[glibc] manual: Clarify undefined behavior of feenableexcept (BZ 31019)

[glibc] manual: Clarify undefined behavior of feenableexcept (BZ 31019)

[Adhemerval Zanella]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e55599e0286655dd5f1f5b48005a17be37cc7f2c

commit e55599e0286655dd5f1f5b48005a17be37cc7f2c
Author: Bruno Haible <bruno@clisp.org>
Date:   Wed Nov 1 02:10:11 2023 +0100

    manual: Clarify undefined behavior of feenableexcept (BZ 31019)
    
    Explain undefined behavior of feenableexcept in a special case.
    
    Reviewed-by: Carlos O'Donell <carlos@redhat.com>

Diff:
---
 manual/arith.texi | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/manual/arith.texi b/manual/arith.texi
index fa7110e992..be24c20493 100644
--- a/manual/arith.texi
+++ b/manual/arith.texi
@@ -1176,6 +1176,12 @@ enabled, the status of the other exceptions is not changed.
 
 The function returns the previous enabled exceptions in case the
 operation was successful, @code{-1} otherwise.
+
+Note: Enabling traps for an exception for which the exception flag is
+currently already set (@pxref{Status bit operations}) has unspecified
+consequences: it may or may not trigger a trap immediately.
+@c It triggers a trap immediately on powerpc*, at the next floating-
+@c instruction on i386, and not at all on the other CPUs.
 @end deftypefun
 
 @deftypefun int fedisableexcept (int @var{excepts})