[llvm-dev] Why not do machine instruction scheduling in SSA form?

[Matthias Braun via llvm-dev]

A motivation for scheduling later is that the program representation is closer to the final instruction stream which makes the machine simulation more accurate. If you schedule too early you do not see the instructions produced by phi elimination and the two address fixup pass.

LiveIntervals should work on MachineSSA form. If it doesn't you should file a bugzilla ticket with more details.

- Matthias

> On Jun 27, 2016, at 4:39 AM, Xing Su via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi LLVM community,
> 
> Currently LLVM backend do pre-RA machine instruction scheduling in non-SSA form, I doubt why not do machine scheduling in SSA machine instruction form? Now LLVM’s machine scheduling uses a list-scheduling algorithm, but if we wang to support more complex scheduling algorithms, for example, modulo scheduling for loops, it seems more easy to accomplish this in SSA form as SSA is more suitable for tracking dependencies and doing code motion.
> 
> I find that LiveIntervals analysis pass, which is required by RegPressureTracker and MachineScheduler, cannot run before phi elimination. Is this one of the reason that machine scheduling is not done in SSA form?
> 
> Any explanation is appreciated. Thanks a lot!
> 
> 
> 
> Cheers,
> Xing
> 
> 
> 
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