[llvm-dev] Help in understanding physreg LiveVariables

[Tyler Kenney via llvm-dev]

Okay, will try it out. Thanks a lot for the help, I think it's becoming
more and more clear that I ought to re-design the frontend and make all the
decisions about which buffers are going to live on the stack and which are
going to live in the register file during a new pre-ISel IR-level pass. It
seems like a design of that nature would be much more consistent with llvm
and allow both ISel and RA to function much more normally, but I'm hoping
to get away with the current design for at least a little while longer
while I finalize some post-ISel stuff.

Cheers,
Tyler




From:	Matthias Braun <mbraun at apple.com>
To:	Tyler Kenney/Marlborough/IBM at IBMUS
Cc:	llvm-dev at lists.llvm.org
Date:	08/23/2016 05:37 PM
Subject:	Re: [llvm-dev] Help in understanding physreg LiveVariables
Sent by:	mbraun at apple.com



This is probably the first target building PHIs in the selection DAG, but I
don't see a reason why it wouldn't work.

- Matthias

      On Aug 23, 2016, at 2:35 PM, Tyler Kenney <tjkenney at us.ibm.com>
      wrote:



      So if I first create the value in an entry BB and then build a
      CopyToReg but then I have to read it in a BB that loops back to
      itself, with it's own CopyToReg at the end, then I have two CopyToReg
      nodes for the same value. In this case, I need to create 3 virt regs,
      1 for each CopyToReg and a third for the CopyFromReg in the beginning
      of the loop BB, right? And then I need to build a PHI instruction at
      the beginning of the loop MBB that relates the 3 virt regs, right? Is
      there anything else I need to do regarding the creation of the new
      phi instruction, do you know?

      Tyler


      <graycol.gif>Matthias Braun ---08/23/2016 05:17:14 PM---> On Aug 23,
      2016, at 2:07 PM, Tyler Kenney <tjkenney at us.ibm.com> wrote: >

      From: Matthias Braun <mbraun at apple.com>
      To: Tyler Kenney/Marlborough/IBM at IBMUS
      Cc: llvm-dev at lists.llvm.org
      Date: 08/23/2016 05:17 PM
      Subject: Re: [llvm-dev] Help in understanding physreg LiveVariables
      Sent by: mbraun at apple.com





                  On Aug 23, 2016, at 2:07 PM, Tyler Kenney <
                  tjkenney at us.ibm.com> wrote:


                  So if I create a value with a DAG.getUndef(myVT); call
                  during instruction legalization, how can I access that
                  value as input in another BB/DAG (also during instruction
                  legalization) without worrying about live-ins and/or phi
                  nodes?

                  Can I create a single virtual register and build both a
                  CopyToReg and a CopyFromReg node with it? I assumed that
                  would break SSA.


      Yes in Selection to you need to use CopyToReg/CoptFromReg to write
      to/from vregs that should cross basic blocks. Yes indeed this
      currently has to respect SSA form, so there should only be a single
      CopyToReg for each vreg and that def should dominate all users.

                  Perhaps I should have said that what stops me is that I
                  don't know how to pass a newly created virtual register
                  from one MBB/DAG to another.
      You have to remember the number somewhere, so you can use CopyFromReg
      on the vreg.

      - Matthias



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