Re: RISC again
On Mon, 25 Mar 1996, Andrew Haley wrote:
> RISC instructions are often a bit less dense than CISC. RISC
> processors handle procedures with a variable number of arguments
> perfectly correctly.
I tried to elaborate on this :-)
> That is: the code does the right thing, and it
> does it quickly.
> Compiler writing for RISC architectures is to a large extent a problem
> which has already been solved. I use such a compiler every day: the
> code it generates isn't perfectly optimum, but it is correct. (In
> fact that's not quite true; I have experienced one optimizer bug which
> generated incorrect code in the course of three years' programming.)
Actually, to the things that matter most -- loop unrolling for floating
point routines -- they are very bad (well, the SGI MIPS compiler for IRIX
5.3) The compiler, given the following C code, produces either far from
optimal machine code, or _incorrect_ code, depending on the switches you
feed it:
/*
* Calculate the dot product of two samples
*/
double ddot(double *v1, double *v2, int n){
int i;
double acc;
for(acc=0,i=0; i<n; i++) acc+=v1[i]*v2[i];
return(acc);
}
This is not a staement about RISC per se, it only says, that the problem
of RISC C compiler writing is far from being solved.
--
Penio Penev <Penev@pisa.Rockefeller.edu> 1-212-327-7423
