llvm dev - Sep 2016 - scheduler options documentation?

I see that there are several options to influence instruction scheduling,
but there doesn't seem to be a lot of information about what they do, for
example:

-misched-topdown -misched-bottomup

The description in MachineScheduler.cpp says:

"Force top-down list scheduling" and "Force bottom-up list
scheduling"

Which isn't too helpful - where might I want to use these? Under what
conditions might they improve an instruction schedule?

Similar questions for the instruction scheduler strategies:

-misched=(default, converge,
ilpmax, ilpmin, or shuffle)

And for the analysis heuristics:

-misched-cluster,
-misched-cyclicpath, -misched-regpressure, and -misched-
fusion

Are these things mutually exclusive or can they be combined? What
combination will give the most rigorous analysis and improve scheduling (at
the cost of longer runtime of llc)

Phil
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As a follow on question: which of these options are already enabled when
compiling with -O3?

Phil

On Mon, Sep 12, 2016 at 10:37 AM, Phil Tomson <phil.a.tomson at gmail.com>
wrote:
> I see that there are several options to influence instruction scheduling,
> but there doesn't seem to be a lot of information about what they do,
for
> example:
>
> -misched-topdown -misched-bottomup
>
> The description in MachineScheduler.cpp says:
>
> "Force top-down list scheduling" and "Force bottom-up list
scheduling"
>
> Which isn't too helpful - where might I want to use these? Under what
> conditions might they improve an instruction schedule?
>
> Similar questions for the instruction scheduler strategies:
>
> -misched=(default, converge,
> ilpmax, ilpmin, or shuffle)
>
> And for the analysis heuristics:
>
> -misched-cluster,
> -misched-cyclicpath, -misched-regpressure, and -misched-
> fusion
>
> Are these things mutually exclusive or can they be combined? What
> combination will give the most rigorous analysis and improve scheduling (at
> the cost of longer runtime of llc)
>
> Phil
>
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I think those should be considered internal options for (llvm codegen)
developers, a normal compiler user should not need to use any of them. Of course
you can look around the llvm sourcecode (esp. in
lib/CodeGen/MachineScheduler.cpp) and see what cl::opt options are available and
how they affect the scheduler operation.

In case of the MachineScheduler all settings are the same regardless of the
optimisation level, however the MachineScheduler is disabled as a whole in
TargetPassConfig.cpp when optimized regalloc is not enabled (which I believe
happens for everything except -O0).

- Matthias
> On Sep 12, 2016, at 10:41 AM, Phil Tomson via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> As a follow on question: which of these options are already enabled when
compiling with -O3?
> 
> Phil
> 
> On Mon, Sep 12, 2016 at 10:37 AM, Phil Tomson <phil.a.tomson at
gmail.com <mailto:phil.a.tomson at gmail.com>> wrote:
> I see that there are several options to influence instruction scheduling,
but there doesn't seem to be a lot of information about what they do, for
example:
> 
> -misched-topdown -misched-bottomup
> 
> The description in MachineScheduler.cpp says:
> 
> "Force top-down list scheduling" and "Force bottom-up list
scheduling"
> 
> Which isn't too helpful - where might I want to use these? Under what
conditions might they improve an instruction schedule?
> 
> Similar questions for the instruction scheduler strategies: 
> 
> -misched=(default, converge,
> ilpmax, ilpmin, or shuffle)
> 
> And for the analysis heuristics: 
> 
> -misched-cluster,
> -misched-cyclicpath, -misched-regpressure, and -misched-
> fusion
> 
> Are these things mutually exclusive or can they be combined? What
combination will give the most rigorous analysis and improve scheduling (at the
cost of longer runtime of llc)
> 
> Phil
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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