From 15823d49b61fc7ddafc2cb10b12f4498d53fb216 Mon Sep 17 00:00:00 2001 From: Hans Wennborg Date: Wed, 22 Apr 2015 23:14:56 +0000 Subject: 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 --- llvm/test/CodeGen/ARM/ifcvt3.ll | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'llvm/test/CodeGen/ARM/ifcvt3.ll') 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 -- cgit v1.2.3