[PATCH] Fold a loop for array processing in ComputeLinearIndex

[Mehdi AMINI]

When processing an array, every Elt has the same layout, it is
useless to recursively call each ComputeLinearIndex on each element.
Just do it once and multiply by the number of elements.

http://reviews.llvm.org/D6832

Files:
  include/llvm/CodeGen/Analysis.h
  lib/CodeGen/Analysis.cpp

Index: include/llvm/CodeGen/Analysis.h
===================================================================
--- include/llvm/CodeGen/Analysis.h
+++ include/llvm/CodeGen/Analysis.h
@@ -31,10 +31,21 @@
 class SelectionDAG;
 struct EVT;
 
-/// ComputeLinearIndex - Given an LLVM IR aggregate type and a sequence
-/// of insertvalue or extractvalue indices that identify a member, return
-/// the linearized index of the start of the member.
+/// \brief Compute the linearized index of a member in a nested
+/// aggregate/struct/array.
 ///
+/// Given an LLVM IR aggregate type and a sequence of insertvalue or
+/// extractvalue indices that identify a member, return the linearized index of
+/// the start of the member, i.e the number of element in memory before the
+/// seeked one. This is disconnected from the number of bytes.
+///
+/// \param Ty is the type indexed by \p Indices.
+/// \param Indices is an optional pointer in the indices list to the current
+/// index.
+/// \param IndicesEnd is the end of the indices list.
+/// \param CurIndex is the current index in the recursion.
+///
+/// \returns \p CurIndex plus the linear index in \p Ty  the indices list.
 unsigned ComputeLinearIndex(Type *Ty,
                             const unsigned *Indices,
                             const unsigned *IndicesEnd,
Index: lib/CodeGen/Analysis.cpp
===================================================================
--- lib/CodeGen/Analysis.cpp
+++ lib/CodeGen/Analysis.cpp
@@ -30,10 +30,9 @@
 
 using namespace llvm;
 
-/// ComputeLinearIndex - Given an LLVM IR aggregate type and a sequence
-/// of insertvalue or extractvalue indices that identify a member, return
-/// the linearized index of the start of the member.
-///
+/// Compute the linearized index of a member in a nested aggregate/struct/array
+/// by recursing and accumulating CurIndex as long as there are indices in the
+/// indices list.
 unsigned llvm::ComputeLinearIndex(Type *Ty,
                                   const unsigned *Indices,
                                   const unsigned *IndicesEnd,
@@ -57,11 +56,17 @@
   // Given an array type, recursively traverse the elements.
   else if (ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
     Type *EltTy = ATy->getElementType();
-    for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i) {
-      if (Indices && *Indices == i)
-        return ComputeLinearIndex(EltTy, Indices+1, IndicesEnd, CurIndex);
-      CurIndex = ComputeLinearIndex(EltTy, nullptr, nullptr, CurIndex);
+    unsigned NumElts = ATy->getNumElements();
+    // Compute the Linear offset when jumping one element of the array
+    unsigned EltLinearOffset = ComputeLinearIndex(EltTy, nullptr, nullptr, 0);
+    if (Indices && *Indices < NumElts) {
+      // If the indice is inside the array, compute the index to the requested
+      // elt and recurse inside the element with the end of the indices list
+      CurIndex += EltLinearOffset* *Indices;
+      return ComputeLinearIndex(EltTy, Indices+1, IndicesEnd, CurIndex);
     }
+    // Out of bound? Assert instead?
+    CurIndex += EltLinearOffset*NumElts;
     return CurIndex;
   }
   // We haven't found the type we're looking for, so keep searching.

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