[PATCH] D31085: [InlineCost] Increase the cost of Switch

[Haicheng Wu via Phabricator via llvm-commits]

haicheng added inline comments.


================
Comment at: lib/Analysis/InlineCost.cpp:1051
+
+  while (!SwitchWorkList.empty()) {
+    unsigned NumCases = SwitchWorkList.back();
----------------
haicheng wrote:
> hans wrote:
> > Why do we have to do this?
> > 
> > The code is basically constructing the tree and throwing it away. It should be possible to compute an estimate for the size of the tree with a closed-form mathematical expression.
> 
> If n is the case number, f(n) is the mapping from the case number to the node number of BTree.  The recursion is 
> f(n) = 1 + f(n/2) + f (n - n/2), when n > 3.  
> So, f(n) is between n + 2^(log2(n) - 1) - 1 and n + 2^(log2(n)) - 1.  The lower bound is about 1.5n - 1 and the upper bound is about 2n - 1
The exact equation is

f(n) = n, n <= 3
f(n) = n + 2^(log(n) - 1) - 1, n > 3 && 2^log(n) <= n <= 1.5*2^(log(n))
f(n) = 2n - 2^(log(n)) - 1, n > 3 && 1.5*2^(log(n)) < n < 2^(log(n)+1)


https://reviews.llvm.org/D31085