[CodeGen] Improve the consistency of instruction fusion

[evandro]

Summary

When either instruction in a fused pair has no other dependency, besides on the other instruction, make sure that other instructions do not get scheduled between them. Additionally, avoid fusing an instruction more than once along the same dependency chain.

[evandro]

Expand the scope of the patch to make it more generic.

[fhahn]

I think this changes makes sense and thanks for getting rid of the if (that's not needed any longer) and making the debug message clearer too! I guess adding a test for the case where SecondSU has dependencies that are scheduled between FirstSU and SecondSU is difficult, as in most fused instruction pairs the second instruction only depends on the first instruction?

I am not sure about excluding control dependencies, but I am probably missing a constraint on control dependencies. Couldn't it be the case that an instruction has a control dependency on FirstSU, so after scheduling FirstSU, we could schedule this instruction instead of SecondSU?

[evandro]

There may be situations when the SecondSU has a couple of predecessors, but only one of them makes a pair and becomes the FirstSU. I think that if one chooses the scheduler to be only either top-down or bottom-up, this change would keep the results consistent.

Control dependencies are weaker than data dependencies and, since I could not find different results, it seemed sensible to trim the graph.

[fhahn]

In D36704#843847, @evandro wrote:

There may be situations when the SecondSU has a couple of predecessors, but only one of them makes a pair and becomes the FirstSU. I think that if one chooses the scheduler to be only either top-down or bottom-up, this change would keep the results consistent.

Yep that's a good call I think.

Control dependencies are weaker than data dependencies and, since I could not find different results, it seemed sensible to trim the graph.

OK. IIRC only adding artificial edges for data dependencies was something I tried in my initial patch, but there were some cases where the results are worse. I'll try to chase this up tomorrow.

[fhahn]

The following tests fail for me on latest master for me (check-llvm-codegen) with this patch applied.

LLVM :: CodeGen/AMDGPU/addrspacecast.ll
LLVM :: CodeGen/AMDGPU/macro-fusion-cluster-vcc-uses.mir
LLVM :: CodeGen/AMDGPU/select-vectors.ll
LLVM :: CodeGen/AMDGPU/udivrem.ll

[evandro]

Looping @arsenm in to understand these miscomparisons.

[evandro]

The AMDGPU tests failed with:

test/CodeGen/AMDGPU/addrspacecast.ll:272:7: error: expected string not found in input
; CI: s_lshr_b32 flat_scratch_hi, [[ADD]], 8
      ^
<stdin>:1645:2: note: scanning from here
 s_waitcnt lgkmcnt(0)
 ^
<stdin>:1645:2: note: with variable "ADD" equal to "s8"
 s_waitcnt lgkmcnt(0)
 ^
<stdin>:1661:41: note: possible intended match here
 .size store_flat_scratch, .Lfunc_end15-store_flat_scratch
                                        ^

test/CodeGen/AMDGPU/fceil64.ll:18:11: error: expected string not found in input
; SI-DAG: s_add_i32 [[SEXP0:s[0-9]+]], [[SEXP]], 0xfffffc01
          ^
<stdin>:27:2: note: scanning from here
 s_addk_i32 s7, 0xfc01
 ^
<stdin>:27:2: note: with variable "SEXP" equal to "s7"
 s_addk_i32 s7, 0xfc01
 ^

test/CodeGen/AMDGPU/macro-fusion-cluster-vcc-uses.mir:205:13: error: expected string not found in input
# GCN-NEXT: %3 = V_MOV_B32_e32 0, implicit %exec
            ^
<stdin>:447:2: note: scanning from here
 dead %6 = V_CNDMASK_B32_e64 %1, %3, %4, implicit %exec
 ^
<stdin>:447:22: note: possible intended match here
 dead %6 = V_CNDMASK_B32_e64 %1, %3, %4, implicit %exec
                     ^

test/CodeGen/AMDGPU/select-vectors.ll:219:8: error: expected string not found in input
; GCN: v_mov_b32_e32 v{{[0-9]+}}, s[[BLO]]
       ^
<stdin>:923:20: note: scanning from here
 v_cndmask_b32_e32 v1, v0, v1, vcc
                   ^
<stdin>:923:20: note: with variable "BLO" equal to "8"
 v_cndmask_b32_e32 v1, v0, v1, vcc
                   ^
<stdin>:925:2: note: possible intended match here
 v_cndmask_b32_e32 v0, v2, v3, vcc
 ^

[evandro]

Ping 🔔

[MatzeB]

llvm/lib/CodeGen/MacroFusion.cpp

38–46

Given the code below that adds all succs of first as succs of second, and vice versa for preds of seconds. How can the situation you are checking here even happen without us failing the cycle check in addEdge anyway?

