Document and verify that emacs_limb_t doesn’t have padding bits.

This is a useful property when doing further bit-twiddling with the
magnitude array before/after calling extract_big_integer or
make_big_integer.  For example, constructing an emacs_limb_t object
using repeated shift-and-add should work as expected, but relies on
the type not having padding bits.  Since the C standard already
guarantees that unsigned integers use a pure binary representation,
not having padding bits is enough to guarantee that the type has
unique object representations in the sense of C++’s
std::has_unique_object_representations.

* doc/lispref/internals.texi (Module Values): Document that
emacs_limb_t doesn’t have padding bits.

* src/emacs-module.c: Verify that emacs_limb_t doesn’t have padding
bits.

1 files changed, 4 insertions, 3 deletions

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 2a4e64dbb56..bccdca65e4e 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -1508,9 +1508,10 @@ overflow in the size calculation.
 @end deftypefn
 
 @deftp {Type alias} emacs_limb_t
-This is an unsigned integer type,
-used as the element type for the magnitude arrays for the big
-integer conversion functions.
+This is an unsigned integer type, used as the element type for the
+magnitude arrays for the big integer conversion functions.  The type
+is guaranteed to have unique object representations, i.e., no padding
+bits.
 @end deftp
 
 @defvr Macro EMACS_LIMB_MAX