[RISCV] Minimally modify incoming state in transferBefore (#72352)

transferBefore currently takes an incoming state and an instruction,
computes
the new state needed for the instruction, and then modifies that new
state to
be more similar to the incoming state.

This patch reverses the approach by instead taking the incoming state
and
modifying only the bits that are demanded by the instruction.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -491,6 +491,8 @@ class VSETVLIInfo {
 
   unsigned getSEW() const { return SEW; }
   RISCVII::VLMUL getVLMUL() const { return VLMul; }
+  bool getTailAgnostic() const { return TailAgnostic; }
+  bool getMaskAgnostic() const { return MaskAgnostic; }
 
   bool hasNonZeroAVL(const MachineRegisterInfo &MRI) const {
     if (hasAVLImm())
@@ -1040,9 +1042,13 @@ bool RISCVInsertVSETVLI::needVSETVLI(const MachineInstr &MI,
   return true;
 }
 
-// Given an incoming state reaching MI, modifies that state so that it is minimally
-// compatible with MI.  The resulting state is guaranteed to be semantically legal
-// for MI, but may not be the state requested by MI.
+static VSETVLIInfo adjustIncoming(VSETVLIInfo PrevInfo, VSETVLIInfo NewInfo,
+                                  DemandedFields &Demanded,
+                                  const MachineRegisterInfo *MRI);
+
+// Given an incoming state reaching MI, minimally modifies that state so that it
+// is compatible with MI. The resulting state is guaranteed to be semantically
+// legal for MI, but may not be the state requested by MI.
 void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
                                         const MachineInstr &MI) const {
   uint64_t TSFlags = MI.getDesc().TSFlags;
@@ -1055,20 +1061,47 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
     return;
 
   const VSETVLIInfo PrevInfo = Info;
-  Info = NewInfo;
+  if (Info.hasSEWLMULRatioOnly() || !Info.isValid() || Info.isUnknown())
+    Info = NewInfo;
 
-  if (!RISCVII::hasVLOp(TSFlags))
+  if (!RISCVII::hasVLOp(TSFlags)) {
+    Info = NewInfo;
     return;
+  }
+
+  DemandedFields Demanded = getDemanded(MI, MRI, ST);
+  const VSETVLIInfo IncomingInfo =
+      adjustIncoming(PrevInfo, NewInfo, Demanded, MRI);
+
+  if (Demanded.usedVL())
+    Info.setAVL(IncomingInfo);
+
+  Info.setVTYPE(
+      ((Demanded.LMUL || Demanded.SEWLMULRatio) ? IncomingInfo : Info)
+          .getVLMUL(),
+      ((Demanded.SEW || Demanded.SEWLMULRatio) ? IncomingInfo : Info).getSEW(),
+      // Prefer tail/mask agnostic since it can be relaxed to undisturbed later
+      // if needed.
+      (Demanded.TailPolicy ? IncomingInfo : Info).getTailAgnostic() ||
+          IncomingInfo.getTailAgnostic(),
+      (Demanded.MaskPolicy ? IncomingInfo : Info).getMaskAgnostic() ||
+          IncomingInfo.getMaskAgnostic());
+}
+
+static VSETVLIInfo adjustIncoming(VSETVLIInfo PrevInfo, VSETVLIInfo NewInfo,
+                                  DemandedFields &Demanded,
+                                  const MachineRegisterInfo *MRI) {
+  VSETVLIInfo Info = NewInfo;
 
   // If we don't use LMUL or the SEW/LMUL ratio, then adjust LMUL so that we
   // maintain the SEW/LMUL ratio. This allows us to eliminate VL toggles in more
   // places.
-  DemandedFields Demanded = getDemanded(MI, MRI, ST);
   if (!Demanded.LMUL && !Demanded.SEWLMULRatio && PrevInfo.isValid() &&
       !PrevInfo.isUnknown()) {
     if (auto NewVLMul = RISCVVType::getSameRatioLMUL(
             PrevInfo.getSEW(), PrevInfo.getVLMUL(), Info.getSEW()))
       Info.setVLMul(*NewVLMul);
+    Demanded.LMUL = true;
   }
 
   // If we only demand VL zeroness (i.e. vmv.s.x and vmv.x.s), then there are
@@ -1080,8 +1113,12 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
   // to prevent extending live range of an avl register operand.
   // TODO: We can probably relax this for immediates.
   if (Demanded.VLZeroness && !Demanded.VLAny && PrevInfo.isValid() &&
-      PrevInfo.hasEquallyZeroAVL(Info, *MRI) && Info.hasSameVLMAX(PrevInfo))
+      PrevInfo.hasEquallyZeroAVL(Info, *MRI) && Info.hasSameVLMAX(PrevInfo)) {
     Info.setAVL(PrevInfo);
+    Demanded.demandVL();
+  }
+
+  return Info;
 }
 
 // Given a state with which we evaluated MI (see transferBefore above for why