Auto merge of #102535 - scottmcm:optimize-split-at-partition-point, r=thomcc

Tell LLVM that `partition_point` returns a valid fencepost

This was already done for a successful `binary_search`, but this way `partition_point` can get similar optimizations.

Demonstration that nightly can't do this optimization today, and leaves in the panicking path: <https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=e1074cd2faf5f68e49cffd728ded243a>

r? `@thomcc`

1 files changed, 20 insertions, 0 deletions

diff --git a/src/test/codegen/binary-search-index-no-bound-check.rs b/src/test/codegen/binary-search-index-no-bound-check.rs
index 2deabcaa6c2..c1766a4a44a 100644
--- a/src/test/codegen/binary-search-index-no-bound-check.rs
+++ b/src/test/codegen/binary-search-index-no-bound-check.rs
@@ -16,3 +16,23 @@ pub fn binary_search_index_no_bounds_check(s: &[u8]) -> u8 {
         42
     }
 }
+
+// Similarly, check that `partition_point` is known to return a valid fencepost.
+
+// CHECK-LABEL: @unknown_split
+#[no_mangle]
+pub fn unknown_split(x: &[i32], i: usize) -> (&[i32], &[i32]) {
+    // This just makes sure that the subsequent function is looking for the
+    // absence of something that might actually be there.
+
+    // CHECK: call core::panicking::panic
+    x.split_at(i)
+}
+
+// CHECK-LABEL: @partition_point_split_no_bounds_check
+#[no_mangle]
+pub fn partition_point_split_no_bounds_check(x: &[i32], needle: i32) -> (&[i32], &[i32]) {
+    // CHECK-NOT: call core::panicking::panic
+    let i = x.partition_point(|p| p < &needle);
+    x.split_at(i)
+}