Switch-to-lookup tables: Don't require a result for the default

case when the lookup table doesn't have any holes.

This means we can build a lookup table for switches like this:

  switch (x) {
    case 0: return 1;
    case 1: return 2;
    case 2: return 3;
    case 3: return 4;
    default: exit(1);
  }

The default case doesn't yield a constant result here, but that doesn't matter,
since a default result is only necessary for filling holes in the lookup table,
and this table doesn't have any holes.

This makes us transform 505 more switches in a clang bootstrap, and shaves 164 KB
off the resulting clang binary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199025 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 50 insertions, 0 deletions

diff --git a/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll b/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
index 368732711a..4ac02dbdf3 100644
--- a/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ b/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -806,3 +806,53 @@ return:
 ; CHECK-NOT: @switch.table
 ; CHECK: switch i32 %c
 }
+
+; If we can build a lookup table without any holes, we don't need a default result.
+declare void @exit(i32)
+define i32 @nodefaultnoholes(i32 %c) {
+entry:
+  switch i32 %c, label %sw.default [
+    i32 0, label %return
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb3
+  ]
+
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: call void @exit(i32 1)
+            unreachable
+return:
+  %x = phi i32 [ -1, %sw.bb3 ], [ 0, %sw.bb2 ], [ 123, %sw.bb1 ], [ 55, %entry ]
+  ret i32 %x
+
+; CHECK-LABEL: @nodefaultnoholes(
+; CHECK: @switch.table
+; CHECK-NOT: switch i32
+}
+
+; This lookup table will have holes, so we cannot build it without default result.
+define i32 @nodefaultwithholes(i32 %c) {
+entry:
+  switch i32 %c, label %sw.default [
+    i32 0, label %return
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+    i32 3, label %sw.bb3
+    i32 5, label %sw.bb3
+  ]
+
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: call void @exit(i32 1)
+            unreachable
+return:
+  %x = phi i32 [ -1, %sw.bb3 ], [ 0, %sw.bb2 ], [ 123, %sw.bb1 ], [ 55, %entry ]
+  ret i32 %x
+
+; CHECK-LABEL: @nodefaultwithholes(
+; CHECK-NOT: @switch.table
+; CHECK: switch i32
+}