Switch lowering: exploit unreachable fall-through when lowering case range cluster

In the example below, we would previously emit two range checks, one for cases
1--3 and one for 4--6. This patch makes us exploit the fact that the
fall-through is unreachable and only one range check is necessary.

  switch i32 %i, label %default [
    i32 1,  label %bb1
    i32 2,  label %bb1
    i32 3,  label %bb1
    i32 4,  label %bb2
    i32 5,  label %bb2
    i32 6,  label %bb2
  ]
  default: unreachable

llvm-svn: 357252

2 files changed, 34 insertions, 9 deletions

diff --git a/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll b/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll
index 9db3cee14e5..9acc5a150b6 100644
--- a/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll
+++ b/llvm/test/CodeGen/AArch64/switch-unreachable-default.ll
@@ -74,17 +74,15 @@ entry:
     i32 64, label %bb5
   ]
 
-; The switch is lowered with a jump table for cases 1--32 and case 64 checked
-; separately. Even though the default of switch is unreachable, the fall-through
-; for the jump table *is* reachable so the range check must be emitted.
+; The switch is lowered with a jump table for cases 1--32 and case 64 handled
+; separately. Even though the default of the switch is unreachable, the
+; fall-through for the jump table *is* reachable so the range check must be
+; emitted.
 ;
 ; CHECK-LABEL: reachable_fallthrough
-; CHECK: sub
-; CHECK: cmp
+; CHECK: sub [[REG:w[0-9]+]], w0, #1
+; CHECK: cmp [[REG]], #31
 ; CHECK: b.hi
-;
-; TODO: Drop the cmp for the 64 case since its fallthrough is unreachable.
-
 
 def: unreachable
 bb1: br label %return
@@ -94,6 +92,6 @@ bb4: br label %return
 bb5: br label %return
 
 return:
-  %p = phi i32 [ 3, %bb1 ], [ 2, %bb2 ], [ 1, %bb3 ], [ 0, %bb4 ], [ 0, %bb5 ]
+  %p = phi i32 [ 3, %bb1 ], [ 2, %bb2 ], [ 1, %bb3 ], [ 0, %bb4 ], [ 42, %bb5 ]
   ret i32 %p
 }
diff --git a/llvm/test/CodeGen/X86/switch.ll b/llvm/test/CodeGen/X86/switch.ll
index a0ebc4eeba2..639b9c0eb8f 100644
--- a/llvm/test/CodeGen/X86/switch.ll
+++ b/llvm/test/CodeGen/X86/switch.ll
@@ -778,3 +778,30 @@ end:
 ; CHECK: btl
 ; CHECK-NOT: btl
 }
+
+
+define void @range_with_unreachable_fallthrough(i32 %i) {
+entry:
+  switch i32 %i, label %default [
+    i32 1,  label %bb1
+    i32 2,  label %bb1
+    i32 3,  label %bb1
+    i32 4,  label %bb2
+    i32 5,  label %bb2
+    i32 6,  label %bb2
+  ]
+bb1: tail call void @g(i32 0) br label %return
+bb2: tail call void @g(i32 1) br label %return
+default: unreachable
+
+return:
+  ret void
+
+; CHECK-LABEL: range_with_unreachable_fallthrough:
+; Since the default is unreachable, either the 1--3 or the 4--6 cluster will be
+; reached. Only one comparison should be emitted.
+; CHECK: cmpl
+; CHECK-NOT: cmpl
+; CHECK: jmp g
+; CHECK: jmp g
+}