[PATCH] Allow blocks to be merged when one has an undef input to a phi.

[Mark Lacey]

Hi All,

This is a relatively small patch I re-submitted last week which seems to have gotten lost in the shuffle. 

Can someone take a look and commit if everything looks good?

Thanks,

Mark


TryToSimplifyUncondBranchFromEmptyBlock was checking that any common
predecessors of the two blocks it is attempting to merge supply the
same incoming values to any phi in the successor block. This change
allows merging in the case where there is one or more incoming values
that are undef. The undef values are rewritten to match the non-undef
value that flows from the other edge.

Merged the tests from
- test/Transforms/SimplifyCFG/2008-05-16-PHIBlockMerge.ll
into
- test/Transforms/SimplifyCFG/EqualPHIEdgeBlockMerge.ll
and also added tests to this file for the situation addressed by this
patch.

<rdar://problem/12801861>
---
lib/Transforms/Utils/Local.cpp                     |  76 ++++++-
.../SimplifyCFG/2008-05-16-PHIBlockMerge.ll        | 131 -----------
.../SimplifyCFG/EqualPHIEdgeBlockMerge.ll          | 240 ++++++++++++++++++++-
3 files changed, 307 insertions(+), 140 deletions(-)
delete mode 100644 test/Transforms/SimplifyCFG/2008-05-16-PHIBlockMerge.ll

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