Rollup merge of #139726 - Amanieu:select_unpredictable_hint, r=dtolnay

Move `select_unpredictable` to the `hint` module

There has been considerable discussion in both the ACP (rust-lang/libs-team#468) and tracking issue (#133962) about whether the `bool::select_unpredictable` method should be in `core::hint` instead.

I believe this is the right move for the following reasons:
- The documentation explicitly says that it is a hint, not a codegen guarantee.
- `bool` doesn't have a corresponding `select` method, and I don't think we should be adding one.
- This shouldn't be something that people reach for with auto-completion unless they specifically understand the interactions with branch prediction. Using conditional moves can easily make code *slower* by preventing the CPU from speculating past the condition due to the data dependency.
- Although currently `core::hint` only contains no-ops, this isn't a hard rule (for example `unreachable_unchecked` is a bit of a gray area). The documentation only status that the module contains "hints to compiler that affects how code should be emitted or optimized". This is consistent with what `select_unpredictable` does.

1 files changed, 4 insertions, 4 deletions

diff --git a/tests/codegen/intrinsics/select_unpredictable.rs b/tests/codegen/intrinsics/select_unpredictable.rs
index 68a02c8342d..2db4ae174b3 100644
--- a/tests/codegen/intrinsics/select_unpredictable.rs
+++ b/tests/codegen/intrinsics/select_unpredictable.rs
@@ -46,21 +46,21 @@ pub fn test_zst(p: bool, a: (), b: ()) -> () {
 pub fn test_int2(p: bool, a: u64, b: u64) -> u64 {
     // CHECK-LABEL: define{{.*}} @test_int2
     // CHECK: select i1 %p, i64 %a, i64 %b, !unpredictable
-    p.select_unpredictable(a, b)
+    core::hint::select_unpredictable(p, a, b)
 }
 
 #[no_mangle]
 pub fn test_pair2(p: bool, a: (u64, u64), b: (u64, u64)) -> (u64, u64) {
     // CHECK-LABEL: define{{.*}} @test_pair2
     // CHECK: select i1 %p, {{.*}}, !unpredictable
-    p.select_unpredictable(a, b)
+    core::hint::select_unpredictable(p, a, b)
 }
 
 #[no_mangle]
 pub fn test_struct2(p: bool, a: Large, b: Large) -> Large {
     // CHECK-LABEL: define{{.*}} @test_struct2
     // CHECK: select i1 %p, {{.*}}, !unpredictable
-    p.select_unpredictable(a, b)
+    core::hint::select_unpredictable(p, a, b)
 }
 
 #[no_mangle]
@@ -68,5 +68,5 @@ pub fn test_zst2(p: bool, a: (), b: ()) -> () {
     // CHECK-LABEL: define{{.*}} @test_zst2
     // CHECK-NEXT: start:
     // CHECK-NEXT: ret void
-    p.select_unpredictable(a, b)
+    core::hint::select_unpredictable(p, a, b)
 }