Dear Julia colleagues,
I'm reviving this old thread because I am also trying to create two
versions of my balanced-tree code, the fast version and the debug version,
and I am trying to figure out the appropriate way for one source-code file
to yield two versions. In C++, the usual technique is a -DDEBUG
compilation flag, followed by #ifdef DEBUG ... #else ... #endif macros.
I thought that maybe in Julia I could accomplish this with ordinary "if"
statements since the compiler could optimize away unused branches, but it
doesn't seem to work:
module testcodegen1
function t1(x::Float64)
s = 0.0
for i = 1 : 20000000
s += sin(x)
end
s
end
function t2(x::Float64)
s = 0.0
for i = 1 : 20000000
if true
s += sin(x)
else
s += sin(2*x)
end
end
s
end
end
Clearly t1 and t2 are equivalent, but t2 has an unreachable code branch
that could be optimized away. However, the compiler doesn't seem to
recognize this:
julia> tic(); testcodegen1.t1(3.0); toc()
tic(); testcodegen1.t1(3.0); toc()
elapsed time: 0.216690647 seconds
0.216690647
julia> tic(); testcodegen1.t2(3.0); toc()
tic(); testcodegen1.t2(3.0); toc()
elapsed time: 0.53437478 seconds
0.53437478
It seems that using ordinary 'if' statements to distinguish the debugging
mode entails a big performance hit.
Thanks for the help,
Steve Vavasis
On Wednesday, May 29, 2013 1:46:21 PM UTC-4, Simon Hardy-Francis wrote:
>
>
>
> Consider this simple Perl script:
>
> use strict;
> use Time::HiRes;
>
> my $t1 = Time::HiRes::time;
> foreach (1..50000000) {
> }
> my $t2 = Time::HiRes::time;
>
> my $t3 = Time::HiRes::time;
> my $debug = 0;
> foreach (1..50000000) {
> if ($debug) { printf qq[my debug line: %u], $_; }
> }
> my $t4 = Time::HiRes::time;
>
> my $t5 = Time::HiRes::time;
> use constant DEBUG => 0;
> foreach (1..50000000) {
> if (DEBUG) { printf qq[my debug line: %u], $_; }
> }
> my $t6 = Time::HiRes::time;
>
> printf qq[%f seconds for plain loop\n], $t2 - $t1;
> printf qq[%f seconds for negative if() loop \n], $t4 - $t3;
> printf qq[%f seconds for compiled out loop \n], $t6 - $t5;
>
> The output is:
>
> $ perl example.pl
> 1.372535 seconds for plain loop
> 1.899815 seconds for negative if() loop
> 1.370118 seconds for compiled out loop
>
> The nice thing is that "if (DEBUG) { printf qq[my debug line: %u], $_; }"
> line gets compiled away and there is no resulting negative if() at
> run-time. This technique is very useful for instrumenting Perl code with
> verbose logging which doesn't slow down production code (by executing many
> negative 'if' statements) when verbosity is dialed down.
>
> Is there an equivalent way to achieve the same performance results in
> julia?
>
> Thanks,
> Simon
>
