Path: utzoo!mnetor!uunet!husc6!purdue!decwrl!megatest!djones
From: djones@megatest.UUCP (Dave Jones)
Newsgroups: comp.lang.c
Subject: Re: Put your code... (was Re: gotos
Message-ID: <504@goofy.megatest.UUCP>
Date: 4 May 88 22:08:50 GMT
References: <211@obie.UUCP>
Organization: Megatest Corporation, San Jose, Ca
Lines: 29

in article <211@obie.UUCP>, wes@obie.UUCP (Barnacle Wes) says:
> 

...

> ...  "common subexpression removal."
> It is quite often used to optimize for space (less code space is
> required, obviously).  It would NOT commonly be used if you are
> looking for the utmost in speed, the time spent jumping into/out of
> the common code does not make for faster programs.
> -- 

Nope, that's not what "common subexpression removal" is.  Consider this
Pascal fragment:


	A[i].k := A[i].k + j;

The expression A[i].k occurs twice.  Furthermore, its "L-value", or
location, must evaluate to the same address when each subexpression is
evaluated.  So a clever compiler will only calculate the address of
A[i].k once, and will leave it in a register to be reused.  There is no 
"jumping into/out of the common code".  

In Pascal programs, this optimization will often increase speed by
thirty percent or more.  Depends on the program.


	-- Dave J.