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DenseMap: make .resize() do the intuitive thing

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In some places, like InstCombine, we resize a DenseMap to fit the elements
we intend to put in it, then insert those elements (to avoid continual
reallocations as it grows). But .resize(foo) doesn't actually do what
people think; it resizes to foo buckets (which is really an
implementation detail the user of DenseMap probably shouldn't care about),
not the space required to fit foo elements. DenseMap grows if 3/4 of its
buckets are full, so this actually causes one forced reallocation every
time instead of avoiding a reallocation.

This patch makes .resize(foo) do the intuitive thing: it grows to the size
necessary to fit foo elements without new allocations.

Also include a test to verify that .resize() actually does what we think it
does.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263522 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Fiona Glaser 2016-03-15 01:50:46 +00:00
parent 875b7560fb
commit 33ae313f2a
2 changed files with 18 additions and 1 deletions
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6
include/llvm/ADT/DenseMap.h
View File

@ -81,8 +81,12 @@ public:
}
unsigned size() const { return getNumEntries(); }
/// Grow the densemap so that it has at least Size buckets. Does not shrink
/// Grow the densemap so that it can contain at least Size items before
/// resizing again. This means somewhat more than Size buckets because
/// densemap resizes upon reaching 3/4 full.
void resize(size_type Size) {
// Size *= (4/3), rounding up.
Size = (Size * 4 + 2) / 3;
incrementEpoch();
if (Size > getNumBuckets())
grow(Size);

13
unittests/ADT/DenseMapTest.cpp
View File

@ -339,6 +339,19 @@ TYPED_TEST(DenseMapTest, ConstIteratorTest) {
EXPECT_TRUE(cit == cit2);
}
// Make sure resize actually gives us enough buckets to insert N items
// without increasing allocation size.
TEST(DenseMapCustomTest, ResizeTest) {
for (unsigned Size = 16; Size < 32; ++Size) {
DenseMap<unsigned, unsigned> Map;
Map.resize(Size);
unsigned MemorySize = Map.getMemorySize();
for (unsigned i = 0; i < Size; ++i)
Map[i] = i;
EXPECT_TRUE(Map.getMemorySize() == MemorySize);
}
}
// Make sure DenseMap works with StringRef keys.
TEST(DenseMapCustomTest, StringRefTest) {
DenseMap<StringRef, int> M;