[PATCH] SROA: Defer migrating the debug info for new allocas until after all partitions are created.

[Adrian Prantl]

Hi chandlerc,

SROA: Defer migrating the debug info for new allocas until after all partitions are created.
As discussed on #llvm today.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6766

Files:
  lib/Transforms/Scalar/SROA.cpp

Index: lib/Transforms/Scalar/SROA.cpp
===================================================================
--- lib/Transforms/Scalar/SROA.cpp
+++ lib/Transforms/Scalar/SROA.cpp
@@ -1224,6 +1224,17 @@
   /// IR. This side-table holds the missing use edges.
   DenseMap<AllocaInst *, DbgDeclareInst *> DbgDeclares;
 
+  struct Piece {
+    AllocaInst *Alloca;
+    uint64_t Offset;
+    uint64_t Size;
+    Piece(AllocaInst *AI, uint64_t O, uint64_t S)
+      : Alloca(AI), Offset(O), Size(S) {}
+  };
+  /// \brief Describes the allocas introduced by rewritePartition in order to
+  /// migrate the debug info.
+  std::vector<Piece> Pieces;
+
 public:
   SROA(bool RequiresDomTree = true)
       : FunctionPass(ID), RequiresDomTree(RequiresDomTree), C(nullptr),
@@ -3498,26 +3509,8 @@
         AI.getName() + ".sroa." + Twine(P.begin() - AS.begin()), &AI);
     ++NumNewAllocas;
 
-    // Migrate debug information from the old alloca to the new alloca
-    // and the individial slices.
-    if (DbgDeclareInst *DbgDecl = DbgDeclares.lookup(&AI)) {
-      DIVariable Var(DbgDecl->getVariable());
-      DIExpression Piece;
-      DIBuilder DIB(*AI.getParent()->getParent()->getParent(),
-                    /*AllowUnresolved*/ false);
-      // Create a piece expression describing the slice, if the new slize is
-      // smaller than the old alloca or the old alloca already was described
-      // with a piece. It would be even better to just compare against the size
-      // of the type described in the debug info, but then we would need to
-      // build an expensive DIRefMap.
-      if (P.size() < DL->getTypeAllocSize(AI.getAllocatedType()) ||
-          DIExpression(DbgDecl->getExpression()).isVariablePiece())
-        Piece = DIB.createPieceExpression(P.beginOffset(), P.size());
-      Instruction *NewDDI = DIB.insertDeclare(NewAI, Var, Piece, &AI);
-      NewDDI->setDebugLoc(DbgDecl->getDebugLoc());
-      DbgDeclares.insert(std::make_pair(NewAI, cast<DbgDeclareInst>(NewDDI)));
-      DeadInsts.insert(DbgDecl);
-    }
+    // Remember the alloca so we can update the debug info for it.
+    Pieces.push_back( Piece(NewAI, P.beginOffset(), P.size()) );
   }
 
   DEBUG(dbgs() << "Rewriting alloca partition "
@@ -3611,6 +3604,7 @@
 
   unsigned NumPartitions = 0;
   bool Changed = false;
+  Pieces.clear();
 
   // Rewrite each parttion.
   for (auto &P : AS.partitions()) {
@@ -3622,6 +3616,26 @@
   MaxPartitionsPerAlloca =
       std::max<unsigned>(NumPartitions, MaxPartitionsPerAlloca);
 
+  // Migrate debug information from the old alloca to the new alloca(s)
+  // and the individial partitions.
+  if (DbgDeclareInst *DbgDecl = DbgDeclares.lookup(&AI)) {
+    DIVariable Var(DbgDecl->getVariable());
+    DIExpression Expr(DbgDecl->getExpression());
+    DIBuilder DIB(*AI.getParent()->getParent()->getParent(),
+                  /*AllowUnresolved*/ false);
+    bool didSplit = Pieces.size() > 1;
+    for (auto Piece : Pieces) {
+      // Create a piece expression describing the new partition or reuse AI's
+      // expression if there is only one partition.
+      if (didSplit)
+        Expr = DIB.createPieceExpression(Piece.Offset, Piece.Size);
+      Instruction *NewDDI = DIB.insertDeclare(Piece.Alloca, Var, Expr, &AI);
+      NewDDI->setDebugLoc(DbgDecl->getDebugLoc());
+      DbgDeclares.insert(std::make_pair(Piece.Alloca,
+                                        cast<DbgDeclareInst>(NewDDI)));
+      DeadInsts.insert(DbgDecl);
+    }
+  }
   return Changed;
 }

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