[PATCH] D75098: Add TCOPY, a terminator form of the COPY instr

[Bill Wendling via Phabricator via llvm-commits]

void added a comment.

In D75098#2021573 <https://reviews.llvm.org/D75098#2021573>, @qcolombet wrote:

> Thanks Bill for the detailed explanations. A couple more questions.
>
> > CodeGen wants registers to be spilled at the end of a block.
>
> That's fast reg alloc only. What is happening with the other allocators?


They don't appear to use the correct registers for the defined values.

>> The spill slot is recorded and used later on when accessing the value. If we just add a COPY into the default block it won't have the necessary information available to load the correct value.
> 
> Why is that? (Why doesn't it have the necessary information to load the correct value.)
> 
> I feel that we are trying to work around a bug/limitation in the fast register allocator where physreg are assumed not to cross basic block boundaries. I wonder if it wouldn't be easier to fix fast reg alloc if that is the problem.

It's not so much working around a bug/limitation in any of the register allocators. What I'm most concerned about is having to specify live-ins to MBBs before register allocation. That invariant was a huge sticking point in the review process for this feature. We're currently violating it because it "seems to work", but I don't want to rely on that.

Because of the live-ins restriction, all live physical registers at the end of an MBB need to be "spilled", or at least recorded in a way so that other blocks can access the information without having their live-ins set. (This is my understanding, it may be inaccurate). The `SelectionDAGBuilder::CopyToExportRegsIfNeeded()` function is one of the ways this is done.

> Out-of-curiosity, how do we deal with invoke? I would expect we have the exact same problems.

Invoke is turned into a regular call with exception handling stuff around it. E.g.:

  $ cat ex.cpp:
  void g();
  int f() {
    try {
      g();
    } catch (int&) {
      return 37;
    }
    return 1;
  }
  
  $ clang++ -mllvm -print-after-all ex.cpp -c -o /dev/null
  ...
  bb.0 (%ir-block.0):
    successors: %bb.1, %bb.2
  
    EH_LABEL <mcsymbol .Ltmp0>
    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
    CALL64pcrel32 @_Z1gv, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b $r13b $r14b $r15b $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp
    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
    EH_LABEL <mcsymbol .Ltmp1>
    JMP_1 %bb.1
  ...


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https://reviews.llvm.org/D75098