Re: Compiler optimization of division and remainder

hbaker@netcom.com (Henry G. Baker) writes:
> [do any compilers actually optimize a/b and a%b into a single operation
> if both appear?]


prener@watson.ibm.com (Dan Prener) wrote:
> IBM's xlc compiler does this, when compiling for the POWER (but not
> the PowerPC) architecture.


Metrowerks CodeWarrior for PowerPC will sort of do this:


        int main(int a, int b) { return(a / b + a % b); }


                00000000: 7CA323D6 divw r5,r3,r4
                00000004: 7C0521D6 mullw r0,r5,r4
                00000008: 7C001850 sub r0,r3,r0
                0000000C: 7C650214 add r3,r5,r0
                00000010: 4E800020 blr


The PowerPC does not compute a remainder result--thus the MULLW/SUB--but
the compiler manages to avoid doing the expensive DIVW twice. It's
actually just a side-effect of performing the CSE optimization at the
machine instruction level--the compiler generates DIVW twice but the
optimizer realizes that they compute the same result and optimizes one of
them away.


--
John McEnerney (mcenerney@metrowerks.com)
Metrowerks PowerPC Compiler Architect
--

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