[PATCH] D15393: [X86] Order the local stack symbols to improve code size and locality.

[Zia Ansari via llvm-commits]

zansari added a comment.

Rights and I believe that's all taken care of, unless I'm missing someone.

The piece of code right before the scope that I showed earlier looks like this:

  // Make sure that the stack protector comes before the local variables on the
  // stack.
  SmallSet<int, 16> ProtectedObjs;
  if (MFI->getStackProtectorIndex() >= 0) {
    StackObjSet LargeArrayObjs;
    StackObjSet SmallArrayObjs;
    StackObjSet AddrOfObjs;
  
    AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), StackGrowsDown,
                      Offset, MaxAlign, Skew);
  
    // Assign large stack objects first.
    for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
      if (MFI->isObjectPreAllocated(i) &&
          MFI->getUseLocalStackAllocationBlock())
        continue;
      if (i >= MinCSFrameIndex && i <= MaxCSFrameIndex)
        continue;
      if (RS && RS->isScavengingFrameIndex((int)i))
        continue;
      if (MFI->isDeadObjectIndex(i))
        continue;
      if (MFI->getStackProtectorIndex() == (int)i)
        continue;
  
      switch (SP->getSSPLayout(MFI->getObjectAllocation(i))) {
      case StackProtector::SSPLK_None:
        continue;
      case StackProtector::SSPLK_SmallArray:
        SmallArrayObjs.insert(i);
        continue;
      case StackProtector::SSPLK_AddrOf:
        AddrOfObjs.insert(i);
        continue;
      case StackProtector::SSPLK_LargeArray:
        LargeArrayObjs.insert(i);
        continue;
      }
      llvm_unreachable("Unexpected SSPLayoutKind.");
    }
  
    AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
                          Offset, MaxAlign, Skew);
    AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
                          Offset, MaxAlign, Skew);
    AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
                          Offset, MaxAlign, Skew);
  }

This walks all objects requiring special allocation due to stack protection and assigns offsets to them. I don't touch this code. Once offset have been assigned to these special objects, they are put in the "ProtectedObjs" bucket which is untouched during my algorithm. My reordering should only operate withing the scope of "everything that's left over at the end".

An interesting "TODO" would be to see whether there's additional opportunities for code size by reordering the way the large arrays get allocated within their own scope.


http://reviews.llvm.org/D15393