Re: Run-time checking and speed

[ncohen@watson.ibm.com (Norman H. Cohen)]

In article <dsmith-1201952049540001@dsmith-ppp.clark.net>, dsmith@clark.net
(Doug Smith) writes: 

|> OK, some personal experience which has been verified countless times by
|> fellow engineers. A small percentage (10%) of the code accounts for (90%)
|> of the processing. In case after case, a dynamic analysis tool identifies
|> the code executed the most, the engineer optimizes and gets almost an order
|> of magnitude speed improvement.
|>
|> In the old days (i.e. early compilers), I measured executable size differences
|> that showed half of the code was constraint checking (Today's compilers do
|> a much better job). By redesigning the code to explicitly check for and
|> raise the correct exceptions, I could then suppress the compiler generated
|> constraint checking.

Static measurements of the proportion of instructions in an object file
devoted to run-time checks can be very misleading.  A good compiler will
move many checks out of inner loops, so that, for the reasons explained
in the first paragraph above, the proportion of run-time-check
instructions among the instructions actually executed will be much
smaller.

--
Norman H. Cohen    ncohen@watson.ibm.com