[CodeGen] Omit range checks from jump tables when lowering switches with unreachable default

During the lowering of a switch that would result in the generation of a
jump table, a range check is performed before indexing into the jump
table, for the switch value being outside the jump table range and a
conditional branch is inserted to jump to the default block. In case the
default block is unreachable, this conditional jump can be omitted. This
patch implements omitting this conditional branch for unreachable
defaults.

Differential Revision: https://reviews.llvm.org/D52002
Reviewers: Hans Wennborg, Eli Freidman, Roman Lebedev

llvm-svn: 355490

1 files changed, 63 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll b/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll
new file mode 100644
index 00000000000..ce11c14d1ca
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll
@@ -0,0 +1,63 @@
+; RUN: llc -O3 -o - %s | FileCheck %s
+
+; Test that the output in the presence of an unreachable default does not have
+; a compare and branch at the top of the switch to handle the default case.
+
+target triple = "aarch64-unknown-linux-gnu"
+
+; Function Attrs: nounwind
+define void @fn(i4) {
+  switch i4 %0, label %default [
+    i4 0, label %case_0
+    i4 1, label %case_1
+    i4 2, label %case_2
+    i4 3, label %case_3
+    i4 4, label %case_4
+    i4 5, label %case_5
+  ]
+
+; CHECK-LABEL: fn:
+; CHECK-NOT:    sub
+; CHECK-NOT:    cmp
+; CHECK-NOT:    b.hi
+; CHECK:        ldrb {{w[0-9]+}}, [{{x[0-9]+}}, {{x[0-9]+}}]
+; CHECK:        add {{x[0-9]+}}, {{x[0-9]+}}, {{x[0-9]+}}, lsl #2
+; CHECK:        br {{x[0-9]+}}
+
+default:
+  unreachable
+
+case_0:
+  tail call void @handle_case_00(i4 %0) #2
+  br label %return_label
+
+case_1:
+  tail call void @handle_case_01(i4 %0) #2
+  br label %return_label
+
+case_2:
+  tail call void @handle_case_02(i4 %0) #2
+  br label %return_label
+
+case_3:
+  tail call void @handle_case_03(i4 %0) #2
+  br label %return_label
+
+case_4:
+  tail call void @handle_case_04(i4 %0) #2
+  br label %return_label
+
+case_5:
+  tail call void @handle_case_05(i4 %0) #2
+  br label %return_label
+
+return_label:
+  ret void
+}
+
+declare  void @handle_case_00(i4)
+declare  void @handle_case_01(i4)
+declare  void @handle_case_02(i4)
+declare  void @handle_case_03(i4)
+declare  void @handle_case_04(i4)
+declare  void @handle_case_05(i4)