From 89a18cb60053a6c1b3b5f817246ddfd917be1e3e Mon Sep 17 00:00:00 2001 From: Scott McMurray Date: Sat, 9 Apr 2022 01:27:47 -0700 Subject: Add `unsigned_offset_from` on pointers Like we have `add`/`sub` which are the `usize` version of `offset`, this adds the `usize` equivalent of `offset_from`. Like how `.add(d)` replaced a whole bunch of `.offset(d as isize)`, you can see from the changes here that it's fairly common that code actually knows the order between the pointers and *wants* a `usize`, not an `isize`. As a bonus, this can do `sub nuw`+`udiv exact`, rather than `sub`+`sdiv exact`, which can be optimized slightly better because it doesn't have to worry about negatives. That's why the slice iterators weren't using `offset_from`, though I haven't updated that code in this PR because slices are so perf-critical that I'll do it as its own change. This is an intrinsic, like `offset_from`, so that it can eventually be allowed in CTFE. It also allows checking the extra safety condition -- see the test confirming that CTFE catches it if you pass the pointers in the wrong order. --- src/test/codegen/intrinsics/offset_from.rs | 36 ++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 src/test/codegen/intrinsics/offset_from.rs (limited to 'src/test/codegen') diff --git a/src/test/codegen/intrinsics/offset_from.rs b/src/test/codegen/intrinsics/offset_from.rs new file mode 100644 index 00000000000..d0de4c8355d --- /dev/null +++ b/src/test/codegen/intrinsics/offset_from.rs @@ -0,0 +1,36 @@ +// compile-flags: -C opt-level=1 +// only-64bit (because we're using [ui]size) + +#![crate_type = "lib"] +#![feature(core_intrinsics)] + +//! Basic optimizations are enabled because otherwise `x86_64-gnu-nopt` had an alloca. +//! Uses a type with non-power-of-two size to avoid normalizations to shifts. + +use std::intrinsics::*; + +type RGB = [u8; 3]; + +// CHECK-LABEL: @offset_from_odd_size +#[no_mangle] +pub unsafe fn offset_from_odd_size(a: *const RGB, b: *const RGB) -> isize { + // CHECK: start + // CHECK-NEXT: ptrtoint + // CHECK-NEXT: ptrtoint + // CHECK-NEXT: sub i64 + // CHECK-NEXT: sdiv exact i64 %{{[0-9]+}}, 3 + // CHECK-NEXT: ret i64 + ptr_offset_from(a, b) +} + +// CHECK-LABEL: @offset_from_unsigned_odd_size +#[no_mangle] +pub unsafe fn offset_from_unsigned_odd_size(a: *const RGB, b: *const RGB) -> usize { + // CHECK: start + // CHECK-NEXT: ptrtoint + // CHECK-NEXT: ptrtoint + // CHECK-NEXT: sub nuw i64 + // CHECK-NEXT: udiv exact i64 %{{[0-9]+}}, 3 + // CHECK-NEXT: ret i64 + ptr_offset_from_unsigned(a, b) +} -- cgit 1.4.1-3-g733a5