Auto merge of #108623 - scottmcm:try-different-as-slice-impl, r=the8472

Move `Option::as_slice` to an always-sound implementation

This approach depends on CSE to not have any branches or selects when the guessed offset is correct -- which it always will be right now -- but to also be *sound* (just less efficient) if the layout algorithms change such that the guess is incorrect.

The codegen test confirms that CSE handles this as expected, leaving the optimal codegen.

cc JakobDegen #108545

1 files changed, 13 insertions, 5 deletions

diff --git a/tests/codegen/option-as-slice.rs b/tests/codegen/option-as-slice.rs
index d5077dbf6cc..d735d558374 100644
--- a/tests/codegen/option-as-slice.rs
+++ b/tests/codegen/option-as-slice.rs
@@ -1,4 +1,4 @@
-// compile-flags: -O
+// compile-flags: -O -Z randomize-layout=no
 // only-x86_64
 
 #![crate_type = "lib"]
@@ -12,17 +12,25 @@ use core::option::Option;
 // CHECK-LABEL: @u64_opt_as_slice
 #[no_mangle]
 pub fn u64_opt_as_slice(o: &Option<u64>) -> &[u64] {
-    // CHECK: start:
     // CHECK-NOT: select
-    // CHECK: ret
+    // CHECK-NOT: br
+    // CHECK-NOT: switch
+    // CHECK-NOT: icmp
     o.as_slice()
 }
 
 // CHECK-LABEL: @nonzero_u64_opt_as_slice
 #[no_mangle]
 pub fn nonzero_u64_opt_as_slice(o: &Option<NonZeroU64>) -> &[NonZeroU64] {
-    // CHECK: start:
     // CHECK-NOT: select
-    // CHECK: ret
+    // CHECK-NOT: br
+    // CHECK-NOT: switch
+    // CHECK-NOT: icmp
+    // CHECK: %[[NZ:.+]] = icmp ne i64 %{{.+}}, 0
+    // CHECK-NEXT: zext i1 %[[NZ]] to i64
+    // CHECK-NOT: select
+    // CHECK-NOT: br
+    // CHECK-NOT: switch
+    // CHECK-NOT: icmp
     o.as_slice()
 }