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.

src/librustc_trans/builder.rs

@@ -344,6 +344,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         }
     }
 
+    pub fn exactudiv(&self, lhs: ValueRef, rhs: ValueRef) -> ValueRef {
+        self.count_insn("exactudiv");
+        unsafe {
+            llvm::LLVMBuildExactUDiv(self.llbuilder, lhs, rhs, noname())
+        }
+    }
+
     pub fn sdiv(&self, lhs: ValueRef, rhs: ValueRef) -> ValueRef {
         self.count_insn("sdiv");
         unsafe {

src/librustc_trans/intrinsic.rs

@@ -289,7 +289,7 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bx: &Builder<'a, 'tcx>,
         "ctlz" | "ctlz_nonzero" | "cttz" | "cttz_nonzero" | "ctpop" | "bswap" |
         "bitreverse" | "add_with_overflow" | "sub_with_overflow" |
         "mul_with_overflow" | "overflowing_add" | "overflowing_sub" | "overflowing_mul" |
-        "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" => {
+        "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" | "exact_div" => {
             let ty = arg_tys[0];
             match int_type_width_signed(ty, cx) {
                 Some((width, signed)) =>
@@ -343,6 +343,12 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bx: &Builder<'a, 'tcx>,
                         "overflowing_add" => bx.add(args[0].immediate(), args[1].immediate()),
                         "overflowing_sub" => bx.sub(args[0].immediate(), args[1].immediate()),
                         "overflowing_mul" => bx.mul(args[0].immediate(), args[1].immediate()),
+                        "exact_div" =>
+                            if signed {
+                                bx.exactsdiv(args[0].immediate(), args[1].immediate())
+                            } else {
+                                bx.exactudiv(args[0].immediate(), args[1].immediate())
+                            },
                         "unchecked_div" =>
                             if signed {
                                 bx.sdiv(args[0].immediate(), args[1].immediate())