[NFC][Regalloc] Ensure Query::interferingVRegs is accurate.

To correctly use Query, one had to first call collectInterferingVRegs to
pre-cache the query result, then call interferingVRegs. Failing the
former, interferingVRegs could be stale. This did cause a bug which was
addressed in D98232, but the underlying usability issue of the Query API
wasn't.

This patch addresses the latter by making collectInterferingVRegs an
implementation detail, and having interferingVRegs play both roles. One
side-effect of this is that interferingVRegs is not const anymore.

Differential Revision: https://reviews.llvm.org/D112882

llvm/include/llvm/CodeGen/LiveIntervalUnion.h

@@ -114,12 +114,19 @@ class LiveIntervalUnion {
     const LiveRange *LR = nullptr;
     LiveRange::const_iterator LRI;  ///< current position in LR
     ConstSegmentIter LiveUnionI;    ///< current position in LiveUnion
-    Optional<SmallVector<LiveInterval *, 4>> InterferingVRegs;
+    SmallVector<LiveInterval *, 4> InterferingVRegs;
     bool CheckedFirstInterference = false;
     bool SeenAllInterferences = false;
     unsigned Tag = 0;
     unsigned UserTag = 0;
 
+    // Count the virtual registers in this union that interfere with this
+    // query's live virtual register, up to maxInterferingRegs.
+    unsigned collectInterferingVRegs(unsigned MaxInterferingRegs);
+
+    // Was this virtual register visited during collectInterferingVRegs?
+    bool isSeenInterference(LiveInterval *VirtReg) const;
+
   public:
     Query() = default;
     Query(const LiveRange &LR, const LiveIntervalUnion &LIU)
@@ -131,7 +138,7 @@ class LiveIntervalUnion {
                const LiveIntervalUnion &NewLiveUnion) {
       LiveUnion = &NewLiveUnion;
       LR = &NewLR;
-      InterferingVRegs = None;
+      InterferingVRegs.clear();
       CheckedFirstInterference = false;
       SeenAllInterferences = false;
       Tag = NewLiveUnion.getTag();
@@ -151,20 +158,12 @@ class LiveIntervalUnion {
     // Does this live virtual register interfere with the union?
     bool checkInterference() { return collectInterferingVRegs(1); }
 
-    // Count the virtual registers in this union that interfere with this
-    // query's live virtual register, up to maxInterferingRegs.
-    unsigned collectInterferingVRegs(
-        unsigned MaxInterferingRegs = std::numeric_limits<unsigned>::max());
-
-    // Was this virtual register visited during collectInterferingVRegs?
-    bool isSeenInterference(LiveInterval *VirtReg) const;
-
-    // Did collectInterferingVRegs collect all interferences?
-    bool seenAllInterferences() const { return SeenAllInterferences; }
-
     // Vector generated by collectInterferingVRegs.
-    const SmallVectorImpl<LiveInterval*> &interferingVRegs() const {
-      return *InterferingVRegs;
+    const SmallVectorImpl<LiveInterval *> &interferingVRegs(
+        unsigned MaxInterferingRegs = std::numeric_limits<unsigned>::max()) {
+      if (!SeenAllInterferences || MaxInterferingRegs < InterferingVRegs.size())
+        collectInterferingVRegs(MaxInterferingRegs);
+      return InterferingVRegs;
     }
   };
 

llvm/lib/CodeGen/LiveIntervalUnion.cpp

@@ -112,7 +112,7 @@ LiveInterval *LiveIntervalUnion::getOneVReg() const {
 // Scan the vector of interfering virtual registers in this union. Assume it's
 // quite small.
 bool LiveIntervalUnion::Query::isSeenInterference(LiveInterval *VirtReg) const {
-  return is_contained(*InterferingVRegs, VirtReg);
+  return is_contained(InterferingVRegs, VirtReg);
 }
 
 // Collect virtual registers in this union that interfere with this
@@ -124,14 +124,11 @@ bool LiveIntervalUnion::Query::isSeenInterference(LiveInterval *VirtReg) const {
 // 2. SeenAllInterferences == true: InterferingVRegs complete, iterators unused.
 // 3. Iterators left at the last seen intersection.
 //
-unsigned LiveIntervalUnion::Query::
-collectInterferingVRegs(unsigned MaxInterferingRegs) {
-  if (!InterferingVRegs)
-    InterferingVRegs.emplace();
-
+unsigned
+LiveIntervalUnion::Query::collectInterferingVRegs(unsigned MaxInterferingRegs) {
   // Fast path return if we already have the desired information.
-  if (SeenAllInterferences || InterferingVRegs->size() >= MaxInterferingRegs)
-    return InterferingVRegs->size();
+  if (SeenAllInterferences || InterferingVRegs.size() >= MaxInterferingRegs)
+    return InterferingVRegs.size();
 