74

Why did you move the SecondSU == ExitSU check into the loop? The condition won't change with different elements.

83–95

This seems unnecessary for SecondSU == ExitSU, maybe shortcut it?

119–122

I'm not sure if (A || (!B && !C)) continue is easier to understand than if (A || B || C) continue...

And I probably should have complained in D34144 already, but this comment doesn't relect reality anymore! And we should document our assumption that fusion can only happen when there is a data dependency or and Order dependency in case of physreg deps (as X86 eflags).

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

38–46

It can happen that an instr can be eligible for fusion for more than one other instruction. This check limits the fusion to the first seen eligible one.

83–95

Methinks that it is then, to avoid the predecessors of the ExitSU from being scheduled between it and the FirstSU.

[MatzeB]

llvm/lib/CodeGen/MacroFusion.cpp

38–46

Hmm ok I was thinking of this:

  A
 / \
B   C

If we see A+B first and fuse it the loop further down will add an extra edge between B and C:

  A
 / \
B  |
 \ |
   C

at this point fusing A+C is pointless because we always must schedule B in between. I was wrong however in thinking that the following addEdge would fail because of this. Nonetheless it would be good to have a check for this situation to avoid adding cluster edges for pairs that cannot be scheduled next to each other anyway. Maybe we should have reachability checks from FirstSU successors to SecondSU and bail if one is found.

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

38–46

What do you mean by reachability exactly? Also, do you think that addEdge() should have this check added?

[MatzeB]

llvm/lib/CodeGen/MacroFusion.cpp

38–46

Having a path between the two SDNodes, there is Topo.IsReachable(A, B). It is used by addEdge, canAddEdge already to avoid situations in which we would form cycles which would be plain invalid.
This case however would not be an invalid ScheduleDAG, just one where we would know beforehand that we cannot fulfill the cluster dependency. So this should be checked outside of addEdge() IMO.

[evandro]

Ping 🔔

[MatzeB]

You can push this if you want, but I still think that first too loops in fuseInstructionPair() are a poor mans version of the reachability check I outlined in the comments (which would catch more cases).

[evandro]

In D36704#893868, @MatzeB wrote:

You can push this if you want, but I still think that first too loops in fuseInstructionPair() are a poor mans version of the reachability check I outlined in the comments (which would catch more cases).

I thought that our comments crossed past each other. Let me give this some thought...

[evandro]

Add comment hopefully reflecting conversation at the US Developers Meeting.

[fhahn]

llvm/lib/CodeGen/MacroFusion.cpp

79

Would it make sense to move this code to a function (maybe just a closure in this function? I suspect the fact that we use isPred/isSucc makes things slightly tricky? :(

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

79

Where would you suggest, as a new method in SUinit?

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

79

Effectively, this test is equivalent to SDep::getKind() == SDep::Order || SDep::getKind() == SDep::Output.

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

79

Hit send before finishing editing the comment above.

Again, effectively, this test is equivalent to SDep::getKind() == SDep::Anti || SDep::getKind() == SDep::Output.

[evandro]

Move frequent check into separate function.

[evandro]

Ping 🔔

[fhahn]

Thanks Evandro. I think this looks good to me now. I still think it's unfortunate that we basically duplicate the loop to add artificial dependences, but I don't see a way to share the code without making it more complicated than it is now. Please wait a bit with committing, to give Matthias a chance to voice additional concerns.

llvm/lib/CodeGen/MacroFusion.cpp

79

Hard to tell in Phabricator, could you check if that could go on the previous line?

[evandro]

llvm/lib/CodeGen/MacroFusion.cpp

79

Yes, it would.

