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Re: Combined top-down and bottom-up instruction scheduler
- From: Jeff Law <law at redhat dot com>
- To: Aditya K <hiraditya at msn dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Cc: "vmakarov at redhat dot com" <vmakarov at redhat dot com>
- Date: Tue, 8 Sep 2015 12:51:24 -0600
- Subject: Re: Combined top-down and bottom-up instruction scheduler
- Authentication-results: sourceware.org; auth=none
- References: <BLU179-W6188AF5C5B442B4282F2F0B6530 at phx dot gbl>
On 09/08/2015 12:39 PM, Aditya K wrote:
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.
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?
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.
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
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.