ray@saturn.ucsc.edu (Ray Swartz) (05/27/89)
I recently was surprised to discover that
while (i < 500000)
++i;
ran much faster than
while (i < 500000))
i++;
The test ran on a 68020 based machine. This test was run without using
the optimizer. Here is yet another example that shows that optimizing
from a higher level language may backfire.
Incidentally, I realize the loops don't do anything, I was testing some
alternatives ways of doing timing loops.poser@csli.Stanford.EDU (Bill Poser) (05/29/89)
In article <7703@saturn.ucsc.edu>, ray@saturn.ucsc.edu (Ray Swartz) writes: > I recently was surprised to discover that > > while (i < 500000) > ++i; > > ran much faster than > > while (i < 500000)) > i++; > > The test ran on a 68020 based machine. This test was run without using > the optimizer. Here is yet another example that shows that optimizing > from a higher level language may backfire. I don't see what this shows about optimizing from a higher level language. Shouldn't we expect a compiler that doesn't generate identical code for these two loops to generate faster code for the ++i case because the question of saving a copy of the unincremented variable does not arise? That is, presumably what is happening is that the compiler is generating (pseudo-assembler) something like: addl2 $1 i for the pre-increment but movl i foo # save unincremented value addl2 $1 i for the post-increment. Of course, a smart compiler will recognize that in these examples the value of i is never used and will not bother with the save. I tried these examples on an HP 9000/350 (68020-based) running HPUX 5.22 and a VAX 11/750 running 4.2BSD and got identical assembler output for the pre- and post-increments (without the optimizer), so it looks like the compiler on Ray Swartz' machine isn't very smart.
cowan@marob.masa.com (John Cowan) (06/01/89)
In article <9146@csli.Stanford.EDU> poser@csli.Stanford.EDU (Bill Poser) writes: [in reference to two loops, one of which pre-increments a variable, and the other post-increments a variable, and nothing else] >Of course, a smart compiler will recognize that in these examples >the value of i is never used and will not bother with the save. > A >really< smart compiler would recognize that the loops don't do anything at all, and kill them both! -- John Cowan <cowan@marob.masa.com> or <cowan@magpie.masa.com> UUCP mailers: ...!uunet!hombre!{marob,magpie}!cowan Fidonet (last resort): 1:107/711 Aiya elenion ancalima!