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

[Jeff Law <law at redhat dot com>]

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.

Jeff