llvm-project/llvm/lib/Transforms/Vectorize
Jiangning Liu 40c1b35292 Fixed a bug in memory dependence checking module of loop vectorization. The following loop should not be vectorized with current algorithm.
{code}
// loop body
   ... = a[i]          (1)
    ... = a[i+1]       (2)
 .......
a[i+1] = ....          (3)
   a[i] = ...          (4)
{code}

The algorithm tries to collect memory access candidates from AliasSetTracker, and then check memory dependences one another. The memory accesses are unique in AliasSetTracker, and a single memory access in AliasSetTracker may map to multiple entries in AccessAnalysis, which could cover both 'read' and 'write'. Originally the algorithm only checked 'write' entry in Accesses if only 'write' exists. This is incorrect and the consequence is it ignored all read access, and finally some RAW and WAR dependence are missed.

For the case given above, if we ignore two reads, the dependence between (1) and (3) would not be able to be captured, and finally this loop will be incorrectly vectorized.

The fix simply inserts a new loop to find all entries in Accesses. Since it will skip most of all other memory accesses by checking the Value pointer at the very beginning of the loop, it should not increase compile-time visibly.

llvm-svn: 225159
2015-01-05 10:08:58 +00:00
..
BBVectorize.cpp [BBVectorize] Remove two more redundant assignments. 2014-12-19 17:21:38 +00:00
CMakeLists.txt SLP Vectorizer: Implement multi-block slp-vectorization. 2013-06-22 21:34:10 +00:00
LLVMBuild.txt Prune redundant dependencies in LLVMBuild.txt. 2013-12-11 00:30:57 +00:00
LoopVectorize.cpp Fixed a bug in memory dependence checking module of loop vectorization. The following loop should not be vectorized with current algorithm. 2015-01-05 10:08:58 +00:00
Makefile
SLPVectorizer.cpp [PM] Split the AssumptionTracker immutable pass into two separate APIs: 2015-01-04 12:03:27 +00:00
Vectorize.cpp [cleanup] Move the Dominators.h and Verifier.h headers into the IR 2014-01-13 09:26:24 +00:00