[llvm] [IR] Require that ptrmask mask matches pointer index size (PR #69343)

[Jessica Clarke via llvm-commits]

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@@ -26965,13 +26966,23 @@ to facilitate alias analysis and underlying-object detection.
 Semantics:
 """"""""""
 
-The result of ``ptrmask(ptr, mask)`` is equivalent to
-``getelementptr ptr, (ptrtoint(ptr) & mask) - ptrtoint(ptr)``. Both the returned
-pointer(s) and the first argument are based on the same underlying object (for more
-information on the *based on* terminology see
-:ref:`the pointer aliasing rules <pointeraliasing>`). If the bitwidth of the
-mask argument does not match the pointer size of the target, the mask is
-zero-extended or truncated accordingly.
+The result of ``ptrmask(%ptr, %mask)`` is equivalent to the following expansion,
+where ``iPtrIdx`` is the index type size of the pointer::
+
+    %intptr = ptrtoint ptr %ptr to iPtrIdx ; this may truncate
+    %masked = and iPtrIdx %intptr, %mask
+    %diff = sub iPtrIdx %masked, %intptr
+    %result = getelementptr i8, ptr %ptr, iPtrIdx %diff
+
+Considering this as an operation on the integer representation of the pointer,
+if the pointer index type size is smaller than the pointer type size, this
+implies that the mask is extended with 1 bits to the pointer type size.
----------------
jrtc27 wrote:

Something like that does indeed sound more appropriate for pointers that aren't just plain integers (though technically our capabilities do still report as being integral, since non-integral pointers are overly-restrictive in their semantics (they imply that ptrtoint gives an unstable value, but ours are as stable as any normal integral pointer)).

https://github.com/llvm/llvm-project/pull/69343