[SYCL][Native CPU][NFC] Increase safety. (#20173)

Since we rely on LoopMasks[...] not creating new map entries, since they
should have been created in advance, use LoopMasks.at(...) instead. This
will ensure that if new changes are introduced that result in any loop
masks being missing, it will fail reliably and be much easier to debug.

This commit fixes one issue with BOSCC, but this is still NFC as at the
moment, BOSCC is never used.

Author: hvdijk

llvm/lib/SYCLNativeCPUUtils/compiler_passes/vecz/source/transform/control_flow_conversion_pass.cpp

@@ -115,6 +115,15 @@ class ControlFlowConversionState::Impl : public ControlFlowConversionState {
     Instruction *persistedCombinedDivergentExitMask = nullptr;
   };
 
+  struct LoopMasksInfoMap : DenseMap<Loop *, LoopMasksInfo> {
+    using DenseMap::DenseMap;
+
+    using DenseMap::at;
+    LoopMasksInfo &at(Loop *val) {
+      return const_cast<LoopMasksInfo &>(DenseMap::at(val));
+    }
+  };
+
   /// @brief Create loop masks for the specified loop and its subloops.
   /// @param[in] L Loop which should have its LoopMasksInfo created.
   void createLoopMasks(Loop *L);
@@ -360,7 +369,7 @@ class ControlFlowConversionState::Impl : public ControlFlowConversionState {
 
   BasicBlock *functionExitBlock = nullptr;
   DenseSet<const Instruction *> blends;
-  DenseMap<Loop *, LoopMasksInfo> LoopMasks;
+  LoopMasksInfoMap LoopMasks;
 };
 
 STATISTIC(VeczCFGFail,
@@ -555,11 +564,6 @@ bool ControlFlowConversionState::Impl::convertToDataFlow() {
   LI = &AM.getResult<LoopAnalysis>(F);
   UVR = &AM.getResult<UniformValueAnalysis>(F);
 
-  // Make sure every loop has an entry in the masks table before we start.
-  for (auto *L : *LI) {
-    createLoopMasks(L);
-  }
-
   if (!VU.choices().linearizeBOSCC()) {
     ROSCCGadget ROSCC(*this);
     ROSCC.run(F);
@@ -579,6 +583,13 @@ bool ControlFlowConversionState::Impl::convertToDataFlow() {
     }
   }
 
+  // Make sure every loop has an entry in the masks table.
+  // Do this after duplicateUniformRegions() since it may create additional
+  // loops.
+  for (auto *L : *LI) {
+    createLoopMasks(L);
+  }
+
   // Reserve space for the masks table and default-construct all entries, to
   // avoid re-hashing/element relocation on access.
   MaskInfos.reserve(F.size());
@@ -960,7 +971,7 @@ bool ControlFlowConversionState::Impl::createExitMasks(BasicBlock &BB,
 }
 
 bool ControlFlowConversionState::Impl::createLoopExitMasks(LoopTag &LTag) {
-  auto &LMask = LoopMasks[LTag.loop];
+  auto &LMask = LoopMasks.at(LTag.loop);
   // If the Loop already has a CombinedExitMasks we have already processed it.
   if (LMask.combinedDivergentExitMask) {
     return true;
@@ -1038,7 +1049,7 @@ bool ControlFlowConversionState::Impl::createLoopExitMasks(LoopTag &LTag) {
       if (exitingLTag->loop != LTag.loop) {
         if (Loop *nestedLoop = nextInnerLoopLeft(exitingBlock, exitBlock)) {
           maskUpdateOperand =
-              LoopMasks[nestedLoop]
+              LoopMasks.at(nestedLoop)
                   .updatedPersistedDivergentExitMasks[exitingBlock];
         }
       }
@@ -1095,7 +1106,7 @@ bool ControlFlowConversionState::Impl::createCombinedLoopExitMask(
   SmallVector<Loop::Edge, 1> exitEdges;
   auto *const Loop = LTag.loop;
   Loop->getExitEdges(exitEdges);
-  auto &LMask = LoopMasks[Loop];
+  auto &LMask = LoopMasks.at(Loop);
   for (const Loop::Edge &EE : exitEdges) {
     BasicBlock *exitingBlock = const_cast<BasicBlock *>(EE.first);
     BasicBlock *exitBlock = const_cast<BasicBlock *>(EE.second);
@@ -1836,7 +1847,7 @@ bool ControlFlowConversionState::Impl::computeDivergentLoopPureExit(
   BasicBlockTag &pureExitTag = DR->getOrCreateTag(pureExit);
 
   // Set the tags.
-  auto &LMask = LoopMasks[LTag.loop];
+  auto &LMask = LoopMasks.at(LTag.loop);
   MaskInfos[pureExit].entryMask = LMask.persistedCombinedDivergentExitMask;
   pureExitTag.outermostExitedLoop = &LTag;
 
@@ -2174,7 +2185,7 @@ bool ControlFlowConversionState::Impl::generateDivergentLoopResults(
 
 bool ControlFlowConversionState::Impl::generateDivergentLoopResultUpdates(
     Value *LLV, LoopTag &LTag) {
-  auto &LMask = LoopMasks[LTag.loop];
+  auto &LMask = LoopMasks.at(LTag.loop);
   Value *mask = LMask.combinedDivergentExitMask;
   VECZ_ERROR_IF(!mask, "Divergent loop does not have an exit mask");
   PHINode *PHI = LTag.loopResultPrevs[LLV];
@@ -2285,7 +2296,7 @@ bool ControlFlowConversionState::Impl::blendDivergentLoopExitMasks(
     }
   }
 
-  auto &LMask = LoopMasks[LTag.loop];
+  auto &LMask = LoopMasks.at(LTag.loop);
   for (const Loop::Edge &EE : exitEdges) {
     BasicBlock *exitingBlock = const_cast<BasicBlock *>(EE.first);
     BasicBlock *exitBlock = const_cast<BasicBlock *>(EE.second);
@@ -3055,7 +3066,7 @@ bool ControlFlowConversionState::Impl::blendInstructions() {
       }
     }
 
-    auto &LMask = LoopMasks[LTag->loop];
+    auto &LMask = LoopMasks.at(LTag->loop);
     for (auto &UPREM : LMask.updatedPersistedDivergentExitMasks) {
       if (UPREM.first != header) {
         blendMap[UPREM.second][header] =
@@ -3122,7 +3133,7 @@ bool ControlFlowConversionState::Impl::blendInstructions() {
         auto *const srcLoop = srcTag.loop;
         if (srcLoop && srcLoop->isLoopDivergent()) {
           if (dst != srcLoop->header) {
-            auto &srcMasks = LoopMasks[srcLoop->loop];
+            auto &srcMasks = LoopMasks.at(srcLoop->loop);
             const auto &headerTag = DR->getTag(srcLoop->header);
 
             // If 'opDef' is an update loop exit mask, set an entry point in