[PATCH] [GVN] Eliminate redundant loads whose addresses are dependent on the result of a select instruction.

[Chad Rosier]

Hi tilmann, nicholas, majnemer,

This patch attempts to eliminate redundant loads whose addresses are dependent upon a select instruction.

First, look for patterns such as:

  (load (gep (select (cond, ptr_a, ptr_b)) idx0, idx1, ..., idxN))
  (load (gep ptr_op, (select (cond, idx_a, idx_b)), idx1 ..., idxN))
  (load (gep ptr_op, idx0, (select (cond, idx_a, idx_b)), ..., idxN))

Then find similar loads and perform a transformation such as:

Constant zero index with different pointer operands:

  a = (load (gep ptr_a 0 0))
  b = (load (gep ptr_b 0 0))
  c = (load (gep (select (cond, ptr_a, ptr_b)) 0 0))
  -->
  a = (load (gep ptr_a, 0, 0))
  b = (load (gep ptr_b, 0, 0))
  c = (select (cond, a, b))

and

GEPs differ by only a single index:

  a = (load (gep ptr_op, idx, idx_a))
  b = (load (gep ptr_op, idx, idx_b))
  c = (load (gep ptr_op, idx, (select (cond, idx_a, idx_b))))
  -->
  a = (load (gep ptr_op, idx, idx_a))
  b = (load (gep ptr_op, idx, idx_b))
  c = (select (cond, a, b))

--------

See the PR for additional information.
http://llvm.org/bugs/show_bug.cgi?id=21052

This work is based on a patch done by Tilmann Scheller (attached to PR21052).

Basically, I moved the patch from InstCombine to GVN, so I would have dependency information.  I also made the solution a bit more general.

This is a WIP, but I wanted to get some feedback before continuing.  After considering the suggestions from Nick L. and David M. (see the bug report), I didn't see how those suggestions would benefit this patch (at least not in local scope).

Here are the tasks that remain:
1. Use use-def and def-use chains rather than iterate over instructions.
2. Allow the base load to be fed by stores as well as loads.
3. Perform additional correctness/perf testing.

Please feel free to comment.

 Chad

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D8120

Files:
  lib/Transforms/Scalar/GVN.cpp
  test/Transforms/GVN/load-gep-select.ll

EMAIL PREFERENCES
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