Debug info: Improve the line table for SwitchStmt variable bindings by

associating the store to it with the first CaseStmt.
The goal is to make the linetable entries as monotone as possible.

Fixes <rdar://problem/15888936> switch line table's first line misplaced.

Swift SVN r13805

lib/SILGen/SILGenPattern.cpp

@@ -1037,6 +1037,8 @@ class ClauseMatrix {
   /// reference into slices of this vector to indicate which ExprPatterns form
   /// part of their guard.
   std::vector<const ExprPattern*> allExprGuards;
+  /// The location of the top-level CaseStmt.
+  SILLocation topLoc;
   
   // The memory layout of data is as follows:
   
@@ -1074,9 +1076,9 @@ public:
   /// given number of rows. The clause matrix will be initialized with zero rows
   /// and a column for every occurrence. Rows can be added using addRows.
   explicit ClauseMatrix(ArrayRef<SILValue> occurrences,
-                        unsigned rowCapacity)
+                        unsigned rowCapacity, SILLocation topLoc)
     : rowCapacity(rowCapacity), columnCapacity(occurrences.size()),
-      rowCount(0), columnCount(columnCapacity)
+      rowCount(0), columnCount(columnCapacity), topLoc(topLoc)
   {
     // Allocate the data buffer.
     data = malloc(getDataSize());
@@ -1096,7 +1098,7 @@ public:
   ClauseMatrix(ClauseMatrix &&m)
     : rowCapacity(m.rowCapacity), columnCapacity(m.columnCapacity),
       rowCount(m.rowCount), columnCount(m.columnCount),
-      data(m.data)
+      data(m.data), topLoc(m.topLoc)
   {
     m.data = nullptr;
   }
@@ -1204,7 +1206,8 @@ public:
     std::copy(getOccurrences().begin() + 1, getOccurrences().end(),
               std::back_inserter(newOccurrences));
 
-    ClauseMatrix specialized{newOccurrences, rowCount - specializer.row};
+    ClauseMatrix specialized{newOccurrences, rowCount - specializer.row,
+        topLoc};
     
     for (unsigned r = specializer.row; r < rowCount; ++r) {
       auto row = (*this)[r];
@@ -1310,14 +1313,14 @@ public:
       case PatternKind::Named:
         // Bind the named variable.
         addVar(cast<NamedPattern>(p)->getDecl(),
-                RegularLocation(const_cast<Pattern*>(p)));
+               RegularLocation(const_cast<Pattern*>(p)));
         break;
           
       case PatternKind::Expr:
         // Bind the ExprPattern's implicit match var.
         // TODO: It'd be nice not to need the temp var for expr patterns.
         addVar(cast<ExprPattern>(p)->getMatchVar(),
-                RegularLocation(const_cast<Pattern*>(p)));
+               RegularLocation(const_cast<Pattern*>(p)));
         break;
 
       case PatternKind::Paren:
@@ -1331,14 +1334,15 @@ public:
       return;
 
     VarDecl *vdToEmit = varToEmit->first;
+    // Why do we bother saving this when we only emit on copy anyway?
     SILLocation locToEmit = varToEmit->second;
     
     // Initialize the variable value.
     InitializationPtr init
-      = gen.emitInitializationForVarDecl(vdToEmit, /*isArgument*/ false, Type());
+      = gen.emitInitializationForVarDecl(vdToEmit, /*isArgument*/false, Type());
     auto mv = ManagedValue::forUnmanaged(v);
     RValue(gen, locToEmit, vdToEmit->getType()->getCanonicalType(), mv)
-      .copyInto(gen, init.get(), locToEmit);
+      .copyInto(gen, init.get(), topLoc);
     
     // Alias the variables to the value.
     for (auto var : varsToAlias) {
@@ -1809,7 +1813,8 @@ void SILGenFunction::emitSwitchStmt(SwitchStmt *S) {
     SILBasicBlock *contBB = createBasicBlock();
     
     // Set up an initial clause matrix.
-    ClauseMatrix clauses(subject, S->getCases().size());
+    ClauseMatrix clauses(subject, S->getCases().size(),
+                         S->getCases()[0]);
     
     for (auto *caseBlock : S->getCases()) {
       caseMap[caseBlock] = {};