[CodeGen] Untangle RegisterCoalescer from LRE's ScannedRemattable flag [nfc[ (#159839)

LiveRangeEdit's rematerialization checking logic is used in two quite
different ways. For SplitKit and InlineSpiller, we're analyzing all defs
associated with a live interval, doing that analysis up front, and then
using the result a bit later. The RegisterCoalescer, we're analysing
exactly one ValNo at a time, and using the legality result immediately.
LRE had a checkRematerializable which existed basically to adapt the
later into the former usage model.

Instead, this change bypasses the ScannedRemat and Remattable
structures, and directly queries the underlying routines. This is easy
to read, and makes it more clear as to which uses actually need the
deferred analysis. (A following change may try to unwind that too, but
it's not strictly NFC.)

Author: preames

llvm/include/llvm/CodeGen/LiveRangeEdit.h

@@ -176,14 +176,10 @@ class LiveRangeEdit : private MachineRegisterInfo::Delegate {
   Register create() { return createFrom(getReg()); }
 
   /// anyRematerializable - Return true if any parent values may be
-  /// rematerializable.
-  /// This function must be called before any rematerialization is attempted.
+  /// rematerializable.  This function must be called before
+  /// canRematerializeAt is called..
   bool anyRematerializable();
 
-  /// checkRematerializable - Manually add VNI to the list of rematerializable
-  /// values if DefMI may be rematerializable.
-  bool checkRematerializable(VNInfo *VNI, const MachineInstr *DefMI);
-
   /// Remat - Information needed to rematerialize at a specific location.
   struct Remat {
     const VNInfo *const ParentVNI;  // parent_'s value at the remat location.

llvm/lib/CodeGen/LiveRangeEdit.cpp

@@ -68,16 +68,6 @@ Register LiveRangeEdit::createFrom(Register OldReg) {
   return VReg;
 }
 
-bool LiveRangeEdit::checkRematerializable(VNInfo *VNI,
-                                          const MachineInstr *DefMI) {
-  assert(DefMI && "Missing instruction");
-  ScannedRemattable = true;
-  if (!TII.isTriviallyReMaterializable(*DefMI))
-    return false;
-  Remattable.insert(VNI);
-  return true;
-}
-
 void LiveRangeEdit::scanRemattable() {
   for (VNInfo *VNI : getParent().valnos) {
     if (VNI->isUnused())
@@ -90,7 +80,8 @@ void LiveRangeEdit::scanRemattable() {
     MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
     if (!DefMI)
       continue;
-    checkRematerializable(OrigVNI, DefMI);
+    if (TII.isTriviallyReMaterializable(*DefMI))
+      Remattable.insert(OrigVNI);
   }
   ScannedRemattable = true;
 }

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1325,9 +1325,7 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   if (!TII->isAsCheapAsAMove(*DefMI))
     return false;
 
-  SmallVector<Register, 8> NewRegs;
-  LiveRangeEdit Edit(&SrcInt, NewRegs, *MF, *LIS, nullptr, this);
-  if (!Edit.checkRematerializable(ValNo, DefMI))
+  if (!TII->isTriviallyReMaterializable(*DefMI))
     return false;
 
   if (!definesFullReg(*DefMI, SrcReg))
@@ -1395,15 +1393,18 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     }
   }
 
-  LiveRangeEdit::Remat RM(ValNo);
-  RM.OrigMI = DefMI;
-  if (!Edit.canRematerializeAt(RM, ValNo, CopyIdx))
+  SmallVector<Register, 8> NewRegs;
+  LiveRangeEdit Edit(&SrcInt, NewRegs, *MF, *LIS, nullptr, this);
+  SlotIndex DefIdx = LIS->getInstructionIndex(*DefMI);
+  if (!Edit.allUsesAvailableAt(DefMI, DefIdx, CopyIdx))
     return false;
 
   DebugLoc DL = CopyMI->getDebugLoc();
   MachineBasicBlock *MBB = CopyMI->getParent();
   MachineBasicBlock::iterator MII =
       std::next(MachineBasicBlock::iterator(CopyMI));
+  LiveRangeEdit::Remat RM(ValNo);
+  RM.OrigMI = DefMI;
   Edit.rematerializeAt(*MBB, MII, DstReg, RM, *TRI, false, SrcIdx, CopyMI);
   MachineInstr &NewMI = *std::prev(MII);
   NewMI.setDebugLoc(DL);