tests/codegen/issue-96497-slice-size-nowrap.rs

   1 // This test case checks that LLVM is aware that computing the size of a slice cannot wrap.
   2 // The possibility of wrapping results in an additional branch when dropping boxed slices
   3 // in some situations, see https://github.com/rust-lang/rust/issues/96497#issuecomment-1112865218
   4
   5 // compile-flags: -O
   6 // min-llvm-version: 15.0
   7
   8 #![crate_type="lib"]
   9
  10 // CHECK-LABEL: @simple_size_of_nowrap
  11 #[no_mangle]
  12 pub fn simple_size_of_nowrap(x: &[u32]) -> usize {
  13     // Make sure the shift used to compute the size has a nowrap flag.
  14
  15     // CHECK: [[A:%.*]] = shl nsw {{.*}}, 2
  16     // CHECK-NEXT: ret {{.*}} [[A]]
  17     core::mem::size_of_val(x)
  18 }
  19
  20 // CHECK-LABEL: @drop_write
  21 #[no_mangle]
  22 pub fn drop_write(mut x: Box<[u32]>) {
  23     // Check that this write is optimized out.
  24     // This depends on the size calculation not wrapping,
  25     // since otherwise LLVM can't tell that the memory is always deallocated if the slice len > 0.
  26
  27     // CHECK-NOT: store i32 42
  28     x[1] = 42;
  29 }