[RISCV] Use DenseMap to track V0 definition. NFC (#84465)

Reviving some of the progress on #71764. To recap, we explored removing
the V0 register copies to simplify the pass, but hit a limitation with
the register allocator due to our use of the vmv0 singleton reg class
and early-clobber constraints.

So since we will have to continue to track the definition of V0
ourselves, this patch simplifies it by storing it in a map. It will
allow us to move about copies to V0 in #71764 without having to do extra
bookkeeping.

Author: lukel97

llvm/lib/Target/RISCV/RISCVFoldMasks.cpp

@@ -47,10 +47,13 @@ class RISCVFoldMasks : public MachineFunctionPass {
   StringRef getPassName() const override { return "RISC-V Fold Masks"; }
 
 private:
-  bool convertToUnmasked(MachineInstr &MI, MachineInstr *MaskDef) const;
-  bool convertVMergeToVMv(MachineInstr &MI, MachineInstr *MaskDef) const;
+  bool convertToUnmasked(MachineInstr &MI) const;
+  bool convertVMergeToVMv(MachineInstr &MI) const;
 
-  bool isAllOnesMask(MachineInstr *MaskDef) const;
+  bool isAllOnesMask(const MachineInstr *MaskDef) const;
+
+  /// Maps uses of V0 to the corresponding def of V0.
+  DenseMap<const MachineInstr *, const MachineInstr *> V0Defs;
 };
 
 } // namespace
@@ -59,10 +62,9 @@ char RISCVFoldMasks::ID = 0;
 
 INITIALIZE_PASS(RISCVFoldMasks, DEBUG_TYPE, "RISC-V Fold Masks", false, false)
 
-bool RISCVFoldMasks::isAllOnesMask(MachineInstr *MaskDef) const {
-  if (!MaskDef)
-    return false;
-  assert(MaskDef->isCopy() && MaskDef->getOperand(0).getReg() == RISCV::V0);
+bool RISCVFoldMasks::isAllOnesMask(const MachineInstr *MaskDef) const {
+  assert(MaskDef && MaskDef->isCopy() &&
+         MaskDef->getOperand(0).getReg() == RISCV::V0);
   Register SrcReg = TRI->lookThruCopyLike(MaskDef->getOperand(1).getReg(), MRI);
   if (!SrcReg.isVirtual())
     return false;
@@ -89,8 +91,7 @@ bool RISCVFoldMasks::isAllOnesMask(MachineInstr *MaskDef) const {
 
 // Transform (VMERGE_VVM_<LMUL> false, false, true, allones, vl, sew) to
 // (VMV_V_V_<LMUL> false, true, vl, sew). It may decrease uses of VMSET.
-bool RISCVFoldMasks::convertVMergeToVMv(MachineInstr &MI,
-                                        MachineInstr *V0Def) const {
+bool RISCVFoldMasks::convertVMergeToVMv(MachineInstr &MI) const {
 #define CASE_VMERGE_TO_VMV(lmul)                                               \
   case RISCV::PseudoVMERGE_VVM_##lmul:                                         \
     NewOpc = RISCV::PseudoVMV_V_V_##lmul;                                      \
@@ -116,7 +117,7 @@ bool RISCVFoldMasks::convertVMergeToVMv(MachineInstr &MI,
     return false;
 
   assert(MI.getOperand(4).isReg() && MI.getOperand(4).getReg() == RISCV::V0);
-  if (!isAllOnesMask(V0Def))
+  if (!isAllOnesMask(V0Defs.lookup(&MI)))
     return false;
 
   MI.setDesc(TII->get(NewOpc));
@@ -133,14 +134,13 @@ bool RISCVFoldMasks::convertVMergeToVMv(MachineInstr &MI,
   return true;
 }
 
-bool RISCVFoldMasks::convertToUnmasked(MachineInstr &MI,
-                                       MachineInstr *MaskDef) const {
+bool RISCVFoldMasks::convertToUnmasked(MachineInstr &MI) const {
   const RISCV::RISCVMaskedPseudoInfo *I =
       RISCV::getMaskedPseudoInfo(MI.getOpcode());
   if (!I)
     return false;
 
-  if (!isAllOnesMask(MaskDef))
+  if (!isAllOnesMask(V0Defs.lookup(&MI)))
     return false;
 
   // There are two classes of pseudos in the table - compares and
@@ -198,20 +198,26 @@ bool RISCVFoldMasks::runOnMachineFunction(MachineFunction &MF) {
   // $v0:vr = COPY %mask:vr
   // %x:vr = Pseudo_MASK %a:vr, %b:br, $v0:vr
   //
-  // Because $v0 isn't in SSA, keep track of it so we can check the mask operand
-  // on each pseudo.
-  MachineInstr *CurrentV0Def;
-  for (MachineBasicBlock &MBB : MF) {
-    CurrentV0Def = nullptr;
-    for (MachineInstr &MI : MBB) {
-      Changed |= convertToUnmasked(MI, CurrentV0Def);
-      Changed |= convertVMergeToVMv(MI, CurrentV0Def);
+  // Because $v0 isn't in SSA, keep track of its definition at each use so we
+  // can check mask operands.
+  for (const MachineBasicBlock &MBB : MF) {
+    const MachineInstr *CurrentV0Def = nullptr;
+    for (const MachineInstr &MI : MBB) {
+      if (MI.readsRegister(RISCV::V0, TRI))
+        V0Defs[&MI] = CurrentV0Def;
 
       if (MI.definesRegister(RISCV::V0, TRI))
         CurrentV0Def = &MI;
     }
   }
 
+  for (MachineBasicBlock &MBB : MF) {
+    for (MachineInstr &MI : MBB) {
+      Changed |= convertToUnmasked(MI);
+      Changed |= convertVMergeToVMv(MI);
+    }
+  }
+
   return Changed;
 }