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author | Philipp Stephani <phst@google.com> | 2019-12-23 15:37:49 +0100 |
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committer | Philipp Stephani <phst@google.com> | 2019-12-23 15:37:49 +0100 |
commit | f8e83d73a259e1809020d47e920a96a1f5803f7a (patch) | |
tree | ae5f8b17390a1090253025d69672f0eca52c5f28 /src | |
parent | 0abdb01be6c1adee1f179b1d5f7a9fb4ee670109 (diff) | |
download | emacs-f8e83d73a259e1809020d47e920a96a1f5803f7a.tar.gz |
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.
Diffstat (limited to 'src')
-rw-r--r-- | src/emacs-module.c | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/src/emacs-module.c b/src/emacs-module.c index f372a153ccf..f2e3f627756 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) |