Re: [patch] cse.c: Speed up cse_reg_info maintanance - Part 3

[Richard Earnshaw <rearnsha at gcc dot gnu dot org>]

On Tue, 2005-02-01 at 23:12, Kazu Hirata wrote:
> Hi,
> 
> Attached is a patch to speed up cse_reg_info maintainance by
> dramatically simplifying the memory allocation.
> 
> The patch replaces this complicated use of xrealloc with xmalloc.
> 
> Here is a timing in seconds for ./cc1 -quiet -O2 -o /dev/null.
> 
>              original patched   diff%
> c-common.i     18.334  18.231 -0.561%
> combine.i      17.475  17.425 -0.286%
> fold-const.i   38.048  37.938 -0.289%
> reload.i       13.497  13.444 -0.392%
> reload1.i      14.258  14.229 -0.203%
> cc1-i files   212.457 212.256 -0.094%
> 
> cc1-i files were compiled only once.
> 
> To my surprise, it's better to free memory when we are not using it,
> that is, between passes of cse_main.  I thought it would be a good
> idea to keep it if similar amount of memory is used over and over.
> Maybe other passes can use the same memory locations for other
> purposes and thereby improve locality!?  xrealloc may have been
> somewhat expensive.

It seems to me, that if it's just the cost of xrealloc copying the dead
memory that's slowing things down, then the following would probably be
a better patch.  This should also go some way to address Mike's concerns
since we now avoid mallocs whenever the table hasn't grown.

I guess there are some versions of realloc that can grow an existing
chunk of memory under some circumstances and that this can be cheaper
than mallocing a new block, but it's likely that the copy will always
dominate the common case.

This is untested, and posted purely by way of suggestion of an
alternative approach.

R.


Index: cse.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/cse.c,v
retrieving revision 1.339
diff -p -r1.339 cse.c
*** cse.c	1 Feb 2005 00:41:56 -0000	1.339
--- cse.c	2 Feb 2005 10:10:18 -0000
*************** init_cse_reg_info (unsigned int nregs)
*** 861,870 ****
  	  new_size = nregs;
  	}
  
!       /* Reallocate the table with NEW_SIZE entries.  */
!       cse_reg_info_table = xrealloc (cse_reg_info_table,
! 				     (sizeof (struct cse_reg_info)
! 				      * new_size));
        cse_reg_info_table_size = new_size;
      }
  
--- 861,874 ----
  	  new_size = nregs;
  	}
  
!       /* Reallocate the table with NEW_SIZE entries.  Don't use xrealloc,
! 	 since we don't need to copy the old data.  */
!       if (cse_reg_info_table)
! 	free (cse_reg_info_table);
! 
!       cse_reg_info_table = xmalloc (cse_reg_info_table,
! 				    (sizeof (struct cse_reg_info)
! 				     * new_size));
        cse_reg_info_table_size = new_size;
      }