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Document an inefficiency in tail merging.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37235 91177308-0d34-0410-b5e6-96231b3b80d8
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Dale Johannesen 2007-05-18 18:46:40 +00:00
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lib/CodeGen/README.txt
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@ -142,3 +142,22 @@ load [T + 4]
load [T + 7]
...
load [T + 15]
//===---------------------------------------------------------------------===//
Tail merging issue:
When we're trying to merge the tails of predecessors of a block I, and there
are more than 2 predecessors, we don't do it optimally. Suppose predecessors
are A,B,C where B and C have 5 instructions in common, and A has 2 in common
with B or C. We want to get:
A:
jmp C3
B:
jmp C2
C:
C2: 3 common to B and C but not A
C3: 2 common to all 3
You get this if B and C are merged first, but currently it might randomly decide
to merge A and B first, which results in not sharing the C2 instructions. We
could look at all N*(N-1) combinations of predecessors and merge the ones with
the most instructions in common first. Usually that will be fast, but it
could get slow on big graphs (e.g. large switches tend to have blocks with many
predecessors).