <pinskia@physics.uc.edu> writes:
>
> > I found that both the C and C++ front-ends do lower &a.b into a +
> > offsetof(b,a) which is really not good for some optimizations which
> > can change the struct a into scaler values.
> >
> > This patch changes the C front-end so there is no lowering and fixes
> > a bug found in c_finish_return with the no lowering turned on.
> > Warning about returning a temporary which is true for &a->b.
>
> Can you give an example of code which is optimized better with your
> patch?

Yes, http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15262 which contains an
example where we miscompile at this point with -O3 before this patch
we would not optimize anything away in main but with it we can optimize
everything away and only have a return left.  There are most likely
a huge number of C++ examples where this can help too because a huge
number of the C++ code usually returns references to fields in the 
classes.


> > +	    addr2 = build1 (ADDR_EXPR, argtype, arg);
> > +	
> > +	    if (!TREE_CONSTANT (addr) || TREE_CONSTANT (addr2))
> > +	      addr = addr2;
>
> Why isn't this
>
>             if (!TREE_CONSTANT (addr))
>               addr = build1 (ADDR_EXPR, argtype, arg);
>
> That avoids constructing a junk tree node in the majority of cases.

That is much better and I will test the patch with that fixed.

I was thinking about doing that but I got sidetracked on other stuff
so I could not test it.

Thanks,
Andrew Pinski