SelectionDAG switch lowering: Replace unreachable default with most popular case.

This can significantly reduce the size of the switch, allowing for more
efficient lowering.

I also worked with the idea of exploiting unreachable defaults by
omitting the range check for jump tables, but always ended up with a
non-neglible binary size increase. It might be worth looking into some more.

SimplifyCFG currently does this transformation, but I'm working towards changing
that so we can optimize harder based on unreachable defaults.

Differential Revision: http://reviews.llvm.org/D6510

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223566 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll

@@ -41,7 +41,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end
@@ -80,7 +80,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end
@@ -119,7 +119,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end

test/CodeGen/X86/asm-label.ll

@@ -24,7 +24,7 @@ if.end:                                           ; preds = %if.then
   br label %cleanup
 
 cleanup:                                          ; preds = %if.end, %if.then9
-  switch i32 undef, label %unreachable [
+  switch i32 undef, label %default [
     i32 0, label %cleanup.cont
     i32 1, label %if.end11
   ]
@@ -35,6 +35,6 @@ cleanup.cont:                                     ; preds = %cleanup
 if.end11:                                         ; preds = %cleanup.cont, %cleanup, %land.lhs.true, %entry
   ret void
 
-unreachable:                                      ; preds = %cleanup
-  unreachable
+default:                                          ; preds = %cleanup
+  br label %if.end11
 }

test/CodeGen/X86/switch-jump-table.ll

@@ -0,0 +1,52 @@
+; RUN: llc -march=x86 < %s | FileCheck %s
+
+
+; An unreachable default destination is replaced with the most popular case label.
+
+define void @sum2(i32 %x, i32* %to) {
+; CHECK-LABEL: sum2:
+; CHECK: movl 4(%esp), [[REG:%e[a-z]{2}]]
+; cmpl $3, [[REG]]
+; CHECK: jbe .LBB0_1
+; CHECK: movl $4
+; CHECK: retl
+; CHECK-LABEL: .LBB0_1:
+; CHECK-NEXT: jmpl *.LJTI0_0(,[[REG]],4)
+
+entry:
+  switch i32 %x, label %default [
+    i32 0, label %bb0
+    i32 1, label %bb1
+    i32 2, label %bb2
+    i32 3, label %bb3
+    i32 4, label %bb4
+    i32 5, label %bb4
+  ]
+bb0:
+  store i32 0, i32* %to
+  br label %exit
+bb1:
+  store i32 1, i32* %to
+  br label %exit
+bb2:
+  store i32 2, i32* %to
+  br label %exit
+bb3:
+  store i32 3, i32* %to
+  br label %exit
+bb4:
+  store i32 4, i32* %to
+  br label %exit
+exit:
+  ret void
+default:
+  unreachable
+
+; The jump table has four entries.
+; CHECK-LABEL: .LJTI0_0:
+; CHECK-NEXT: .long  .LBB0_2
+; CHECK-NEXT: .long  .LBB0_3
+; CHECK-NEXT: .long  .LBB0_4
+; CHECK-NEXT: .long  .LBB0_5
+; CHECK-NOT: .long
+}