From 50f6a9ed87e47c7a8ff6aefcde01a33821e80e20 Mon Sep 17 00:00:00 2001 From: Erik Desjardins Date: Mon, 13 Jun 2022 21:49:59 -0400 Subject: use unchecked mul to compute slice sizes ...since slice sizes can't signed wrap see https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html > The total size len * mem::size_of::() of the slice must be no larger than isize::MAX. --- src/test/codegen/issue-96497-slice-size-nowrap.rs | 29 +++++++++++++++++++++++ 1 file changed, 29 insertions(+) create mode 100644 src/test/codegen/issue-96497-slice-size-nowrap.rs (limited to 'src/test/codegen') diff --git a/src/test/codegen/issue-96497-slice-size-nowrap.rs b/src/test/codegen/issue-96497-slice-size-nowrap.rs new file mode 100644 index 00000000000..a5dbef93460 --- /dev/null +++ b/src/test/codegen/issue-96497-slice-size-nowrap.rs @@ -0,0 +1,29 @@ +// This test case checks that LLVM is aware that computing the size of a slice cannot wrap. +// The possibility of wrapping results in an additional branch when dropping boxed slices +// in some situations, see https://github.com/rust-lang/rust/issues/96497#issuecomment-1112865218 + +// compile-flags: -O +// min-llvm-version: 14.0 + +#![crate_type="lib"] + +// CHECK-LABEL: @simple_size_of_nowrap +#[no_mangle] +pub fn simple_size_of_nowrap(x: &[u32]) -> usize { + // Make sure the shift used to compute the size has a nowrap flag. + + // CHECK: [[A:%.*]] = shl nsw {{.*}}, 2 + // CHECK-NEXT: ret {{.*}} [[A]] + core::mem::size_of_val(x) +} + +// CHECK-LABEL: @drop_write +#[no_mangle] +pub fn drop_write(mut x: Box<[u32]>) { + // Check that this write is optimized out. + // This depends on the size calculation not wrapping, + // since otherwise LLVM can't tell that the memory is always deallocated if the slice len > 0. + + // CHECK-NOT: store i32 42 + x[1] = 42; +} -- cgit 1.4.1-3-g733a5