Add tests for LLVM 20 slice bounds check optimization

Author: reez12g

tests/codegen/slice-last-elements-optimization.rs

@@ -0,0 +1,37 @@
+//@ compile-flags: -Copt-level=3
+//@ only-x86_64
+//@ min-llvm-version: 20
+#![crate_type = "lib"]
+
+// This test verifies that LLVM 20 properly optimizes the bounds check
+// when accessing the last few elements of a slice with proper conditions.
+// Previously, this would generate an unreachable branch to
+// slice_start_index_len_fail even when the bounds check was provably safe.
+
+// CHECK-LABEL: @last_four_initial(
+#[no_mangle]
+pub fn last_four_initial(s: &[u8]) -> &[u8] {
+    // Previously this would generate a branch to slice_start_index_len_fail
+    // that is unreachable. The LLVM 20 fix should eliminate this branch.
+    // CHECK-NOT: slice_start_index_len_fail
+    // CHECK-NOT: unreachable
+    let start = if s.len() <= 4 { 0 } else { s.len() - 4 };
+    &s[start..]
+}
+
+// CHECK-LABEL: @last_four_optimized(
+#[no_mangle]
+pub fn last_four_optimized(s: &[u8]) -> &[u8] {
+    // This version was already correctly optimized before the fix in LLVM 20.
+    // CHECK-NOT: slice_start_index_len_fail
+    // CHECK-NOT: unreachable
+    if s.len() <= 4 { &s[0..] } else { &s[s.len() - 4..] }
+}
+
+// Just to verify we're correctly checking for the right thing
+// CHECK-LABEL: @test_bounds_check_happens(
+#[no_mangle]
+pub fn test_bounds_check_happens(s: &[u8], i: usize) -> &[u8] {
+    // CHECK: slice_start_index_len_fail
+    &s[i..]
+}