[Power9] Disable removing extra swaps on P9 since it should not be needed.

[stefanp]

Summary

On power 8 we sometimes insert swaps to deal with the difference between Little Endian and Big Endian. The swap removal pass is supposed to clean up these swaps. On power 9 we don't need this pass since we do not need to insert the swaps in the first place.

[nemanjai]

LGTM.

lib/Target/PowerPC/PPCTargetMachine.cpp

369 ↗

(On Diff #103964)

Please add a comment such as

// Power ISA 3.0 has load/store instructions that preserve
// element order so swaps aren't introduced. The pass to remove
// them is thereby not necessary.

[inouehrs]

lib/Target/PowerPC/PPCTargetMachine.cpp

371 ↗

(On Diff #103964)

I feel it is better to use Subtarget.needsSwapsForVSXMemOps() to make the meaning of the check clearer.

[nemanjai]

lib/Target/PowerPC/PPCTargetMachine.cpp

371 ↗

(On Diff #103964)

This is actually a really good point. I forgot that we added that function. Please use that instead.

[jtony]

lib/Target/PowerPC/PPCTargetMachine.cpp

371 ↗

(On Diff #103964)

I agree with Hiroshi, this is a good way to increase code readability.

[stefanp]

Sorry... We can't do what I wanted to do in the first place because I don't have access to the PPCSubtarget object in the PPCTargetMachine code.
I've moved the guard into the PPCVSXSwapRemoval code.

[echristo]

This is how you should do it. Though I'm not sure it isn't just better to check for "processor >= power9", but *shrug* for now.

I've also marked getSubtargetImpl() as delete so you won't be tempted again. :)

llvm/trunk/lib/Target/PowerPC/PPCVSXSwapRemoval.cpp

 
 public:
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
     if (skipFunction(*MF.getFunction()))
       return false;
 
     // If we don't have VSX on the subtarget, don't do anything.
+    // Also, on Power 9 the load and store ops preserve element order and so
+    // the swaps are not required.
     const PPCSubtarget &STI = MF.getSubtarget<PPCSubtarget>();
-    if (!STI.hasVSX())
+    if (!STI.hasVSX() || !STI.needsSwapsForVSXMemOps())
       return false;
 
     bool Changed = false;
     initialize(MF);
 
     if (gatherVectorInstructions()) {
       formWebs();
       recordUnoptimizableWebs();