sed Question

unihiekka · 09-25-2009, 03:09 AM

Hi!

I have just generated a very large (header) file for C/C++, containing the definition of a rather lengthy mathematical function, with Mathematica. However, since the math function is to be evaluated many times, I want to optimize it. Currently, I have it as a switch-case, but in each case there are often the same basic evaluations several times, e.g.

Code:

...

case intnum:
z = A*pow(x,2)*(B*pow(y,3.8)*pow(x,2)-C*pow(y,3.8)*pow(x,6));
break;
...

where z is the return value (usually double, but it's templated to type Dbl, as the function supports multiple precision), and x,y, A B C are doubles as well. Since pow(x,2) and pow(y,3.8) are in the evaluation more than once, I would like to replace it with something like:

Code:

...

case intnum:
Dbl X2 = pow(x,2);
Dbl Y38 = pow(y,3.8);
z = A*X2*(B*Y38*pow(x,2)-C*Y38*pow(x,6));
break;
...

Is it possible to use sed for this and what would a script look like? I need to loop through all instances, and the check has to be done within each case-break block, and if (and only if) there is an expression of the form pow(xy,exp), with xy either x or y and exp a numerical value, it should replace it with some new definition, and add the corresponding line right after the case statement (or just before z = ...). I'm quite sure sed can do this, but this is way beyond me...
I don't really care about the name, but if being specific helps, then something like xbdpPadp or ybdpPadp would be nice, where bdp is the part before the decimal point and adp is the part after the decimal point. So, above it would be x2d or x2d0, and y3d8.

-----
Yes, I know pow(x,6) = pow(X2,3), but it's only evaluated once in my example, so it's not really a big deal in my computation.

lutusp · 09-25-2009, 03:23 AM

Quote:

Originally Posted by unihiekka

Hi!

I have just generated a very large (header) file for C/C++, containing the definition of a rather lengthy mathematical function, with Mathematica. However, since the math function is to be evaluated many times, I want to optimize it. Currently, I have it as a switch-case, but in each case there are often the same basic evaluations several times, e.g.

Code:

...

case intnum:
z = A*pow(x,2)*(B*pow(y,3.8)*pow(x,2)-C*pow(y,3.8)*pow(x,6));
break;
...

where z is the return value (usually double, but it's templated to type Dbl, as the function supports multiple precision), and x,y, A B C are doubles as well. Since pow(x,2) and pow(y,3.8) are in the evaluation more than once, I would like to replace it with something like:

Code:

...

case intnum:
Dbl X2 = pow(x,2);
Dbl Y38 = pow(y,3.8);
z = A*X2*(B*Y38*pow(x,2)-C*Y38*pow(x,6));
break;
...

Is it possible to use sed for this and what would a script look like? I need to loop through all instances, and the check has to be done within each case-break block, and if there is a pow(xy,exp), with xy either x or y and exp a value of type Dbl, it should replace it with some new definition (I don't really care about its name), and add the corresponding line right after the case statement (or just before z = ...). I'm quite sure sed can do this, but this is way beyond me...

-----
Yes, I know pow(x,6) = pow(X2,3), but it's only evaluated once in my example, so it's not really a big deal in my computation.

I do quite a lot of this kind of thing (optimizing expressions derived from Mathematica and other computer algebra systems) and I must say I have not found any alternatives to hand-tuning, e.g. locating and manually separating common subexpressions.

I don't think this sort of thing can be meaningfully automated, at least not with present methods and tools. And I'm not saying this lightly -- I've given this quite a lot of thought, and the outcome matters to me as well.

unihiekka · 09-25-2009, 05:43 AM

It's good to know that I'm not alone with my problem. Just a question, though, is my request somewhat irrelevant since 'most' compilers do common subexpression replacement automatically if you turn the optimization options on? See e.g. here

Still, I would be interested to know if there are sed/perl/bash/... gurus out there who would have a brilliant idea.

unihiekka · 09-25-2009, 12:08 PM

For mathematica, this seems a nice way to get the job done.