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Re: Jump optimizations (was Re: g77 performance on ALPHA)


On Thu, Sep 02, 1999 at 04:23:02PM -0400, John Wehle wrote:
> Were you speaking of both cases, or just about:

Both.

> The gcse pass runs before loop.  Are you suggesting adding another
> pass, or just making jump smarter by using information and routines
> supplied by gcse and lcm?

A new pass, perhaps run directly after cse2.  Or maybe better,
"run again after cse2", as gcse could certainly make use of a
tidier flow graph.

Much of jump.c should be ditched and rewritten -- adding
anything to jump_optimize et al is a losing proposition.

> Any feel for how useful value-range
> propogation is (outside of fixing the unroll problem)?

According to Law, and some friends of his that worked on it, it
doesn't do a whole lot for general C code.  There are a few more
cases that show up in Fortran code, e.g.

	do i=1,n
	  do j=1,n
	    ...
	  enddo
	enddo

where we currently have an extra check (1 < N) for the inner loop.

The biggest win is with anything that does array bounds checks.
If you do this, the Java folk will sing your praises.



r~


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