[PATCH] [DAGCombine] Fix bug in MergeConsecutiveStores

[Akira Hatanaka]

Hi spatel, nadav,

This bug manifests when there are two loads and two stores that are chained as follows in a DAG,

(ld v3f32) -> (st f32) -> (ld v3f32) -> (st f32)

and the store's values are extracted from the preceding vector loads.

The current code in trunk creates a build_vector node and then inserts the new merged store node between the two load nodes. This creates a cycle between the merged store node, the build_vector node, the extract_vector_elt node, and the second vector load (set a breakpoint at DAGCombiner.cpp:10056 to visualize the transformation), which eventually results in a crash during type legalization.

This patch fixes the bug by inserting the new merged store at the position of the last store node in the chain.

http://reviews.llvm.org/D8849

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/AArch64/merge-store.ll
  test/CodeGen/X86/2012-11-28-merge-store-alias.ll

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