Revert "[RegAlloc] Relax the split constrain on MBB prolog" (#169990)

Reverts #168259

breaks hip buildot

Author: ronlieb

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -774,7 +774,8 @@ bool RAGreedy::addSplitConstraints(InterferenceCache::Cursor Intf,
       // Abort if the spill cannot be inserted at the MBB' start
       if (((BC.Entry == SpillPlacement::MustSpill) ||
            (BC.Entry == SpillPlacement::PrefSpill)) &&
-          !SA->canSplitBeforeProlog(BC.Number))
+          SlotIndex::isEarlierInstr(BI.FirstInstr,
+                                    SA->getFirstSplitPoint(BC.Number)))
         return false;
     }
 
@@ -829,7 +830,11 @@ bool RAGreedy::addThroughConstraints(InterferenceCache::Cursor Intf,
     BCS[B].Number = Number;
 
     // Abort if the spill cannot be inserted at the MBB' start
-    if (!SA->canSplitBeforeProlog(Number))
+    MachineBasicBlock *MBB = MF->getBlockNumbered(Number);
+    auto FirstNonDebugInstr = MBB->getFirstNonDebugInstr();
+    if (FirstNonDebugInstr != MBB->end() &&
+        SlotIndex::isEarlierInstr(LIS->getInstructionIndex(*FirstNonDebugInstr),
+                                  SA->getFirstSplitPoint(Number)))
       return false;
     // Interference for the live-in value.
     if (Intf.first() <= Indexes->getMBBStartIdx(Number))

llvm/lib/CodeGen/SplitKit.cpp

@@ -147,54 +147,6 @@ InsertPointAnalysis::getLastInsertPointIter(const LiveInterval &CurLI,
   return LIS.getInstructionFromIndex(LIP);
 }
 
-bool InsertPointAnalysis::canSplitBeforeProlog(const LiveInterval &CurLI,
-                                               const MachineBasicBlock &MBB) {
-  const TargetInstrInfo *TII = MBB.getParent()->getSubtarget().getInstrInfo();
-
-  for (auto &MI : MBB) {
-    if (MI.isPHI() || MI.isPosition() || MI.isDebugInstr() ||
-        MI.isPseudoProbe())
-      continue;
-
-    if (!TII->isBasicBlockPrologue(MI))
-      return true;
-
-    for (auto &MO : MI.operands()) {
-      if (!MO.isReg() || !MO.isDef() || !MO.getReg().isVirtual())
-        continue;
-
-      // For the AMDGPU target if a MBB contains exec mask restore preamble,
-      // SplitEditor may get state when it cannot insert a spill instruction
-      // at the begin of the MBB.
-      // E.g. for a MIR
-      // bb.100:
-      //     %1 = S_OR_SAVEEXEC_B64 %2, implicit-def $exec, implicit-def $scc,
-      //          implicit $exec
-      //     ...
-      //     use %1
-      // If the regalloc try to allocate a virtreg to the physreg already
-      // assigned to virtreg %1 and the pyhsreg is computed as the best
-      // candidate for split, it may insert COPY instruction.
-      //  bb.100:
-      //     %1 = S_OR_SAVEEXEC_B64 %2, implicit-def $exec, implicit-def $scc,
-      //          implicit $exec
-      //     %2 = COPY %orig
-      //     ...
-      //     use %1
-      // Thus %1 and %orig still have interference. We may add cost for the
-      // physreg candidate or abandon the candidate.
-      const MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
-      const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
-      const TargetRegisterClass *RC = MRI.getRegClass(MO.getReg());
-      const TargetRegisterClass *CurRC = MRI.getRegClass(CurLI.reg());
-      if (TRI->getCommonSubClass(RC, CurRC))
-        return false;
-    }
-  }
-
-  return true;
-}
-
 //===----------------------------------------------------------------------===//
 //                                 Split Analysis
 //===----------------------------------------------------------------------===//

llvm/lib/CodeGen/SplitKit.h

@@ -89,9 +89,6 @@ class LLVM_LIBRARY_VISIBILITY InsertPointAnalysis {
     return Res;
   }
 
-  /// Return true if we can split \pCurLI before \pMBB's prolog.
-  bool canSplitBeforeProlog(const LiveInterval &CurLI,
-                            const MachineBasicBlock &MBB);
 };
 
 /// SplitAnalysis - Analyze a LiveInterval, looking for live range splitting
@@ -250,11 +247,6 @@ class LLVM_LIBRARY_VISIBILITY SplitAnalysis {
   SlotIndex getFirstSplitPoint(unsigned Num) {
     return IPA.getFirstInsertPoint(*MF.getBlockNumbered(Num));
   }
-
-  bool canSplitBeforeProlog(unsigned Num) {
-    MachineBasicBlock *BB = MF.getBlockNumbered(Num);
-    return IPA.canSplitBeforeProlog(*CurLI, *BB);
-  }
 };
 
 /// SplitEditor - Edit machine code and LiveIntervals for live range