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Add high level description of MachineInstr bundles.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146589 91177308-0d34-0410-b5e6-96231b3b80d8
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Evan Cheng 2011-12-14 21:32:14 +00:00
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@ -50,6 +50,7 @@
<li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
class</a></li>
<li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
<li><a href="#machineinstrbundle"><tt>MachineInstr Bundles</tt></a></li>
</ul>
</li>
<li><a href="#mc">The "MC" Layer</a>
@ -761,6 +762,88 @@ ret
</div>
<!-- ======================================================================= -->
<h3>
<a name="machineinstrbundle"><tt>MachineInstr Bundles</tt></a>
</h3>
<div>
<p>LLVM code generator can model sequences of instructions as MachineInstr
bundles. A MI bundle can model a VLIW group / pack which contains an
arbitrary number of parallel instructions. It can also be used to model
a sequential list of instructions (potentially with data dependencies) that
cannot be legally separated (e.g. ARM Thumb2 IT blocks).</p>
<p>Conceptually a MI bundle is a MI with a number of other MIs nested within:
</p>
<div class="doc_code">
<pre>
--------------
| Bundle | ---------
-------------- \
| ----------------
| | MI |
| ----------------
| |
| ----------------
| | MI |
| ----------------
| |
| ----------------
| | MI |
| ----------------
|
--------------
| Bundle | --------
-------------- \
| ----------------
| | MI |
| ----------------
| |
| ----------------
| | MI |
| ----------------
| |
| ...
|
--------------
| Bundle | --------
-------------- \
|
...
</pre>
</div>
<p> MI bundle support does not change the physical representations of
MachineBasicBlock and MachineInstr. All the MIs (including top level and
nested ones) are stored as sequential list of MIs. The "bundled" MIs are
marked with the 'InsideBundle' flag. A top level MI with the special BUNDLE
opcode is used to represent the start of a bundle. It's legal to mix BUNDLE
MIs with indiviual MIs that are not inside bundles nor represent bundles.
</p>
<p> MachineInstr passes should operate on a MI bundle as a single unit. Member
methods have been taught to correctly handle bundles and MIs inside bundles.
The MachineBasicBlock iterator has been modified to skip over bundled MIs to
enforce the bundle-as-a-single-unit concept. An alternative iterator
instr_iterator has been added to MachineBasicBlock to allow passes to
iterate over all of the MIs in a MachineBasicBlock, including those which
are nested inside bundles. The top level BUNDLE instruction must have the
correct set of register MachineOperand's that represent the cumulative
inputs and outputs of the bundled MIs.</p>
<p> Packing / bundling of MachineInstr's should be done as part of the register
allocation super-pass. More specifically, the pass which determines what
MIs should be bundled together must be done after code generator exits SSA
form (i.e. after two-address pass, PHI elimination, and copy coalescing).
Bundles should only be finalized (i.e. adding BUNDLE MIs and input and
output register MachineOperands) after virtual registers have been
rewritten into physical registers. This requirement eliminates the need to
add virtual register operands to BUNDLE instructions which would effectively
double the virtual register def and use lists.</p>
</div>
<!-- *********************************************************************** -->