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Re: Combined top-down and bottom-up instruction scheduler


On 09/08/2015 01:24 PM, Aditya K wrote:


----------------------------------------
Subject: Re: Combined top-down and bottom-up instruction scheduler
To: hiraditya@msn.com; gcc@gcc.gnu.org
CC: vmakarov@redhat.com
From: law@redhat.com
Date: Tue, 8 Sep 2015 12:51:24 -0600

On 09/08/2015 12:39 PM, Aditya K wrote:
IIUC, in the haifa-sched.c, the default scheduling algorithm seems to
be top-down (before reload). Is there a way to schedule the other way
(bottom up), or both ways?
Not that I'm aware of. Note that region scheduling allows insns to move
between basic blocks to help fill the bubbles that can occur at the end
of a block.


As a use case for bottom-up or some other heuristic: Currently, the
first priority in the selection is given to the longest path, in some
cases this may produce code with stalls at the end of the basic
block. Whereas in the case of combined top-down + bottom-up
scheduling we would end up having stalls in the middle of the basic
block.
GCC's original scheduler worked bottom-up until ~1997. IBM Haifa's work
turned it into a top-down model and was a small, but clear improvement.

There's certainly better things that can be done than strictly top-down
or bottom-up, but revamping the scheduler again hasn't been seen as a
major win for the most common processors GCC targets these days. Thus
it hasn't been a significant area of focus.

Do you have pointers on places to look for if I want to explore bottom-up, or maybe a combination of the two.

Not immediately handy. I'd comb through PLDI through the 1990s and early 2000s and possibly Morgan's compiler book.

jeff


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