[RISCV][InsertVSETVLI] Rework code structure to make reasoning about undefined lanes explicit [NFC]

We already have several places in this code which reason about whether the inactive lanes are defined, and are about to add one more in D151653. Let's go ahead and common the code so that we don't have the same concept repeating in multiply places.

Differential Revision: https://reviews.llvm.org/D152844

Author: preames

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -136,6 +136,26 @@ static bool isMaskRegOp(const MachineInstr &MI) {
   return Log2SEW == 0;
 }
 
+/// Return true if the inactive elements in the result are entirely undefined.
+/// Note that this is different from "agnostic" as defined by the vector
+/// specification.  Agnostic requires each lane to either be undisturbed, or
+/// take the value -1; no other value is allowed.
+static bool hasUndefinedMergeOp(const MachineInstr &MI,
+                                const MachineRegisterInfo &MRI) {
+
+  unsigned UseOpIdx;
+  if (!MI.isRegTiedToUseOperand(0, &UseOpIdx))
+    // If there is no passthrough operand, then the pass through
+    // lanes are undefined.
+    return true;
+
+  // If the tied operand is an IMPLICIT_DEF, the pass through lanes
+  // are undefined.
+  const MachineOperand &UseMO = MI.getOperand(UseOpIdx);
+  MachineInstr *UseMI = MRI.getVRegDef(UseMO.getReg());
+  return UseMI && UseMI->isImplicitDef();
+}
+
 /// Which subfields of VL or VTYPE have values we need to preserve?
 struct DemandedFields {
   // Some unknown property of VL is used.  If demanded, must preserve entire
@@ -315,14 +335,13 @@ DemandedFields getDemanded(const MachineInstr &MI,
     Res.LMUL = false;
     Res.SEWLMULRatio = false;
     Res.VLAny = false;
-    // For vmv.s.x and vfmv.s.f, if writing to an implicit_def operand, we don't
+    // For vmv.s.x and vfmv.s.f, if the merge operand is *undefined*, we don't
     // need to preserve any other bits and are thus compatible with any larger,
     // etype and can disregard policy bits.  Warning: It's tempting to try doing
     // this for any tail agnostic operation, but we can't as TA requires
     // tail lanes to either be the original value or -1.  We are writing
     // unknown bits to the lanes here.
-    auto *VRegDef = MRI->getVRegDef(MI.getOperand(1).getReg());
-    if (VRegDef && VRegDef->isImplicitDef()) {
+    if (hasUndefinedMergeOp(MI, *MRI)) {
       Res.SEW = DemandedFields::SEWGreaterThanOrEqual;
       Res.TailPolicy = false;
     }
@@ -691,9 +710,9 @@ static VSETVLIInfo computeInfoForInstr(const MachineInstr &MI, uint64_t TSFlags,
                                        const MachineRegisterInfo *MRI) {
   VSETVLIInfo InstrInfo;
 
-  bool TailAgnostic, MaskAgnostic;
-  unsigned UseOpIdx;
-  if (MI.isRegTiedToUseOperand(0, &UseOpIdx)) {
+  bool TailAgnostic = true;
+  bool MaskAgnostic = true;
+  if (!hasUndefinedMergeOp(MI, *MRI)) {
     // Start with undisturbed.
     TailAgnostic = false;
     MaskAgnostic = false;
@@ -708,27 +727,13 @@ static VSETVLIInfo computeInfoForInstr(const MachineInstr &MI, uint64_t TSFlags,
       MaskAgnostic = Policy & RISCVII::MASK_AGNOSTIC;
     }
 
-    // If the tied operand is an IMPLICIT_DEF we can use TailAgnostic and
-    // MaskAgnostic.
-    const MachineOperand &UseMO = MI.getOperand(UseOpIdx);
-    MachineInstr *UseMI = MRI->getVRegDef(UseMO.getReg());
-    if (UseMI && UseMI->isImplicitDef()) {
-      TailAgnostic = true;
-      MaskAgnostic = true;
-    }
     // Some pseudo instructions force a tail agnostic policy despite having a
     // tied def.
     if (RISCVII::doesForceTailAgnostic(TSFlags))
       TailAgnostic = true;
 
     if (!RISCVII::usesMaskPolicy(TSFlags))
       MaskAgnostic = true;
-  } else {
-    // If there is no tied operand,, there shouldn't be a policy operand.
-    assert(!RISCVII::hasVecPolicyOp(TSFlags) && "Unexpected policy operand");
-    // No tied operand use agnostic policies.
-    TailAgnostic = true;
-    MaskAgnostic = true;
   }
 
   RISCVII::VLMUL VLMul = RISCVII::getLMul(TSFlags);
@@ -890,23 +895,20 @@ bool RISCVInsertVSETVLI::needVSETVLI(const MachineInstr &MI,
 
   DemandedFields Used = getDemanded(MI, MRI);
 
-  // A slidedown/slideup with an IMPLICIT_DEF merge op can freely clobber
+  // A slidedown/slideup with an *undefined* merge op can freely clobber
   // elements not copied from the source vector (e.g. masked off, tail, or
   // slideup's prefix). Notes:
   // * We can't modify SEW here since the slide amount is in units of SEW.
   // * VL=1 is special only because we have existing support for zero vs
   //   non-zero VL.  We could generalize this if we had a VL > C predicate.
   // * The LMUL1 restriction is for machines whose latency may depend on VL.
-  // * As above, this is only legal for IMPLICIT_DEF, not TA.
+  // * As above, this is only legal for tail "undefined" not "agnostic".
   if (isVSlideInstr(MI) && Require.hasAVLImm() && Require.getAVLImm() == 1 &&
-      isLMUL1OrSmaller(CurInfo.getVLMUL())) {
-    auto *VRegDef = MRI->getVRegDef(MI.getOperand(1).getReg());
-    if (VRegDef && VRegDef->isImplicitDef()) {
-      Used.VLAny = false;
-      Used.VLZeroness = true;
-      Used.LMUL = false;
-      Used.TailPolicy = false;
-    }
+      isLMUL1OrSmaller(CurInfo.getVLMUL()) && hasUndefinedMergeOp(MI, *MRI)) {
+    Used.VLAny = false;
+    Used.VLZeroness = true;
+    Used.LMUL = false;
+    Used.TailPolicy = false;
   }
 
   if (CurInfo.isCompatible(Used, Require, *MRI))