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From: "David Livshin" <dlivshin@internet-zahav.net>
To: "Richard Henderson" <rth@redhat.com>
Cc: "Alan Lehotsky" <apl@alum.mit.edu>, <gcc@gcc.gnu.org>
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Subject: Re: gcc port to StarCore
Date: Thu, 02 May 2002 05:18:00 -0000
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----- Original Message -----
From: Richard Henderson <rth@redhat.com>
To: David Livshin <dlivshin@zahav.net.il>
Cc: Alan Lehotsky <apl@alum.mit.edu>; <gcc@gcc.gnu.org>
Sent: Thursday, May 02, 2002 8:22 AM
Subject: Re: gcc port to StarCore


> On Thu, May 02, 2002 at 08:38:23AM +0200, David Livshin wrote:
> > However this doesn't seem to be the reason for the problem I have
because
> > the pattern:
> >
> > ( define_expand "cmpsi"
>
> No, the define_expand is irrelevant.  It's the define_insn
> that matters here.
>
>
> r~

Are you saying that in order to benefit from constant propagation in the:

x = 10;
y = 100;
if ( x > y )
...

'define_expand "cmpsi"' may not be used?

Would you please direct me to a material ( source code and/or document )
that may clarify the way this optimization is implemented by the GNU
compiler. Right now I even cant guess what it might be. For example I tried
in md:

( define_insn "addsi3"
  [ ( set
      ( match_operand:SI 0 "register_operand"             "" )
      ( plus:SI ( match_operand:SI 1 "general_operand"   "" )
                ( match_operand:SI 2 "general_operand" "" )
      )
    )
  ]
....

and then compiled:

x = 10;
y = 100;
z = x + y;
...

It seems from the resulting code:

 moveu.l #10,d0
 move.l d0,_x
 moveu.l #100,d2
 move.l d2,_y
 add #10,d2,d2
 move.l d2,_z

that compiler understands that 'x' is equal to 10 but misses the fact that
'y' is also a constant and 'z' is always equal to 110( or something inside
the compiler forces the generation of the add instruction ). Just in-case
you are wondering: description of "movsi"  in my md file allows load of any
constant to any register.

Thank you,

David