Switch lowering: extract jump tables and bit tests before building binary tree (PR22262)

This is a re-commit of r235101, which also fixes the problems with the previous patch:

- Switches with only a default case and non-fallthrough were handled incorrectly

- The previous patch tickled a bug in PowerPC Early-Return Creation which is fixed here.

> This is a major rewrite of the SelectionDAG switch lowering. The previous code
> would lower switches as a binary tre, discovering clusters of cases
> suitable for lowering by jump tables or bit tests as it went along. To increase
> the likelihood of finding jump tables, the binary tree pivot was selected to
> maximize case density on both sides of the pivot.
>
> By not selecting the pivot in the middle, the binary trees would not always
> be balanced, leading to performance problems in the generated code.
>
> This patch rewrites the lowering to search for clusters of cases
> suitable for jump tables or bit tests first, and then builds the binary
> tree around those clusters. This way, the binary tree will always be balanced.
>
> This has the added benefit of decoupling the different aspects of the lowering:
> tree building and jump table or bit tests finding are now easier to tweak
> separately.
>
> For example, this will enable us to balance the tree based on profile info
> in the future.
>
> The algorithm for finding jump tables is quadratic, whereas the previous algorithm
> was O(n log n) for common cases, and quadratic only in the worst-case. This
> doesn't seem to be major problem in practice, e.g. compiling a file consisting
> of a 10k-case switch was only 30% slower, and such large switches should be rare
> in practice. Compiling e.g. gcc.c showed no compile-time difference.  If this
> does turn out to be a problem, we could limit the search space of the algorithm.
>
> This commit also disables all optimizations during switch lowering in -O0.
>
> Differential Revision: http://reviews.llvm.org/D8649

llvm-svn: 235560

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/test/CodeGen/Generic/MachineBranchProb.ll b/llvm/test/CodeGen/Generic/MachineBranchProb.ll
index 83277c98989..f10bd395abe 100644
--- a/llvm/test/CodeGen/Generic/MachineBranchProb.ll
+++ b/llvm/test/CodeGen/Generic/MachineBranchProb.ll
@@ -17,9 +17,9 @@ entry:
 ; CHECK: BB#0: derived from LLVM BB %entry
 ; CHECK: Successors according to CFG: BB#2(64) BB#4(14)
 ; CHECK: BB#4: derived from LLVM BB %entry
-; CHECK: Successors according to CFG: BB#1(10) BB#5(4)
+; CHECK: Successors according to CFG: BB#1(4) BB#5(10)
 ; CHECK: BB#5: derived from LLVM BB %entry
-; CHECK: Successors according to CFG: BB#1(4) BB#3(7)
+; CHECK: Successors according to CFG: BB#1(10) BB#3(7)
 
 sw.bb:
   br label %return