[GISel]: Allow Unused G_PHIs to be DCEd

[aditya_nandakumar]

Summary

currently G_PHIs which are not used are still not cleaned up by DCE (as it's currently not considered safe to move). This patch changes that for G_PHI (not the PHI instruction).

[dsanders]

LGTM with a nit. Also we should consider whether we ought to be calling isSafeToMove() when we're not moving things.

lib/CodeGen/GlobalISel/Utils.cpp

140

As a side note, isSafeToMove() doesn't seem like the right check in any case since we're not planning to move it. We really want a mayHaveSideEffects() or isSafeToErase(). That said, the contents would be largely the same so it makes little difference aside from clearer labelling of the intent.

141

We should use isPhi() instead. It's just as valid to delete dead PHI's we have in the late stages of the pipeline as it is to delete dead G_PHI's in the early stages

[aditya_nandakumar]

Submitted in r344811

lib/CodeGen/GlobalISel/Utils.cpp

bool llvm::constrainSelectedInstRegOperands(MachineInstr &I,
   return true;
 }
 
 bool llvm::isTriviallyDead(const MachineInstr &MI,
                            const MachineRegisterInfo &MRI) {
   // If we can move an instruction, we can remove it.  Otherwise, it has
   // a side-effect of some sort.
   bool SawStore = false;
-  if (!MI.isSafeToMove(/*AA=*/nullptr, SawStore))
+  if (!MI.isSafeToMove(/*AA=*/nullptr, SawStore) &&
+      MI.getOpcode() != TargetOpcode::G_PHI)
     return false;
 
   // Instructions without side-effects are dead iff they only define dead vregs.
   for (auto &MO : MI.operands()) {
     if (!MO.isReg() || !MO.isDef())
       continue;
 
     unsigned Reg = MO.getReg();

test/CodeGen/AArch64/GlobalISel/legalize-phi.mir

body:             |
   bb.2:
     successors: %bb.3(0x80000000)
 
     %7(s32) = G_ADD %0, %6
     %8(s1) = G_TRUNC %7(s32)
 
   bb.3:
     %9(s1) = G_PHI %5(s1), %bb.1, %8(s1), %bb.2
+    %11:_(s1) = G_PHI %5(s1), %bb.1, %8(s1), %bb.2
     %10(s32) = G_ZEXT %9(s1)
     $w0 = COPY %10(s32)
     RET_ReallyLR implicit $w0
 
 ...
 ---
 name:            legalize_phi_ptr
 alignment:       2