On 10/10/20 8:00 AM, Marc Nieper-Wißkirchen wrote:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97364
Looking at that bug report and related material (particularly Richard Biener's 2012 comment <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51971#c1>) the GCC folks seem to have decided long ago that const and pure attributes were intended more as directives for common subexpression elimination than as independent notions in their own right, it's just that they haven't gotten around to (or are embarrassed to :-) document that decision. In the meantime, to try to help out Gnulib hackers I installed the attached commentary changes. It was cleaner to document 'const' first so I rearranged the #defines.
>From e3665e43f2a042b26fa65cdba80dd89846d95540 Mon Sep 17 00:00:00 2001 From: Paul Eggert <egg...@cs.ucla.edu> Date: Sat, 10 Oct 2020 11:48:16 -0700 Subject: [PATCH] attribute: improve const, pure doc MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Problem reported by Marc Nieper-Wißkirchen in: https://lists.gnu.org/r/bug-gnulib/2020-10/msg00035.html * lib/attribute.h (ATTRIBUTE_CONST, ATTRIBUTE_PURE): Improv doc. See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51971#c1 --- ChangeLog | 8 ++++++++ lib/attribute.h | 21 ++++++++++++--------- 2 files changed, 20 insertions(+), 9 deletions(-) diff --git a/ChangeLog b/ChangeLog index 875f3551a..b479cf0c7 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,11 @@ +2020-10-10 Paul Eggert <egg...@cs.ucla.edu> + + attribute: improve const, pure doc + Problem reported by Marc Nieper-Wißkirchen in: + https://lists.gnu.org/r/bug-gnulib/2020-10/msg00035.html + * lib/attribute.h (ATTRIBUTE_CONST, ATTRIBUTE_PURE): Improv doc. See: + https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51971#c1 + 2020-10-05 Paul Eggert <egg...@cs.ucla.edu> thread: pacify GCC on Solaris 10 diff --git a/lib/attribute.h b/lib/attribute.h index b97514655..48d9826c8 100644 --- a/lib/attribute.h +++ b/lib/attribute.h @@ -170,18 +170,21 @@ /* Applies to: function. */ #define ATTRIBUTE_ALWAYS_INLINE _GL_ATTRIBUTE_ALWAYS_INLINE -/* The function does not affect observable state, and always returns a value. - Compilers can omit duplicate calls with the same arguments if - observable state is not changed between calls. (This attribute is - looser than ATTRIBUTE_CONST.) */ +/* It is OK for a compiler to omit duplicate calls with the same arguments. + This attribute is safe for a function that neither depends on + nor affects observable state, and always returns exactly once - + e.g., does not loop forever, and does not call longjmp. + (This attribute is stricter than ATTRIBUTE_PURE.) */ /* Applies to: functions. */ -#define ATTRIBUTE_PURE _GL_ATTRIBUTE_PURE +#define ATTRIBUTE_CONST _GL_ATTRIBUTE_CONST -/* The function neither depends on nor affects observable state, - and always returns a value. Compilers can omit duplicate calls with - the same arguments. (This attribute is stricter than ATTRIBUTE_PURE.) */ +/* It is OK for a compiler to omit duplicate calls with the same + arguments if observable state is not changed between calls. + This attribute is safe for a function that does not affect + observable state, and always returns exactly once. + (This attribute is looser than ATTRIBUTE_CONST.) */ /* Applies to: functions. */ -#define ATTRIBUTE_CONST _GL_ATTRIBUTE_CONST +#define ATTRIBUTE_PURE _GL_ATTRIBUTE_PURE /* The function is rarely executed. */ /* Applies to: functions. */ -- 2.25.1
