Question

Two Parts two my question. Which is more efficient/faster:

int a,b,c,d,e,f;
int a1,b1,c1,d1,e1,f1;
int SumValue=0; // oops forgot zero
// ... define all values
SumValue=a*a1+b*b1+c*c1+d*d1+e*e1*f*f1;

or

Sumvalue+=a*a1+b*b1+c*c1;
Sumvalue+=d*d1+e*e1*f*f1;

I’m guessing the first one is. My second question is why.

I guess a third question is, at any point would it be necessary to break up an addition operation (besides compiler limitations on number of line continuations etc…).

Edit

Is the only time I would see a slow down when then entire arithmetic operation could not fit in the cache? I think this is impossible – compiler probably gets mad about two many line continuations before this could happen. Maybe I’ll have to play tomorrow and see.

Answer 1

Did you measure that? The optimized machine code for both approaches will probably be very similar, if not the same.

EDIT: I just tested this, the results are what I expected:

$ gcc -O2 -S math1.c  # your first approach
$ gcc -O2 -S math2.c  # your second approach
$ diff -u math1.s math2.s

--- math1.s 2010-10-26 19:35:06.487021094 +0200
+++ math2.s 2010-10-26 19:35:08.918020954 +0200
@@ -1,4 +1,4 @@
-   .file   "math1.c"
+   .file   "math2.c"
    .section    .rodata.str1.1,"aMS",@progbits,1
 .LC0:
    .string "%d\n"

That’s it. Identical machine code.