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.

llvm-svn: 223049

3 files changed, 58 insertions, 6 deletions

diff --git a/llvm/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll b/llvm/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll
index 10dc927200b..9cd150a2f56 100644
--- a/llvm/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll
+++ b/llvm/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
diff --git a/llvm/test/CodeGen/X86/asm-label.ll b/llvm/test/CodeGen/X86/asm-label.ll
index 1fc6e2eaf2b..1da66e74d34 100644
--- a/llvm/test/CodeGen/X86/asm-label.ll
+++ b/llvm/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
 }
diff --git a/llvm/test/CodeGen/X86/switch-jump-table.ll b/llvm/test/CodeGen/X86/switch-jump-table.ll
new file mode 100644
index 00000000000..7ae45b56641
--- /dev/null
+++ b/llvm/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
+}