[LoopStrengthReduce] Don't use a DenseSet<int64_t> when we might add any valid int64_t to the set.

Summary:
SmallSetVector uses DenseSet, but that means we need to reserve some
values for the empty and tombstone keys.

It seems to me we should have a general way to let us store full-range
ints inside of DenseSets, and furthermore that we probably shouldn't
silently let you add ints into DenseSets without explicitly promising
that they're in range.  But that's a battle for another day; for now,
just fix this code, since we currently do something Very Bad when
compiling ffmpeg.

Fixes PR30914.

Reviewers: jeremyhu

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D26323

llvm-svn: 286038

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index 9354d40b98c..e28c374549c 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1646,7 +1646,12 @@ class LSRInstance {
   Instruction *IVIncInsertPos;
 
   /// Interesting factors between use strides.
-  SmallSetVector<int64_t, 8> Factors;
+  ///
+  /// We explicitly use a SetVector which contains a SmallSet, instead of the
+  /// default, a SmallDenseSet, because we need to use the full range of
+  /// int64_ts, and there's currently no good way of doing that with
+  /// SmallDenseSet.
+  SetVector<int64_t, SmallVector<int64_t, 8>, SmallSet<int64_t, 8>> Factors;
 
   /// Interesting use types, to facilitate truncation reuse.
   SmallSetVector<Type *, 4> Types;