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This patch relaxes the post-dominance requirement for accesses to objects visible after the function returns. Instead of requiring the killing def to post-dominate the access to eliminate, the set of 'killing blocks' (= blocks that completely overwrite the original access) is collected. If all paths from the access to eliminate and an exit block go through a killing block, the access can be removed. To check this property, we first get the common post-dominator block for the killing blocks. If this block does not post-dominate the access block, there may be a path from DomAccess to an exit block not involving any killing block. Otherwise we have to check if there is a path from the DomAccess to the common post-dominator, that does not contain a killing block. If there is no such path, we can remove DomAccess. For this check, we start at the common post-dominator and then traverse the CFG backwards. Paths are terminated when we hit a killing block or a block that is not executed between DomAccess and a killing block according to the post-order numbering (if the post order number of a block is greater than the one of DomAccess, the block cannot be in in a path starting at DomAccess). This gives the following improvements on the total number of stores after DSE for MultiSource, SPEC2K, SPEC2006: Tests: 237 Same hash: 206 (filtered out) Remaining: 31 Metric: dse.NumRemainingStores Program base new100 diff test-suite...CFP2000/188.ammp/188.ammp.test 3624.00 3544.00 -2.2% test-suite...ch/g721/g721encode/encode.test 128.00 126.00 -1.6% test-suite.../Benchmarks/Olden/mst/mst.test 73.00 72.00 -1.4% test-suite...CFP2006/433.milc/433.milc.test 3202.00 3163.00 -1.2% test-suite...000/186.crafty/186.crafty.test 5062.00 5010.00 -1.0% test-suite...-typeset/consumer-typeset.test 40460.00 40248.00 -0.5% test-suite...Source/Benchmarks/sim/sim.test 642.00 639.00 -0.5% test-suite...nchmarks/McCat/09-vor/vor.test 642.00 644.00 0.3% test-suite...lications/sqlite3/sqlite3.test 35664.00 35563.00 -0.3% test-suite...T2000/300.twolf/300.twolf.test 7202.00 7184.00 -0.2% test-suite...lications/ClamAV/clamscan.test 19475.00 19444.00 -0.2% test-suite...INT2000/164.gzip/164.gzip.test 2199.00 2196.00 -0.1% test-suite...peg2/mpeg2dec/mpeg2decode.test 2380.00 2378.00 -0.1% test-suite.../Benchmarks/Bullet/bullet.test 39335.00 39309.00 -0.1% test-suite...:: External/Povray/povray.test 36951.00 36927.00 -0.1% test-suite...marks/7zip/7zip-benchmark.test 67396.00 67356.00 -0.1% test-suite...6/464.h264ref/464.h264ref.test 31497.00 31481.00 -0.1% test-suite...006/453.povray/453.povray.test 51441.00 51416.00 -0.0% test-suite...T2006/401.bzip2/401.bzip2.test 4450.00 4448.00 -0.0% test-suite...Applications/kimwitu++/kc.test 23481.00 23471.00 -0.0% test-suite...chmarks/MallocBench/gs/gs.test 6286.00 6284.00 -0.0% test-suite.../CINT2000/254.gap/254.gap.test 13719.00 13715.00 -0.0% test-suite.../Applications/SPASS/SPASS.test 30345.00 30338.00 -0.0% test-suite...006/450.soplex/450.soplex.test 15018.00 15016.00 -0.0% test-suite...ications/JM/lencod/lencod.test 27780.00 27777.00 -0.0% test-suite.../CINT2006/403.gcc/403.gcc.test 105285.00 105276.00 -0.0% There might be potential to pre-compute some of the information of which blocks are on the path to an exit for each block, but the overall benefit might be comparatively small. On the set of benchmarks, 15738 times out of 20322 we reach the CFG check, the CFG check is successful. The total number of iterations in the CFG check is 187810, so on average we need less than 10 steps in the check loop. Bumping the threshold in the loop from 50 to 150 gives a few small improvements, but I don't think they warrant such a big bump at the moment. This is all pending further tuning in the future. Reviewers: dmgreen, bryant, asbirlea, Tyker, efriedma, george.burgess.iv Reviewed By: george.burgess.iv Differential Revision: https://reviews.llvm.org/D78932
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