[llvm-commits] Target independent VLIW Packetizer

[Sundeep]

Hi All,

I have been working on a target independent VLIW packetizer in LLVM. On VLIW
machines, an instruction packet represents a group of instructions which can
be executed in parallel. The compiler is responsible for identifying
independent machine instructions and grouping those together into packets.
The VLIW packetizer in LLVM is implemented as follows:

Implementation Details:

1. The VLIW packetizer is implemented after all target specific passes are
run. This ensures that all pseudo instructions and spill code is enumerated
before packetization.

2. The VLIW packetizer extends "ScheduleDAGInstrs" class. The Packetizer
uses "BuildSchedGraph" API to build dependence graph. The dependence graph
along with deterministic finite automaton (DFA) is used to group independent
instructions into packets.

3. The instruction packets are finalized using "finalizeBundle" API. This
API inserts BUNDLE instructions and updates each packet instruction with
"insideBundle" bit.

4. The Hexagon backend assembly printer is updated to handle BUNDLE
instruction and print packet semantics correctly. Other VLIW targets will
also have to update the assembly printer.

The VLIW packetizer roughly implements the following algorithm:

  1 Instantiate DFA resource tracker
  2 For all basic blocks (BB) in Machine Function
  3 do
  4   reset DFA
  5   CurrentPacket = {}
  6   For all machine instructions (I) in BB
  7   do
  8     avail = DFA.IsAvailable(I)
  9     if (avail == true) {
 10       For all machine instructions (Ip) in CurrentPacket
 11       do
 12         if (DependenceGraph.HasDependence(I, Ip)) {
 13           if (Target.CanPruneDependence(I, Ip)) {
 14             // End current packet
 15             finalizeBundle
 16             reset DFA
 17             reset CurrentPacket
 18           }
 19         }
 20       done
 21     }
 22     else {
 23       // End current packet
 24       finalizeBundle
 25       reset DFA
 26       reset CurrentPacket
 27     }
 28
 29     // Add I to CurrentPacket
 30     CurrentPacket.Add(I)
 31   done
 32 done

I have already verified that it builds and runs regression test suites clean
on x86. I attaching the initial patch for review and comments.

Thanks,
Sundeep
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