Rollup merge of #136690 - Voultapher:use-more-explicit-and-reliable-ptr-select, r=thomcc

Use more explicit and reliable ptr select in sort impls

Using `if ...` with the intent to avoid branches can be surprising to readers and carries the risk of turning into jumps/branches generated by some future compiler version, breaking crucial optimizations.

This commit replaces their usage with the explicit and IR annotated `bool::select_unpredictable`.

Author: matthiaskrgr

library/core/src/slice/sort/shared/smallsort.rs

@@ -387,7 +387,7 @@ unsafe fn swap_if_less<T, F>(v_base: *mut T, a_pos: usize, b_pos: usize, is_less
 where
     F: FnMut(&T, &T) -> bool,
 {
-    // SAFETY: the caller must guarantee that `a` and `b` each added to `v_base` yield valid
+    // SAFETY: the caller must guarantee that `a_pos` and `b_pos` each added to `v_base` yield valid
     // pointers into `v_base`, and are properly aligned, and part of the same allocation.
     unsafe {
         let v_a = v_base.add(a_pos);
@@ -404,16 +404,16 @@ where
         // The equivalent code with a branch would be:
         //
         // if should_swap {
-        //     ptr::swap(left, right, 1);
+        //     ptr::swap(v_a, v_b, 1);
         // }
 
         // The goal is to generate cmov instructions here.
-        let left_swap = if should_swap { v_b } else { v_a };
-        let right_swap = if should_swap { v_a } else { v_b };
+        let v_a_swap = should_swap.select_unpredictable(v_b, v_a);
+        let v_b_swap = should_swap.select_unpredictable(v_a, v_b);
 
-        let right_swap_tmp = ManuallyDrop::new(ptr::read(right_swap));
-        ptr::copy(left_swap, v_a, 1);
-        ptr::copy_nonoverlapping(&*right_swap_tmp, v_b, 1);
+        let v_b_swap_tmp = ManuallyDrop::new(ptr::read(v_b_swap));
+        ptr::copy(v_a_swap, v_a, 1);
+        ptr::copy_nonoverlapping(&*v_b_swap_tmp, v_b, 1);
     }
 }
 
@@ -640,26 +640,21 @@ pub unsafe fn sort4_stable<T, F: FnMut(&T, &T) -> bool>(
         //  1,  1 |  c   b    a         d
         let c3 = is_less(&*c, &*a);
         let c4 = is_less(&*d, &*b);
-        let min = select(c3, c, a);
-        let max = select(c4, b, d);
-        let unknown_left = select(c3, a, select(c4, c, b));
-        let unknown_right = select(c4, d, select(c3, b, c));
+        let min = c3.select_unpredictable(c, a);
+        let max = c4.select_unpredictable(b, d);
+        let unknown_left = c3.select_unpredictable(a, c4.select_unpredictable(c, b));
+        let unknown_right = c4.select_unpredictable(d, c3.select_unpredictable(b, c));
 
         // Sort the last two unknown elements.
         let c5 = is_less(&*unknown_right, &*unknown_left);
-        let lo = select(c5, unknown_right, unknown_left);
-        let hi = select(c5, unknown_left, unknown_right);
+        let lo = c5.select_unpredictable(unknown_right, unknown_left);
+        let hi = c5.select_unpredictable(unknown_left, unknown_right);
 
         ptr::copy_nonoverlapping(min, dst, 1);
         ptr::copy_nonoverlapping(lo, dst.add(1), 1);
         ptr::copy_nonoverlapping(hi, dst.add(2), 1);
         ptr::copy_nonoverlapping(max, dst.add(3), 1);
     }
-
-    #[inline(always)]
-    fn select<T>(cond: bool, if_true: *const T, if_false: *const T) -> *const T {
-        if cond { if_true } else { if_false }
-    }
 }
 
 /// SAFETY: The caller MUST guarantee that `v_base` is valid for 8 reads and