Re: Compiler optimization of division and remainder

Henry G. Baker <hbaker@netcom.com> wrote:
>On another newsgroup, a poster claimed that some compilers could
>optimize a program which used both a/b and a%b (both quotient and
>remainder with the _same_ arguments) in such a way that the compiler
>would perform only a single hardware division operation and arrange to
>use both the quotient and remainder from this single operation.
>
>[...]
>
>I would be interested in a) references to real, existing compilers
>which are capable of doing such a thing; and b)
>papers/tech-reports/theses, etc. which say how to do this.


The Norcroft/ARM C compiler performs this optimisation, since division
is done by a run-time routine which also delivers the remainder. I
cannot say for sure, but I suspect that this is done using CSE.


For example:


int f(int y,int x)
{
return x/y + x%y;
}


is compiled to:


                 f
e92d4000 STMDB sp!,{lr}
ebfffffd BL __rt_sdiv
e0800001 ADD a1,a1,a2
e8bd8000 LDMIA sp!,{pc}


Mike.
              _____________________________________________________________________
\ x / Michael Williams Advanced RISC Machines Limited
|\/|\/\ michael.williams@armltd.co.uk Fulbourn Road, Cambridge, CB1 4JN, UK
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