[llvm] [SimplifyCFG] Replace unreachable switch lookup table holes with poison (PR #94990)

[via llvm-commits]

================
@@ -2151,3 +2152,222 @@ return:                                           ; preds = %sw.default, %entry,
   %retval.0 = phi { i8, i8 } [ undef, %entry ], [ undef, %entry ], [ undef, %entry ], [ %1, %sw.default ]
   ret { i8, i8 } %retval.0
 }
+
+; The switch has a hole which falls through to an unreachable default case, but it can still be optimized into a constant load because
+; the poison value used for the hole is ignored.
+define i32 @constant_hole_unreachable_default(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  switch i32 %x, label %sw.default [
+  i32 0, label %bb0
+  i32 2, label %bb0
+  i32 3, label %bb0
+  i32 4, label %bb0
+  ]
+
+sw.default: unreachable
+bb0: br label %return
+
+return:
+  %res = phi i32 [ 1, %bb0 ]
+  ret i32 %res
+}
+
+; The switch has a hole which falls through to an unreachable default case and the first case explicitly returns undef, but it can still
+; be optimized into a constant load because the undef values are ignored.
+define i32 @constant_hole_unreachable_default_firstundef(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default_firstundef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  switch i32 %x, label %sw.default [
+  i32 0, label %bb.undef
+  i32 2, label %bb0
+  i32 3, label %bb0
+  i32 4, label %bb0
+  ]
+
+sw.default: unreachable
+bb.undef: br label %return
+bb0: br label %return
+
+return:
+  %res = phi i32 [ undef, %bb.undef ], [ 1, %bb0 ]
+  ret i32 %res
+}
+
+; The switch has a hole which falls through to an unreachable default case and the first case explicitly returns undef, but it can still
+; be optimized into a constant load because the undef values are ignored.
+define i32 @constant_hole_unreachable_default_lastundef(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default_lastundef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  switch i32 %x, label %sw.default [
+  i32 0, label %bb0
+  i32 2, label %bb0
+  i32 3, label %bb0
+  i32 4, label %bb.undef
+  ]
+
+sw.default: unreachable
+bb.undef: br label %return
+bb0: br label %return
+
+return:
+  %res = phi i32 [ undef, %bb.undef ], [ 1, %bb0 ]
+  ret i32 %res
+}
+
+; The switch has a hole which falls through to an unreachable default case and the first case explicitly returns poison, but it can still
+; be optimized into a constant load because the poison values are ignored.
+define i32 @constant_hole_unreachable_default_firstpoison(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default_firstpoison(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  switch i32 %x, label %sw.default [
+  i32 0, label %bb.poison
+  i32 2, label %bb0
+  i32 3, label %bb0
+  i32 4, label %bb0
+  ]
+
+sw.default: unreachable
+bb.poison: br label %return
+bb0: br label %return
+
+return:
+  %res = phi i32 [ poison, %bb.poison ], [ 1, %bb0 ]
+  ret i32 %res
+}
+
+; The switch has a hole which falls through to an unreachable default case and the first case explicitly returns poison, but it can still
+; be optimized into a constant load because the poison values are ignored.
+define i32 @constant_hole_unreachable_default_lastpoison(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default_lastpoison(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  switch i32 %x, label %sw.default [
+  i32 0, label %bb0
+  i32 2, label %bb0
+  i32 3, label %bb0
+  i32 4, label %bb.poison
+  ]
+
+sw.default: unreachable
+bb.poison: br label %return
+bb0: br label %return
+
+return:
+  %res = phi i32 [ poison, %bb.poison ], [ 1, %bb0 ]
+  ret i32 %res
+}
+
+define i32 @constant_hole_unreachable_default_undef_poison(i32 %x) {
+; CHECK-LABEL: @constant_hole_unreachable_default_undef_poison(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i32 poison
----------------
DianQK wrote:

It should return `undef` here.

https://github.com/llvm/llvm-project/pull/94990