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, 1 insertions, 1 deletions

diff --git a/llvm/test/CodeGen/ARM/struct-byval-frame-index.ll b/llvm/test/CodeGen/ARM/struct-byval-frame-index.ll
index bca797d6dce..52f70fe1e0f 100644
--- a/llvm/test/CodeGen/ARM/struct-byval-frame-index.ll
+++ b/llvm/test/CodeGen/ARM/struct-byval-frame-index.ll
@@ -194,7 +194,7 @@ lor.lhs.false459:                                 ; preds = %if.end454
   %18 = load i32, i32* %mb_type, align 4
   switch i32 %18, label %for.inc503 [
     i32 9, label %if.then475
-    i32 10, label %if.then475
+    i32 11, label %if.then475
     i32 13, label %if.then475
     i32 14, label %if.then475
   ]