[PATCH] [GC] CodeGenPrep transform: simplify offsetable relocate

[Sanjoy Das]

================
Comment at: lib/CodeGen/CodeGenPrepare.cpp:564
@@ +563,3 @@
+  // Assume that the first relocate in the basic block is relocating the base
+  // object. TODO: what can be done to get rid of this assumption?
+  IntrinsicInst *MasterRelocate = RelocateCalls[0];
----------------
reames wrote:
> sanjoy wrote:
> > This seems limiting, but correct; and a strict improvement over what we have.  We could easily have a `DenseMap<CallInst *, CallInst *>` that we build in a first pass that maps derived gc relocates to base gc relocates; and write the rest of the algorithm in terms of that.
> Er, maybe I'm misreading Sanjoy's comment, but I think his first sentence is wrong.  There is no guarantee that the first gc.relocate encountered is a base relocation,
> There is no guarantee that the first gc.relocate encountered is a base relocation,

I agree.  What I meant was that if the first `gc.relocate` is not a base relocation, then the worst case scenario is that we will miss the optimization, because that fact is checked later.

http://reviews.llvm.org/D6883

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