[llvm-dev] Bundling MachineInstr instructions before register allocation seems to always give errors

[Alex Susu via llvm-dev]

   Hello.
     I come back to this more than 1 year old topic on bundles. I need to create bundles 
before register allocation to avoid generating COPY instructions in the bundle.

     Is now the bundles API more reliable - were the patches for pre-ra support discussed 
below made mainstream? I ask because, as already discussed, the solution to avoid 
generating COPY instructions, etc in a block of MachineInstr is to bundle them - this was 
discussed also at http://lists.llvm.org/pipermail/llvm-dev/2016-October/106553.html 
("spills, reloads, splits or further scheduling sneaking instruction in between").

   Thank you,
     Alex

On 2/24/2017 6:09 AM, Matthias Braun wrote:
> In theory it should be possible. In practice you will run into issues because nobody is
> really doing that right now so some things like liveness updates are semi-broken. I am
> currently working on some patches myself for bundling some things pre-ra myself. I
> should put up some fixes for the infrastructure soon(ish).
>
> - Matthias
>
>> On Feb 23, 2017, at 6:45 AM, Alex Susu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Hello. I am having difficulties to bundle MachineInstr instructions, before register
>> allocation (RA). More exactly, I registered a simple pass in addPreRegAlloc() that is
>> trying to create bundles.
>>
>> I see that it is written at
>> http://llvm.org/docs/CodeGenerator.html#machineinstr-bundles: <<Packing / bundling of
>> MachineInstr’s should be done as part of the register allocation super-pass. More
>> specifically, the pass which determines what MIs should be bundled together must be
>> done after code generator exits SSA form (i.e. after two-address pass, PHI
>> elimination, and copy coalescing). Bundles should only be finalized (i.e. adding
>> BUNDLE MIs and input and output register MachineOperands) after virtual registers
>> have been rewritten into physical registers. This requirement eliminates the need to
>> add virtual register operands to BUNDLE instructions which would effectively double
>> the virtual register def and use lists.>>
>>
>> I presume this is the reason why I'm getting always errors after creating bundles in
>> the pass registered in addPreRegAlloc(). More exactly, my pass seems to be invoked
>> before the "Eliminate PHI nodes for register allocation" pass.
>>
>> If however, there is a simple possibility to bundle MachineInstr before RA, please
>> let me know.
>>
>> Thank you, Alex
>>
>> PS: Is there a possibility to bundle MachineInstr instructions before RA and use
>> llvm::finalizeBundle() on these bundles in a pass invoked after RA?
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>
>