[sdcc-devel] [ sdcc-Feature Requests-603246 ] Inefficient multiply by a constant

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Feature Requests item #603246, was opened at 2002-09-01 20:53
Message generated for change (Comment added) made by frief
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Category: None
Group: None
Status: Open
Priority: 5
Submitted By: Wim Lewis (wiml)
Assigned to: Nobody/Anonymous (nobody)
Summary: Inefficient multiply by a constant

Initial Comment:
The following C code:

xdata at 0x7FC6 char FOO;
char fun(unsigned char tmp)
{
    tmp = 2 * (tmp-1);
    return ((xdata char *)(&FOO))[tmp];
}

is compiled inefficiently by the version of SDCC I got
from CVS on 1-September-2002.

My older copy of SDCC (2.2.1, which I'm trying to
upgrade from) generates reasonably efficient code
involving a decrement and an add. The new version
(2.3.2-devel) generates much more elaborate code using
an actual MUL instruction, instead of optimizing the
MUL to an ADD.



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>Comment By: Frieder Ferlemann (frief)
Date: 2004-01-22 22:24

Message:
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Hi Maarten, 
 
> , although more optimizations are always possible. 
 
what you're probably alluding to is the sequence with 
the redundant xch xch pair: 
 
;     genLeftShift 
;     genLeftShiftLiteral 
;     genlshTwo 
        mov     a,r4 
        xch     a,r3 
        add     a,acc 
        xch     a,r3 
        rlc     a 
        mov     r4,a 
 
There is a peephole optimization to catch a similar 
case (peephole 252 for variables in direct data). 
Unfortunately it doesn't catch this one... 
While another peephole could be defined this is probably 
not the proper level to do it: there would have to 
be one for xdata, idata, and yes, some people would 
even want to left shift a long int too:) 
 
> , although more optimizations are always possible. 
 
Yeah, there is another one. And it would make sense 
to find a peephole optimization for it :^) 
 
This is it: 
 
        mov     r4,#0x00 
;     genLeftShift 
;     genLeftShiftLiteral 
;     genlshTwo 
        mov     a,r4 
 

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Comment By: Maarten Brock (maartenbrock)
Date: 2004-01-22 18:54

Message:
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user_id=888171

Dear developers,

Tried this with SDCC 2.3.7 (2004-01-21) and it uses no MUL, 
only ADD A,ACC and RLC A along with the necessary MOV's. 
Produced code looks perfectly ok to me, although more 
optimizations are always possible.

I think this RFE can be closed.

Maarten

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