[MIR] Update MIRLangRef with bundled instructions

[MatzeB]

docs/MIRLangRef.rst

393–395

Ugh, I never noticed the MI syntax makes a distinction between "inside" and "outside" of a bundle (I assume that notion comes from MachineInstr::isInsideBundle()). Unfortunately that notion doesn't always make sense as we can have bundles without a header. In which case the current syntax can be somewhat confusing :-(

Because of this I would recommend to not talk about "instructions starting a bundle" or "being outside/inside of a bundle" to not confuse the matter even more. Maybe something like this instead:

The first instruction is often a bundle header. The instructions between ``{`` and ``}`` are bundled with the first instruction.

I would tweak the example a bit to be something that would actually be valid MI:

BUNDLE implicit-def %r0, implicit-def %r1, implicit %r2 {
  %r0 = SOME_OP %r2
  %r1 = ANOTHER_OP internal %0
}

397–404

Did this syntax emerge by accident (I don't see what it is good for)? Should we maybe not document it?

[thegameg]

Go with Matthias' explanation.

docs/MIRLangRef.rst

393–395

Thanks, makes more sense.

397–404

We explicitly handle this case in the MIR parser, but I can't see why. Should we get rid of it?

[MatzeB]

LGTM

docs/MIRLangRef.rst

.. code-block:: text
     %fp = frame-setup ADDXri %sp, 0, 0
 
 .. code-block:: text
 
     %x21, %x20 = frame-destroy LDPXi %sp
 
 .. _registers:
 
+Bundled Instructions
+^^^^^^^^^^^^^^^^^^^^
+
+The syntax for bundled instructions is the following:
+
+.. code-block:: text
+
+    %r0 = IMPLICIT_DEF {
+      %r1 = IMPLICIT_DEF
+      %r2 = IMPLICIT_DEF
+    }
+
+where the first instruction ``%r0 = IMPLICIT_DEF`` is **starting** the bundle,
+but it is **outside** the bundle. The two other instructions are **inside** the
+bundle. The ``}`` ends the bundle.
+
+The following syntax where the first instruction **inside** the bundle is on
+the same line is also accepted:
+
+.. code-block:: text
+
+    %r0 = IMPLICIT_DEF { %r1 = IMPLICIT_DEF
+      %r2 = IMPLICIT_DEF
+    }
+
 Registers
 ---------
 
 Registers are one of the key primitives in the machine instructions
 serialization language. They are primarly used in the
 :ref:`register machine operands <register-operands>`,
 but they can also be used in a number of other places, like the
 :ref:`basic block's live in list <bb-liveins>`.

 For an int eq predicate ``ICMP_EQ``, the syntax is:
 
 .. code-block:: text
 
    %2:gpr(s32) = G_ICMP intpred(eq), %0, %1
 
 .. TODO: Describe the parsers default behaviour when optional YAML attributes
    are missing.
-.. TODO: Describe the syntax for the bundled instructions.
 .. TODO: Describe the syntax for virtual register YAML definitions.
 .. TODO: Describe the machine function's YAML flag attributes.
 .. TODO: Describe the syntax for the register mask machine operands.
 .. TODO: Describe the frame information YAML mapping.
 .. TODO: Describe the syntax of the stack object machine operands and their
    YAML definitions.
 .. TODO: Describe the syntax of the block address machine operands.
 .. TODO: Describe the syntax of the metadata machine operands, and the
    instructions debug location attribute.
 .. TODO: Describe the syntax of the register live out machine operands.
 .. TODO: Describe the syntax of the machine memory operands.