Assume slice len is bounded by allocation size

Uses assume to check the length against a constant upper bound. The
inlined result then informs the optimizer of the sound value range.

This was tried with unreachable_unchecked before which introduces a
branch. This has the advantage of not being executed in sound code but
complicates basic blocks. It resulted in ~2% increased compile time in
some worst cases.

Add a codegen test for the assumption, testing the issue from #67186

1 files changed, 24 insertions, 0 deletions

diff --git a/src/test/codegen/len-is-bounded.rs b/src/test/codegen/len-is-bounded.rs
new file mode 100644
index 00000000000..bb74fc3b275
--- /dev/null
+++ b/src/test/codegen/len-is-bounded.rs
@@ -0,0 +1,24 @@
+// min-llvm-version: 11.0
+// compile-flags: -O -C panic=abort
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn len_range(a: &[u8], b: &[u8]) -> usize {
+    // CHECK-NOT: panic
+    a.len().checked_add(b.len()).unwrap()
+}
+
+#[no_mangle]
+pub fn len_range_on_non_byte(a: &[u16], b: &[u16]) -> usize {
+    // CHECK-NOT: panic
+    a.len().checked_add(b.len()).unwrap()
+}
+
+pub struct Zst;
+
+#[no_mangle]
+pub fn zst_range(a: &[Zst], b: &[Zst]) -> usize {
+    // Zsts may be arbitrarily large.
+    // CHECK: panic
+    a.len().checked_add(b.len()).unwrap()
+}