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

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
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.


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