[PATCH] D105284: Greedy set cover implementation of `Merger::Merge`

[Aristotelis Koutsouridis via Phabricator via llvm-commits]

arisKoutsou added inline comments.


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Comment at: compiler-rt/lib/fuzzer/FuzzerMerge.cpp:358
+    if (MaxNumFeatures == 0)
+      continue;
+
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morehouse wrote:
> Could this continue cause an infinite loop?  i.e. when `Remaining` is empty
It wouldn't cause an infinite loop when Remaining is empty because the `while` condition `(CoveredSize != AllFeatures.size())` would be false. On the other hand, I noticed that there would be a problem if a feature had a value greater than `1 << 21`, which is the size of the bitvector. In that case, there could be an infinite loop because the `while` condition would never become false. I am addressing this problem in the latest patch.


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Comment at: compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp:953
+                      "",
+                      true));
+  EXPECT_EQ(M.SetCoverMerge(Features, &NewFeatures, Cov, &NewCov, &NewFiles),
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morehouse wrote:
> IIUC, this test doesn't quite do what's intended.
> 
> We choose input C because it has the most unique features, but after that A only has `{0}` as a unique feature while B has `{4, 5}`.  So we do in fact choose B, but not because it is smaller.
Correct, I will adjust the features correctly so that we can test breaking ties between files with equal number of features.


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Comment at: compiler-rt/test/fuzzer/set_cover_merge.test:48
+CHECK_OVERLAP: MERGE-OUTER: 10 files, 1 in the initial corpus
+CHECK_OVERLAP: MERGE-OUTER: 2 new files with 6 new features added
+# Make sure that we are prefering smaller files (T3/3 over T3/a).
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morehouse wrote:
> I think we would get the same results with `-merge`.  Perhaps we should make some feature-poor inputs smaller so that `-merge` would pick those first, while `-set_cover_merge` picks the feature-rich ones.
Should I also include a test of `-merge=1` in this source file to highlight the difference in behavior?


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  https://reviews.llvm.org/D105284/new/

https://reviews.llvm.org/D105284