[gcc r12-6662] libstdc++: Improve comments describing --enable-fully-dynamic-string

[Jonathan Wakely]

https://gcc.gnu.org/g:5f3c0ee908b5e10f5226be0bad8b4ac55d41d985

commit r12-6662-g5f3c0ee908b5e10f5226be0bad8b4ac55d41d985
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jan 17 23:53:37 2022 +0000

    libstdc++: Improve comments describing --enable-fully-dynamic-string
    
    libstdc++-v3/ChangeLog:
    
            * acinclude.m4 (GLIBCXX_ENABLE_FULLY_DYNAMIC_STRING): Improve
            comments.
            * configure: Regenerate.

Diff:
---
 libstdc++-v3/acinclude.m4 | 17 +++++++++++------
 libstdc++-v3/configure    |  4 ++--
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 336c690f07b..930861e6a94 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -499,19 +499,24 @@ AC_DEFUN([GLIBCXX_CHECK_LFS], [
 
 
 dnl
-dnl Check for whether a fully dynamic basic_string implementation should
-dnl be turned on, that does not put empty objects in per-process static
-dnl memory (mostly useful together with shared memory allocators, see PR
-dnl libstdc++/16612 for details).
+dnl Check whether the old Copy-On-Write basic_string should allocate a new
+dnl empty representation for every default-constructed basic_string. Without
+dnl this option, COW strings share a single empty rep in static storage,
+dnl but this only works if the linker can guarantee the static storage has
+dnl a unique definition in the process. It also doesn't work if basic_string
+dnl objects are stored in shared memory (see PR libstdc++/16612).
+dnl When fully dynamic strings are enabled, the static storage is not used
+dnl and a new empty string with reference-count == 1 is allocated each time.
+dnl Enabling this changes the libstdc++.so ABI.
 dnl
 dnl --enable-fully-dynamic-string defines _GLIBCXX_FULLY_DYNAMIC_STRING to 1
 dnl --disable-fully-dynamic-string defines _GLIBCXX_FULLY_DYNAMIC_STRING to 0
-dnl otherwise undefined
+dnl otherwise the macro is not defined.
 dnl  +  Usage:  GLIBCXX_ENABLE_FULLY_DYNAMIC_STRING[(DEFAULT)]
 dnl       Where DEFAULT is either `yes' or `no'.
 dnl
 AC_DEFUN([GLIBCXX_ENABLE_FULLY_DYNAMIC_STRING], [
-  GLIBCXX_ENABLE(fully-dynamic-string,$1,,[do not put empty strings in per-process static memory])
+  GLIBCXX_ENABLE(fully-dynamic-string,$1,,[do not put empty strings in per-process static memory], [permit yes|no])
   if test $enable_fully_dynamic_string = yes; then
     enable_fully_dynamic_string_def=1
   else
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 34f6718cfd0..4c9be007521 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -18679,8 +18679,8 @@ if test "${enable_fully_dynamic_string+set}" = set; then :
   enableval=$enable_fully_dynamic_string;
       case "$enableval" in
        yes|no) ;;
-       *) as_fn_error $? "Argument to enable/disable fully-dynamic-string must be yes or no" "$LINENO" 5 ;;
-      esac
+       *) as_fn_error $? "Unknown argument to enable/disable fully-dynamic-string" "$LINENO" 5 ;;
+	  	        esac
 
 else
   enable_fully_dynamic_string=no