SimplifyCFG: don't remove unreachable default switch destinations

An unreachable default destination can be exploited by other optimizations, and
SDag lowering is now prepared to handle them efficiently.

For example, branches to the unreachable destination will be optimized away,
such as in the case of range checks for switch lookup tables.

On 64-bit Linux, this reduces the size of a clang bootstrap by 80 kB (and
Chromium by 30 kB).

llvm-svn: 223050

2 files changed, 7 insertions, 33 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll b/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
index 21428c62f53..22b144b5cb8 100644
--- a/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
+++ b/llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
@@ -46,32 +46,6 @@ T:
         ret i32 2
 }
 
-; PR9450
-define i32 @test4(i32 %v, i32 %w) {
-; CHECK: entry:
-; CHECK-NEXT:  switch i32 %v, label %T [
-; CHECK-NEXT:    i32 3, label %V
-; CHECK-NEXT:    i32 2, label %U
-; CHECK-NEXT:  ]
-
-entry:
-        br label %SWITCH
-V:
-        ret i32 7
-SWITCH:
-        switch i32 %v, label %default [
-                 i32 1, label %T
-                 i32 2, label %U
-                 i32 3, label %V
-        ]
-default:
-        unreachable
-U:
-        ret i32 %w
-T:
-        ret i32 2
-}
-
 
 ;; We can either convert the following control-flow to a select or remove the
 ;; unreachable control flow because of the undef store of null. Make sure we do
diff --git a/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll b/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
index 95c9cc539d2..27c4839846e 100644
--- a/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ b/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -21,8 +21,8 @@ target triple = "x86_64-unknown-linux-gnu"
 ; The table for @cprop
 ; CHECK: @switch.table5 = private unnamed_addr constant [7 x i32] [i32 5, i32 42, i32 126, i32 -452, i32 128, i32 6, i32 7]
 
-; The table for @unreachable
-; CHECK: @switch.table6 = private unnamed_addr constant [5 x i32] [i32 0, i32 0, i32 0, i32 1, i32 -1]
+; The table for @unreachable_case
+; CHECK: @switch.table6 = private unnamed_addr constant [9 x i32] [i32 0, i32 0, i32 0, i32 2, i32 -1, i32 1, i32 1, i32 1, i32 1]
 
 ; A simple int-to-int selection switch.
 ; It is dense enough to be replaced by table lookup.
@@ -752,7 +752,7 @@ return:
 ; CHECK: %switch.gep = getelementptr inbounds [7 x i32]* @switch.table5, i32 0, i32 %switch.tableidx
 }
 
-define i32 @unreachable(i32 %x)  {
+define i32 @unreachable_case(i32 %x)  {
 entry:
   switch i32 %x, label %sw.default [
     i32 0, label %sw.bb
@@ -770,15 +770,15 @@ sw.bb: br label %return
 sw.bb1: unreachable
 sw.bb2: br label %return
 sw.bb3: br label %return
-sw.default: unreachable
+sw.default: br label %return
 
 return:
-  %retval.0 = phi i32 [ 1, %sw.bb3 ], [ -1, %sw.bb2 ], [ 0, %sw.bb ]
+  %retval.0 = phi i32 [ 1, %sw.bb3 ], [ -1, %sw.bb2 ], [ 0, %sw.bb ], [ 2, %sw.default ]
   ret i32 %retval.0
 
-; CHECK-LABEL: @unreachable(
+; CHECK-LABEL: @unreachable_case(
 ; CHECK: switch.lookup:
-; CHECK: getelementptr inbounds [5 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
+; CHECK: getelementptr inbounds [9 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
 }
 
 ; Don't create a table with illegal type