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/ARM/ifcvt3.ll b/llvm/test/CodeGen/ARM/ifcvt3.ll
index 5da63dc5f02..e53d989ad52 100644
--- a/llvm/test/CodeGen/ARM/ifcvt3.ll
+++ b/llvm/test/CodeGen/ARM/ifcvt3.ll
@@ -4,8 +4,8 @@
 
 define i32 @t1(i32 %a, i32 %b, i32 %c, i32 %d) {
 ; CHECK-LABEL: t1:
-; CHECK: cmp r2, #1
-; CHECK: cmpne r2, #7
+; CHECK: cmp r2, #7
+; CHECK: cmpne r2, #1
 	switch i32 %c, label %cond_next [
 		 i32 1, label %cond_true
 		 i32 7, label %cond_true