master f8e83d7: Document and verify that emacs_limb_t doesn’t have padding bits.

[Philipp Stephani]

branch: master
commit f8e83d73a259e1809020d47e920a96a1f5803f7a
Author: Philipp Stephani <address@hidden>
Commit: Philipp Stephani <address@hidden>

    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.
---
 doc/lispref/internals.texi | 7 ++++---
 src/emacs-module.c         | 7 +++++++
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 2a4e64d..bccdca6 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
diff --git a/src/emacs-module.c b/src/emacs-module.c
index f372a15..f2e3f62 100644
--- a/src/emacs-module.c
+++ b/src/emacs-module.c
@@ -827,6 +827,13 @@ enum
   module_bignum_count_max = min (SIZE_MAX, PTRDIFF_MAX) / sizeof (emacs_limb_t)
 };
 
+/* Verify that emacs_limb_t indeed has unique object
+   representations.  */
+verify (CHAR_BIT == 8);
+verify ((sizeof (emacs_limb_t) == 4 && EMACS_LIMB_MAX == 0xFFFFFFFF)
+        || (sizeof (emacs_limb_t) == 8
+            && EMACS_LIMB_MAX == 0xFFFFFFFFFFFFFFFF));
+
 static bool
 module_extract_big_integer (emacs_env *env, emacs_value arg, int *sign,
                             ptrdiff_t *count, emacs_limb_t *magnitude)