[PATCH] D129997: [Local] Allow creating callbr with duplicate successors

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers added a comment.

Basically, before early-tailduplication we have:

  bb.3.if.else.i.i:
  ; predecessors: %bb.2
    successors: %bb.5(0x80000000); %bb.5(100.00%)
  
    INLINEASM_BR &"" [sideeffect] [attdialect], $0:[imm], blockaddress(@ceph_con_v2_try_read, %ir-block.if.end.i)
    B %bb.5
  
  bb.5.if.end.i (address-taken, inlineasm-br-indirect-target):
  ; predecessors: %bb.3
    successors: %bb.1(0x80000000); %bb.1(100.00%)
  
    LIFETIME_END %stack.0.skip.i.i
    B %bb.1

after:

  bb.3.if.else.i.i:
  ; predecessors: %bb.2
    successors: %bb.1(0x80000000); %bb.1(100.00%)
  
    INLINEASM_BR &"" [sideeffect] [attdialect], $0:[imm], blockaddress(@ceph_con_v2_try_read, %ir-block.if.end.i)
    LIFETIME_END %stack.0.skip.i.i
    B %bb.1
  
  bb.5.if.end.i (address-taken, inlineasm-br-indirect-target):
    successors: %bb.1(0x80000000); %bb.1(100.00%)
  
    LIFETIME_END %stack.0.skip.i.i
    B %bb.1

`TailDuplicator::tailDuplicate`<llvm/lib/CodeGen/TailDuplicator.cpp> has code like:

   866     // Remove PredBB's unconditional branch.                                    
   867     TII->removeBranch(*PredBB);
  ...
   887     // Update the CFG.                                                          
   888     PredBB->removeSuccessor(PredBB->succ_begin());

which is going to mess up the TailBB's predecessor list. (Notice: it's broken in the after example).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D129997/new/

https://reviews.llvm.org/D129997