Re: missed optimization, would be very helpful

[Andreas Schwab <schwab at LS5 dot informatik dot uni-dortmund dot de>]

Zack Weinberg <zack@rabi.columbia.edu> writes:

|> In a loop of the form
|> 
|> while (condition) (*int_ptr)++;
|> 
|> gcc generates a read-mod-write cycle to the memory location *int_ptr at each
|> iteration of the loop.  Modern chips really don't like that; I got a ~30%
|> speedup on one piece of code by rewriting it to bump a register and write
|> the memory location at the end.  The pointer isn't volatile, so I believe
|> there is nothing stopping gcc from doing this optimization.
|> 
|> This is egcs-1.1.1; it might've been fixed more recently but I can't test
|> that since the Dec 24 CVS is unreliable on my system.
|> 
|> (If anyone wants an example, look at cpplib.c:adjust_position.)

This is a bad example, because linep and colp may alias each other, and a
char * can alias everything.

-- 
Andreas Schwab                                      "And now for something
schwab@issan.cs.uni-dortmund.de                      completely different"
schwab@gnu.org