Re: CSE not combining equivalent expressions.

["Pranav Bhandarkar" <pranav dot bhandarkar at gmail dot com>]

On 1/18/07, Richard Kenner <kenner@vlsi1.ultra.nyu.edu> wrote:
> > I'm not immediately aware of too many cases where lowering the IL is
> > going to expose new opportunities to track and optimize nonzero/zero
> > bits stuff.
>
> Bitfield are the big one.  If you have both bitfield and logical operations,
> you can often merge the logical operations with those used to retrieve
> and/or store the field.
>
> Things such as
>
>         x.y |= 1;
>
> where Y is a bitfield and X non-BLKmode would be a large sequence of logical
> operations that can all be replaced by a single OR insn at the RTL level
> but presents no optimization opportunities at the tree level.
I have found another case where a zero / sign extend is inhibiting optimization

extern char b;
extern char c;
int main()
{

 b <<= 1;
 b <<= 1;
 b <<= 1;
 c = b;

 return 0;
}

Here again a zero extend gets generated after every  'ashift' whereas
we are interested only in the lower order 8 bits. However when i try
the same thing with int instead of char i.e. there is no need for
extension then the operations get converted into b<<=3 instead of 3
instructions.
regards,
Pranav