   // Set up iterators on the first call.
   if (!CheckedFirstInterference) {
@@ -160,14 +157,14 @@ collectInterferingVRegs(unsigned MaxInterferingRegs) {
       LiveInterval *VReg = LiveUnionI.value();
       if (VReg != RecentReg && !isSeenInterference(VReg)) {
         RecentReg = VReg;
-        InterferingVRegs->push_back(VReg);
-        if (InterferingVRegs->size() >= MaxInterferingRegs)
-          return InterferingVRegs->size();
+        InterferingVRegs.push_back(VReg);
+        if (InterferingVRegs.size() >= MaxInterferingRegs)
+          return InterferingVRegs.size();
       }
       // This LiveUnion segment is no longer interesting.
       if (!(++LiveUnionI).valid()) {
         SeenAllInterferences = true;
-        return InterferingVRegs->size();
+        return InterferingVRegs.size();
       }
     }
 
@@ -188,7 +185,7 @@ collectInterferingVRegs(unsigned MaxInterferingRegs) {
     LiveUnionI.advanceTo(LRI->start);
   }
   SeenAllInterferences = true;
-  return InterferingVRegs->size();
+  return InterferingVRegs.size();
 }
 
 void LiveIntervalUnion::Array::init(LiveIntervalUnion::Allocator &Alloc,

llvm/lib/CodeGen/RegAllocBasic.cpp

@@ -217,9 +217,7 @@ bool RABasic::spillInterferences(LiveInterval &VirtReg, MCRegister PhysReg,
   // Collect interferences assigned to any alias of the physical register.
   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
-    Q.collectInterferingVRegs();
-    for (unsigned i = Q.interferingVRegs().size(); i; --i) {
-      LiveInterval *Intf = Q.interferingVRegs()[i - 1];
+    for (auto *Intf : reverse(Q.interferingVRegs())) {
       if (!Intf->isSpillable() || Intf->weight() > VirtReg.weight())
         return false;
       Intfs.push_back(Intf);

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -955,11 +955,12 @@ bool RAGreedy::canEvictInterference(
   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
     // If there is 10 or more interferences, chances are one is heavier.
-    if (Q.collectInterferingVRegs(10) >= 10)
+    const auto &Interferences = Q.interferingVRegs(10);
+    if (Interferences.size() >= 10)
       return false;
 
     // Check if any interfering live range is heavier than MaxWeight.
-    for (LiveInterval *Intf : reverse(Q.interferingVRegs())) {
+    for (LiveInterval *Intf : reverse(Interferences)) {
       assert(Register::isVirtualRegister(Intf->reg()) &&
              "Only expecting virtual register interference from query");
 
@@ -1037,7 +1038,6 @@ bool RAGreedy::canEvictInterferenceInRange(const LiveInterval &VirtReg,
 
   for (MCRegUnitIterator Units(PhysReg, TRI); Units.isValid(); ++Units) {
     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
-    Q.collectInterferingVRegs();
 
     // Check if any interfering live range is heavier than MaxWeight.
     for (const LiveInterval *Intf : reverse(Q.interferingVRegs())) {
@@ -1127,7 +1127,6 @@ void RAGreedy::evictInterference(LiveInterval &VirtReg, MCRegister PhysReg,
     // should be fast, we may need to recalculate if when different physregs
     // overlap the same register unit so we had different SubRanges queried
     // against it.
-    Q.collectInterferingVRegs();
     ArrayRef<LiveInterval*> IVR = Q.interferingVRegs();
     Intfs.append(IVR.begin(), IVR.end());
   }
@@ -2547,8 +2546,9 @@ bool RAGreedy::mayRecolorAllInterferences(
     LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, *Units);
     // If there is LastChanceRecoloringMaxInterference or more interferences,
     // chances are one would not be recolorable.
-    if (Q.collectInterferingVRegs(LastChanceRecoloringMaxInterference) >=
-        LastChanceRecoloringMaxInterference && !ExhaustiveSearch) {
+    if (Q.interferingVRegs(LastChanceRecoloringMaxInterference).size() >=
+            LastChanceRecoloringMaxInterference &&
+        !ExhaustiveSearch) {
       LLVM_DEBUG(dbgs() << "Early abort: too many interferences.\n");
       CutOffInfo |= CO_Interf;
       return false;