Re: Is ther any sense in *= and matrices?

[18k11tm001@sneakemail.com (Russ)]

"Bobby D. Bryant" <bdbryant@mail.utexas.edu> wrote in message news:<pan.2003.06.09.14.53.15.912223@mail.utexas.edu>...
> On Sun, 08 Jun 2003 21:27:13 -0700, Russ wrote:
> 
> > "Bobby D. Bryant" <bdbryant@mail.utexas.edu> wrote in message
> > news:<pan.2003.06.08.22.56.37.297066@mail.utexas.edu>...
> >> On Sun, 08 Jun 2003 11:51:31 -0700, Russ wrote:
> >> 
> >> > "John R. Strohm" <strohm@airmail.net> wrote in message
> >> > news:<bbumef$fnq@library2.airnews.net>...
>  
> >> >> With all due respect, ladies and gentlemen, it has been known for a
> >> >> very long time that the difference in "efficiency" between A := A +
> >> >> B and A += B is lost in the noise floor compared to the improvements
> >> >> that can be gotten by improving the algorithms involved.
> >> > 
> >> > Oh, really? I just did a test in C++ with 3x3 matrices. I added them
> >> > together 10,000,000 times using "+", then "+=". The "+=" version took
> >> > about 19 seconds, and the "+" version took about 55 seconds. That's
> >> > just shy of a factor of 3, folks. If that's your "noise floor," I
> >> > can't help wonder what kind of "algorithms" you are dealing with!
> >> 
> >> I'm just curious why the compiler didn't generate the same code for
> >> both versions.
> > 
> > The compiler didn't generate the code. I did.
> 
> No, I'm talking about the compiler's machine-code output.  You wrote in
> C++, right?

Yes, I wrote it in C++. I would not have expected the compiler to
generate the same code for both versions. I did not use any
optimization flag, by the way, but even if I had, I still would not
have expected the same executable. If you think it should, your beef
is with the gcc folks.