You might find this to be a fun example of how simple-seeming optimizations can
be much more trouble than you might expect.
For _years_, in Matlab (and perhaps other languages), x^2 was much, much
slower than x*x. You might think it's surprising that a company with
Mathworks' resources couldn't fix this for so long. In Julia we optimize it:
julia> foo1(x) = x*x
foo1 (generic function with 1 method)
julia> foo2(x) = x^2
foo2 (generic function with 1 method)
julia> foo3(x) = x^3
foo3 (generic function with 1 method)
julia> @code_llvm foo1(5)
define i64 @julia_foo1_21463(i64) {
top:
%1 = mul i64 %0, %0
ret i64 %1
}
julia> @code_llvm foo2(5)
define i64 @julia_foo2_21471(i64) {
top:
%1 = mul i64 %0, %0
ret i64 %1
}
julia> @code_llvm foo3(5)
define i64 @julia_foo3_21472(i64) {
top:
%1 = mul i64 %0, %0
%2 = mul i64 %1, %0
ret i64 %2
}
("mul" means multiply, which means it's expanding x^3 as (x*x)*x.)
BUT, this seemingly-trivial optimization proved to be a lot of hassle: see
https://github.com/JuliaLang/julia/issues/6506
and the discussions that preceded it.
Best,
--Tim
On Wednesday, May 11, 2016 02:34:56 AM Ford Ox wrote:
> From docs:
>
> macro r_str(p)
>
> > Regex(p)
> > end
> >
> >
> > That’s all. This macro says that the literal contents of the string
> > literal r"^\s*(?:#|$)" should be passed to the
> > @r_str macro and the result of that expansion should be placed in the
> > syntax tree where the string literal occurs. In
> > other words, the expression r"^\s*(?:#|$)" is equivalent to placing the
> > following object directly into the syntax
> > tree:
> > Regex("^\\s*(?:#|\$)")
> >
> >
> > Not only is the string literal form shorter and far more convenient, but
> > it is also more efficient: since the regular
> > expression is compiled and the Regex object is actually created when the
> > code is compiled, the compilation occurs
> > only once, rather than every time the code is executed. Consider if the
> > regular expression occurs in a loop:
> > for line = lines
> > m = match(r"^\s*(?:#|$)", line)
> > if m == nothing
> > # non-comment
> > else
> > # comment
> > end
> > end
> >
> > Shouldn't the compiler optimize this code *on its own* like this ?
>
> re = Regex("^\\s*(?:#|\$)")
> for line = lines
> m = match(re, line)
> if m == nothing
> # non-comment
> else
> # comment
> end
> end
>
> This is kind of optimization which is really easy to detect and and execute.
> How is that so that julia is not able to do that?
