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.

1 files changed, 36 insertions, 0 deletions

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)
+}