Auto merge of #49297 - scottmcm:offset-from, r=dtolnay

Introduce unsafe offset_from on pointers

Adds intrinsics::exact_div to take advantage of the unsafe, which reduces the implementation from
```asm
    sub rcx, rdx
    mov rax, rcx
    sar rax, 63
    shr rax, 62
    lea rax, [rax + rcx]
    sar rax, 2
    ret
```
down to
```asm
    sub rcx, rdx
    sar rcx, 2
    mov rax, rcx
    ret
```
(for `*const i32`)

See discussion on the `offset_to` tracking issue https://github.com/rust-lang/rust/issues/41079

Some open questions
- Would you rather I split the intrinsic PR from the library PR?
- Do we even want the safe version of the API?  https://github.com/rust-lang/rust/issues/41079#issuecomment-374426786  I've added some text to its documentation that even if it's not UB, it's useless to use it between pointers into different objects.

and todos
- [x] ~~I need to make a codegen test~~ Done
- [x] ~~Can the subtraction use nsw/nuw?~~ No, it can't https://github.com/rust-lang/rust/pull/49297#discussion_r176697574
- [x] ~~Should there be `usize` variants of this, like there are now `add` and `sub` that you almost always want over `offset`?  For example, I imagine `sub_ptr` that returns `usize` and where it's UB if the distance is negative.~~ Can wait for later; C gives a signed result https://github.com/rust-lang/rust/issues/41079#issuecomment-375842235, so we might as well, and this existing to go with `offset` makes sense.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index a5644d6f9e2..e815d151aeb 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -1492,3 +1492,11 @@ LLVMRustBuildVectorReduceFMax(LLVMBuilderRef, LLVMValueRef, bool) {
   return nullptr;
 }
 #endif
+
+#if LLVM_VERSION_LT(4, 0)
+extern "C" LLVMValueRef
+LLVMBuildExactUDiv(LLVMBuilderRef B, LLVMValueRef LHS,
+                   LLVMValueRef RHS, const char *Name) {
+  return wrap(unwrap(B)->CreateExactUDiv(unwrap(LHS), unwrap(RHS), Name));
+}
+#endif