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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235560 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 34 insertions, 1 deletions

diff --git a/test/CodeGen/PowerPC/early-ret.ll b/test/CodeGen/PowerPC/early-ret.ll
index 7d3e225a1e2..52cf464b9fd 100644
--- a/test/CodeGen/PowerPC/early-ret.ll
+++ b/test/CodeGen/PowerPC/early-ret.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s -mtriple=powerpc64-unknown-linux-gnu -mcpu=pwr7 | FileCheck %s
+; RUN: llc < %s -mtriple=powerpc64-unknown-linux-gnu | FileCheck %s
 target datalayout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-f128:128:128-v128:128:128-n32:64"
 target triple = "powerpc64-unknown-linux-gnu"
 
@@ -45,4 +45,37 @@ if.end3:                                          ; preds = %if.then, %if.then2,
 ; CHECK: blr
 }
 
+
+@.str0 = private unnamed_addr constant [2 x i8] c"a\00"
+@.str1 = private unnamed_addr constant [2 x i8] c"b\00"
+@.str2 = private unnamed_addr constant [2 x i8] c"c\00"
+@.str3 = private unnamed_addr constant [2 x i8] c"d\00"
+@.str4 = private unnamed_addr constant [2 x i8] c"e\00"
+define i8* @dont_assert(i32 %x) {
+; LLVM would assert due to moving an early return into the jump table block and
+; removing one of its predecessors despite that block ending with an indirect
+; branch.
+entry:
+  switch i32 %x, label %sw.epilog [
+    i32 1, label %return
+    i32 2, label %sw.bb1
+    i32 3, label %sw.bb2
+    i32 4, label %sw.bb3
+    i32 255, label %sw.bb4
+  ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.bb4: br label %return
+sw.epilog: br label %return
+return:
+  %retval.0 = phi i8* [ null, %sw.epilog ],
+                      [ getelementptr inbounds ([2 x i8], [2 x i8]* @.str4, i64 0, i64 0), %sw.bb4 ],
+                      [ getelementptr inbounds ([2 x i8], [2 x i8]* @.str3, i64 0, i64 0), %sw.bb3 ],
+                      [ getelementptr inbounds ([2 x i8], [2 x i8]* @.str2, i64 0, i64 0), %sw.bb2 ],
+                      [ getelementptr inbounds ([2 x i8], [2 x i8]* @.str1, i64 0, i64 0), %sw.bb1 ],
+                      [ getelementptr inbounds ([2 x i8], [2 x i8]* @.str0, i64 0, i64 0), %entry ]
+  ret i8* %retval.0
+}
+
 attributes #0 = { nounwind }