[pruned-liveness] Change the internal machinery of updateForUse to use only scalar logic.

I did this by doing the following:

1. I renamed computeUseBlockLiveness to computeScalarUseBlockLiveness and
changed it to take a specific bit that it is testing for.

2. I changed the logic that already existed in this code path that worked scalar
by scalar to use scalar logic rather than call the broken multi-bit at a time
code path.

3. We took advantage of resultingFoundLiveness now only returning the requested
bits instead of all bits. This ensures that we do not run
computeScalarUseBlockLiveness for those unneeded dead bits resulting in liveness
being inappropriately propagated into predecessors.

Author: gottesmm

include/swift/SIL/PrunedLiveness.h

@@ -283,7 +283,7 @@ class PrunedLiveBlocks {
     auto liveness = getBlockLiveness(block, bitNo);
     if (liveness != Dead)
       return liveness;
-    computeUseBlockLiveness(block, bitNo, bitNo + 1);
+    computeScalarUseBlockLiveness(block, bitNo);
     return getBlockLiveness(block, bitNo);
   }
 
@@ -348,8 +348,14 @@ class PrunedLiveBlocks {
     }
   }
 
-  void computeUseBlockLiveness(SILBasicBlock *userBB, unsigned startBitNo,
-                               unsigned endBitNo);
+private:
+  /// A helper routine that as a fast path handles the scalar case. We do not
+  /// handle the mult-bit case today since the way the code is written today
+  /// assumes we process a bit at a time.
+  ///
+  /// TODO: Make a multi-bit query for efficiency reasons.
+  void computeScalarUseBlockLiveness(SILBasicBlock *userBB,
+                                     unsigned startBitNo);
 };
 
 /// If inner borrows are 'Contained', then liveness is fully described by the

lib/SIL/Utils/PrunedLiveness.cpp

@@ -21,16 +21,15 @@
 
 using namespace swift;
 
-/// Mark blocks live during a reverse CFG traversal from one specific block
-/// containing a user.
-void PrunedLiveBlocks::computeUseBlockLiveness(SILBasicBlock *userBB,
-                                               unsigned startBitNo,
-                                               unsigned endBitNo) {
+void PrunedLiveBlocks::computeScalarUseBlockLiveness(SILBasicBlock *userBB,
+                                                     unsigned startBitNo) {
+  unsigned endBitNo = startBitNo + 1;
+
   // If, we are visiting this block, then it is not already LiveOut. Mark it
   // LiveWithin to indicate a liveness boundary within the block.
   markBlockLive(userBB, startBitNo, endBitNo, LiveWithin);
 
-  SmallVector<IsLive, 8> predLivenessInfo;
+  // Specialize the given code for scalar code.
   BasicBlockWorklist worklist(userBB->getFunction());
   worklist.push(userBB);
 
@@ -40,19 +39,15 @@ void PrunedLiveBlocks::computeUseBlockLiveness(SILBasicBlock *userBB,
     // Traversal terminates at any previously visited block, including the
     // blocks initialized as definition blocks.
     for (auto *predBlock : block->getPredecessorBlocks()) {
-      SWIFT_DEFER { predLivenessInfo.clear(); };
-      getBlockLiveness(predBlock, startBitNo, endBitNo, predLivenessInfo);
-      for (unsigned i : indices(predLivenessInfo)) {
-        switch (predLivenessInfo[i]) {
-        case Dead:
-          worklist.pushIfNotVisited(predBlock);
-          LLVM_FALLTHROUGH;
-        case LiveWithin:
-          markBlockLive(predBlock, startBitNo, endBitNo, LiveOut);
-          break;
-        case LiveOut:
-          break;
-        }
+      switch (getBlockLiveness(predBlock, startBitNo)) {
+      case Dead:
+        worklist.pushIfNotVisited(predBlock);
+        LLVM_FALLTHROUGH;
+      case LiveWithin:
+        markBlockLive(predBlock, startBitNo, endBitNo, LiveOut);
+        break;
+      case LiveOut:
+        break;
       }
     }
   }
@@ -66,22 +61,26 @@ void PrunedLiveBlocks::computeUseBlockLiveness(SILBasicBlock *userBB,
 void PrunedLiveBlocks::updateForUse(
     SILInstruction *user, unsigned startBitNo, unsigned endBitNo,
     SmallVectorImpl<IsLive> &resultingLivenessInfo) {
+  resultingLivenessInfo.clear();
+
   SWIFT_ASSERT_ONLY(seenUse = true);
 
   auto *bb = user->getParent();
   getBlockLiveness(bb, startBitNo, endBitNo, resultingLivenessInfo);
 
-  for (auto isLive : resultingLivenessInfo) {
-    switch (isLive) {
+  for (auto pair : llvm::enumerate(resultingLivenessInfo)) {
+    unsigned index = pair.index();
+    unsigned specificBitNo = startBitNo + index;
+    switch (pair.value()) {
     case LiveOut:
     case LiveWithin:
       continue;
     case Dead: {
       // This use block has not yet been marked live. Mark it and its
       // predecessor blocks live.
-      computeUseBlockLiveness(bb, startBitNo, endBitNo);
-      resultingLivenessInfo.clear();
-      return getBlockLiveness(bb, startBitNo, endBitNo, resultingLivenessInfo);
+      computeScalarUseBlockLiveness(bb, specificBitNo);
+      resultingLivenessInfo.push_back(getBlockLiveness(bb, specificBitNo));
+      continue;
     }
     }
     llvm_unreachable("covered switch");