[evandro]

@MatzeB?

llvm/lib/CodeGen/MacroFusion.cpp

 
 using namespace llvm;
 
 static cl::opt<bool> EnableMacroFusion("misched-fusion", cl::Hidden,
   cl::desc("Enable scheduling for macro fusion."), cl::init(true));
 
 static void fuseInstructionPair(ScheduleDAGMI &DAG, SUnit &FirstSU,
                                 SUnit &SecondSU) {
   // Create a single weak edge between the adjacent instrs. The only effect is
   // to cause bottom-up scheduling to heavily prioritize the clustered instrs.
   DAG.addEdge(&SecondSU, SDep(&FirstSU, SDep::Cluster));
 
   // Adjust the latency between the anchor instr and its
   // predecessors.
   for (SDep &IDep : SecondSU.Preds)
     if (IDep.getSUnit() == &FirstSU)
       IDep.setLatency(0);
 
   // Adjust the latency between the dependent instr and its
   // predecessors.
   for (SDep &IDep : FirstSU.Succs)
     if (IDep.getSUnit() == &SecondSU)
       IDep.setLatency(0);
 
   DEBUG(dbgs() << DAG.MF.getName() << "(): Macro fuse ";
         FirstSU.print(dbgs(), &DAG); dbgs() << " - ";
         SecondSU.print(dbgs(), &DAG); dbgs() << " /  ";
         dbgs() << DAG.TII->getName(FirstSU.getInstr()->getOpcode()) << " - " <<
                   DAG.TII->getName(SecondSU.getInstr()->getOpcode()) << '\n'; );
 
   if (&SecondSU != &DAG.ExitSU)
-    // Make instructions dependent on FirstSU also dependent on SecondSU to
+    // Make data dependencies from FirstSU also dependent on SecondSU to
     // prevent them from being scheduled between FirstSU and and SecondSU.
     for (const SDep &SI : FirstSU.Succs) {
-      if (SI.getSUnit() == &SecondSU)
+      if (SI.isCtrl() || SI.getSUnit() == &SecondSU)
         continue;
       DEBUG(dbgs() << "  Copy Succ ";
             SI.getSUnit()->print(dbgs(), &DAG); dbgs() << '\n';);
       DAG.addEdge(SI.getSUnit(), SDep(&SecondSU, SDep::Artificial));
     }
 
   ++NumFused;
 }
 
 namespace {
 
 /// \brief Post-process the DAG to create cluster edges between instrs that may
 /// be fused by the processor into a single operation.
 class MacroFusion : public ScheduleDAGMutation {
   ShouldSchedulePredTy shouldScheduleAdjacent;
   bool FuseBlock;
   bool scheduleAdjacentImpl(ScheduleDAGMI &DAG, SUnit &AnchorSU);
 
 public:
   MacroFusion(ShouldSchedulePredTy shouldScheduleAdjacent, bool FuseBlock)
     : shouldScheduleAdjacent(shouldScheduleAdjacent), FuseBlock(FuseBlock) {}
 
   void apply(ScheduleDAGInstrs *DAGInstrs) override;
 };
 
 } // end anonymous namespace
 
 void MacroFusion::apply(ScheduleDAGInstrs *DAGInstrs) {
   ScheduleDAGMI *DAG = static_cast<ScheduleDAGMI*>(DAGInstrs);
 
   if (FuseBlock)
     // For each of the SUnits in the scheduling block, try to fuse the instr in
     // it with one in its predecessors.
     for (SUnit &ISU : DAG->SUnits)
         scheduleAdjacentImpl(*DAG, ISU);
 
   if (DAG->ExitSU.getInstr())
     // Try to fuse the instr in the ExitSU with one in its predecessors.
     scheduleAdjacentImpl(*DAG, DAG->ExitSU);
 }
 
 /// \brief Implement the fusion of instr pairs in the scheduling DAG,
 /// anchored at the instr in AnchorSU..
 bool MacroFusion::scheduleAdjacentImpl(ScheduleDAGMI &DAG, SUnit &AnchorSU) {
   const MachineInstr &AnchorMI = *AnchorSU.getInstr();
   const TargetInstrInfo &TII = *DAG.TII;
   const TargetSubtargetInfo &ST = DAG.MF.getSubtarget();
 
   // Check if the anchor instr may be fused.
   if (!shouldScheduleAdjacent(TII, ST, nullptr, AnchorMI))
     return false;
 
   // Explorer for fusion candidates among the dependencies of the anchor instr.
   for (SDep &Dep : AnchorSU.Preds) {
     // Ignore dependencies that don't enforce ordering.
     if (Dep.getKind() == SDep::Anti || Dep.getKind() == SDep::Output ||
         Dep.isWeak())
       continue;
 
     SUnit &DepSU = *Dep.getSUnit();
     if (DepSU.isBoundaryNode())
       continue;
 
     const MachineInstr *DepMI = DepSU.getInstr();
     if (!shouldScheduleAdjacent(TII, ST, DepMI, AnchorMI))
       continue;