[llvm] [memprof] Speed up caller-callee pair extraction (PR #116184)

[Kazu Hirata via llvm-commits]

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@@ -1303,6 +1303,10 @@ Error IndexedMemProfReader::deserializeV3(const unsigned char *Start,
   FrameBase = Ptr;
   CallStackBase = Start + CallStackPayloadOffset;
 
+  // Compute the number of elements in the radix tree array.
+  RadixTreeSize = (RecordPayloadOffset - CallStackPayloadOffset) /
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kazutakahirata wrote:

Sure.  I'll add a comment.  By the way, `RadixTreeSize` is used to reserve enough bits in a `BitVector`; I'm not computing the exact number of elements.  If I get a couple of extra elements because of padding, that's totally OK.  I'll include this in the comment also.

https://github.com/llvm/llvm-project/pull/116184