[clang] [CIR] Upstream pointer subtraction handling (PR #163306)

[Andy Kaylor via cfe-commits]

================
@@ -4044,6 +4044,37 @@ def CIR_ExpectOp : CIR_Op<"expect", [
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// PtrDiffOp
+//===----------------------------------------------------------------------===//
+
+def CIR_PtrDiffOp : CIR_Op<"ptr_diff", [Pure, SameTypeOperands]> {
+  let summary = "Pointer subtraction arithmetic";
+  let description = [{
+    The cir.ptr_diff operation computes the difference between two pointers that
+    have the same element type
+
+    The result reflects the ABI-defined size of the pointed-to type. For example,
+    subtracting two !cir.ptr<!u64i> values may yield 1, representing an 8-byte
----------------
andykaylor wrote:

I'm opening a can of worms here, but a more informative example would be `_BitInt(24)`, which we currently represent in CIR as `!u24i`. We handle that incorrectly in the incubator, treating it as a 3-byte type, though for the purposes of target data layout it is generally going to be a 4-byte type.

https://godbolt.org/z/6zneqYzYP

See also: https://github.com/llvm/llvm-project/issues/159731

I'm not suggesting that you need to fix the associated problems in this PR. I'd just like to see the description here changed to reflect the way the operation should behave. I believe that `cir::MissingFeatures::llvmLoweringPtrDiffConsidersPointee` captures the problem.

https://github.com/llvm/llvm-project/pull/163306