[PATCH] MIR Serialization: Serialize MBB operands.

Duncan P. N. Exon Smith dexonsmith at apple.com
Tue Jun 23 12:43:23 PDT 2015

> On 2015-Jun-22, at 11:38, Alex Lorenz <arphaman at gmail.com> wrote:
> Hi dexonsmith, bob.wilson, bogner,
> This patch is based on a previous serialization patch that serializes null register operands (http://reviews.llvm.org/D10580).
> This patch serializes machine basic block operands.
> The following syntax is used for basic block operands:
>    %bb.<name>
>    %bb.<index>
> The name is printed when the machine basic block has a name, i.e. when its basic block has a name. Otherwise, a number is printed, which is just an index into a list of machine basic blocks for that function.
> Example:
>    JG_1 %bb.if.cond
>    JG_1 %bb.2

This is ambiguous in general.  IIUC, it's legal for two different
`MachineBasicBlock`s to point at the same `BasicBlock` (see, e.g.,
`BranchFolder::SplitMBBAt()`), so the name isn't a sufficient identifier.

I'm not such a big fan of your numbered identifiers either, since they're
going to be hard to read, the numbering isn't explicit in the source, and
the numbering is fragile (removing a basic block in the middle changes
all the numbers following).

Here's a different straw man proposal (I don't love it, so I hope you can
refine it to something better).

First, change the `name:` field to include an ID number.  Suppose you
have two BBs, %then and %else.  %then has two MBBs.  You'd get something
like the following:

    - name: 0.then
    - name: 0.else
    - name: 1.then

The syntax is `<num> ['.' <bb-name>]`.  (Maybe the `<num>` should be
specified in a separate field?)

Second, change the references to use both these numbers and the names.
Maybe, for these:


Unnamed basic blocks fall out naturally as:

    %bb.0 ; reference to the first unnamed basic block
    %bb.1 ; reference to the second unnamed basic block

These aren't alternate names, these are the only names.  And the `0` and
`1` are somehow defined explicitly in MIR.

Thoughts?  Other ideas?

>  rL LLVM
> http://reviews.llvm.org/D10608
> Files:
>  lib/CodeGen/MIRParser/MILexer.cpp
>  lib/CodeGen/MIRParser/MILexer.h
>  lib/CodeGen/MIRParser/MIParser.cpp
>  lib/CodeGen/MIRParser/MIParser.h
>  lib/CodeGen/MIRParser/MIRParser.cpp
>  lib/CodeGen/MIRPrinter.cpp
>  test/CodeGen/MIR/X86/large-index-number-error.mir
>  test/CodeGen/MIR/X86/machine-basic-block-operands.mir
>  test/CodeGen/MIR/X86/unknown-machine-basic-block.mir
>  test/CodeGen/MIR/X86/unknown-named-machine-basic-block.mir
>  http://reviews.llvm.org/settings/panel/emailpreferences/
> <D10608.28137.patch>

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