[PATCH] D18344: Adjust initial size in StringMap constructor to guarantee no grow()

[David Blaikie via llvm-commits]

Could you include a test that demonstrates non-growth (I think someone
contributed a similar patch for DenseMap's growth recently - could check
how that was tested (I think it could be tested better with a move-counting
object, for example, but not a strict requirement))?

On Mon, Mar 21, 2016 at 11:15 PM, Mehdi AMINI via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> joker.eph created this revision.
> joker.eph added a reviewer: dblaikie.
> joker.eph added a subscriber: llvm-commits.
>
> StringMap ctor accepts an initialize size, but expect it to be
> rounded to the next power of 2. The ctor can handle that directly
> instead of expecting clients to round it. Also, since the map will
> resize itself when 75% full, take this into account an initialize
> a larger initial size to avoid any growth.
>
> http://reviews.llvm.org/D18344
>
> Files:
>   lib/Support/StringMap.cpp
>   unittests/ADT/StringMapTest.cpp
>
> Index: unittests/ADT/StringMapTest.cpp
> ===================================================================
> --- unittests/ADT/StringMapTest.cpp
> +++ unittests/ADT/StringMapTest.cpp
> @@ -231,12 +231,12 @@
>    // moved to a different bucket during internal rehashing. This depends
> on
>    // the particular key, and the implementation of StringMap and
> HashString.
>    // Changes to those might result in this test not actually checking
> that.
> -  StringMap<uint32_t> t(1);
> -  EXPECT_EQ(1u, t.getNumBuckets());
> +  StringMap<uint32_t> t(0);
> +  EXPECT_EQ(0u, t.getNumBuckets());
>
>    StringMap<uint32_t>::iterator It =
>      t.insert(std::make_pair("abcdef", 42)).first;
> -  EXPECT_EQ(2u, t.getNumBuckets());
> +  EXPECT_EQ(16u, t.getNumBuckets());
>    EXPECT_EQ("abcdef", It->first());
>    EXPECT_EQ(42u, It->second);
>  }
> Index: lib/Support/StringMap.cpp
> ===================================================================
> --- lib/Support/StringMap.cpp
> +++ lib/Support/StringMap.cpp
> @@ -22,6 +22,13 @@
>
>    // If a size is specified, initialize the table with that many buckets.
>    if (InitSize) {
> +    // The table will grow when the number of entries reach 3/4 of the
> number of
> +    // buckets. To guarantee that "InitSize" number of entries can be
> inserted
> +    // in the table without growing, we allocate just what is needed here.
> +    InitSize = (InitSize * 4 + 2) / 3;
> +    // Size has to be a power of 2
> +    if (!isPowerOf2_32(InitSize))
> +      InitSize = NextPowerOf2(InitSize);
>      init(InitSize);
>      return;
>    }
>
>
>
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