elaborate, add check for exact bounds

Author: erikdesjardins

src/test/codegen/issue-69101-bounds-check.rs

@@ -2,6 +2,12 @@
 // compile-flags: -O
 #![crate_type = "lib"]
 
+// Make sure no bounds checks are emitted in the loop when upfront slicing
+// ensures that the slices are big enough.
+// In particular, bounds checks were not always optimized out if the upfront
+// check was for a greater len than the loop requires.
+// (i.e. `already_sliced_no_bounds_check` was not always optimized even when
+// `already_sliced_no_bounds_check_exact` was)
 // CHECK-LABEL: @already_sliced_no_bounds_check
 #[no_mangle]
 pub fn already_sliced_no_bounds_check(a: &[u8], b: &[u8], c: &mut [u8]) {
@@ -13,7 +19,18 @@ pub fn already_sliced_no_bounds_check(a: &[u8], b: &[u8], c: &mut [u8]) {
     }
 }
 
-// make sure we're checking for the right thing: there can be a panic if the slice is too small
+// CHECK-LABEL: @already_sliced_no_bounds_check_exact
+#[no_mangle]
+pub fn already_sliced_no_bounds_check_exact(a: &[u8], b: &[u8], c: &mut [u8]) {
+    // CHECK: slice_index_len_fail
+    // CHECK-NOT: panic_bounds_check
+    let _ = (&a[..1024], &b[..1024], &mut c[..1024]);
+    for i in 0..1024 {
+        c[i] = a[i] ^ b[i];
+    }
+}
+
+// Make sure we're checking for the right thing: there can be a panic if the slice is too small.
 // CHECK-LABEL: @already_sliced_bounds_check
 #[no_mangle]
 pub fn already_sliced_bounds_check(a: &[u8], b: &[u8], c: &mut [u8]) {