hide private functions with __attribute__((visibility("hidden")))

This allows us to enforce API boundaries and potentially enables
compiler optimisations.

We've been always hiding non-public symbols on Windows. This commit
brings that to the other platforms.

Configure

@@ -391,6 +391,7 @@ d_attribute_nonnull=''
 d_attribute_noreturn=''
 d_attribute_pure=''
 d_attribute_unused=''
+d_attribute_visibility=''
 d_attribute_warn_unused_result=''
 d_printf_format_null=''
 d_backtrace=''
@@ -11337,6 +11338,34 @@ set d_attribute_always_inline
 eval $setvar
 $rm -f attrib*
 
+: Look for GCC-style attribute visibility
+case "$d_attribute_visibility" in
+'')
+echo " "
+echo "Checking whether your compiler can handle __attribute__((visibility)) ..." >&4
+$cat >attrib.c <<'EOCP'
+#include <stdio.h>
+__attribute__((visibility("hidden"))) int I_will_be_hidden(void);
+EOCP
+if $cc $ccflags -c attrib.c >attrib.out 2>&1 ; then
+        if $compiler_warning attrib.out >/dev/null 2>&1; then
+                echo "Your C compiler doesn't support __attribute__((visibility))."
+                val="$undef"
+        else
+                echo "Your C compiler supports __attribute__((visibility))."
+                val="$define"
+        fi
+else
+        echo "Your C compiler doesn't seem to understand __attribute__ at all."
+        val="$undef"
+fi
+;;
+*) val="$d_attribute_visibility" ;;
+esac
+set d_attribute_visibility
+eval $setvar
+$rm -f attrib*
+
 : see if getpgrp exists
 set getpgrp d_getpgrp
 eval $inlibc
@@ -24598,6 +24627,7 @@ d_attribute_nonnull='$d_attribute_nonnull'
 d_attribute_noreturn='$d_attribute_noreturn'
 d_attribute_pure='$d_attribute_pure'
 d_attribute_unused='$d_attribute_unused'
+d_attribute_visibility='$d_attribute_visibility'
 d_attribute_warn_unused_result='$d_attribute_warn_unused_result'
 d_backtrace='$d_backtrace'
 d_bsd='$d_bsd'

Cross/config.sh-arm-linux

@@ -124,6 +124,7 @@ d_attribute_nonnull='undef'
 d_attribute_noreturn='undef'
 d_attribute_pure='undef'
 d_attribute_unused='undef'
+d_attribute_visibility='undef'
 d_attribute_warn_unused_result='undef'
 d_backtrace='undef'
 d_bsd='undef'

Cross/config.sh-arm-linux-n770

@@ -123,6 +123,7 @@ d_attribute_nonnull='undef'
 d_attribute_noreturn='undef'
 d_attribute_pure='undef'
 d_attribute_unused='undef'
+d_attribute_visibility='undef'
 d_attribute_warn_unused_result='undef'
 d_backtrace='undef'
 d_bsd='undef'

Porting/config.sh

@@ -141,6 +141,7 @@ d_attribute_nonnull='define'
 d_attribute_noreturn='define'
 d_attribute_pure='define'
 d_attribute_unused='define'
+d_attribute_visibility='undef'
 d_attribute_warn_unused_result='define'
 d_backtrace='define'
 d_bsd='undef'

config_h.SH

@@ -1495,6 +1495,10 @@ sed <<!GROK!THIS! >$CONFIG_H -e 's!^#undef\(.*/\)\*!/\*#define\1 \*!' -e 's!^#un
  *	Can we handle GCC attribute for functions that should always be
  *	inlined.
  */
+/* HASATTRIBUTE_VISIBILITY:
+ *	Can we handle GCC attribute for functions that should have a
+ *	different visibility.
+ */
 #$d_attribute_deprecated HASATTRIBUTE_DEPRECATED	/**/
 #$d_attribute_format HASATTRIBUTE_FORMAT	/**/
 #$d_printf_format_null PRINTF_FORMAT_NULL_OK	/**/
@@ -1505,6 +1509,7 @@ sed <<!GROK!THIS! >$CONFIG_H -e 's!^#undef\(.*/\)\*!/\*#define\1 \*!' -e 's!^#un
 #$d_attribute_unused HASATTRIBUTE_UNUSED	/**/
 #$d_attribute_warn_unused_result HASATTRIBUTE_WARN_UNUSED_RESULT	/**/
 #$d_attribute_always_inline HASATTRIBUTE_ALWAYS_INLINE	/**/
+#$d_attribute_visibility HASATTRIBUTE_VISIBILITY	/**/
 
 /* HAS_BACKTRACE:
  *	This symbol, if defined, indicates that the backtrace() routine is

configure.com

@@ -6124,6 +6124,7 @@ $ WC "d_attribute_nonnull='undef'"
 $ WC "d_attribute_noreturn='undef'"
 $ WC "d_attribute_pure='undef'"
 $ WC "d_attribute_unused='undef'"
+$ WC "d_attribute_visibility='undef'"
 $ WC "d_attribute_warn_unused_result='undef'"
 $ WC "d_prctl='undef'"
 $ WC "d_prctl_set_name='undef'"

perl.h

@@ -364,6 +364,9 @@ Now a no-op.
 #    if PERL_GCC_VERSION_GE(4,7,0)
 #      define HASATTRIBUTE_ALWAYS_INLINE
 #    endif
+#    if PERL_GCC_VERSION_GE(3,3,0)
+#      define HASATTRIBUTE_VISIBILITY
+#    endif
 #  endif
 #endif /* #ifndef PERL_MICRO */
 
@@ -397,6 +400,10 @@ Now a no-op.
 #    define __attribute__always_inline__      __attribute__((always_inline))
 #  endif
 #endif
+#if defined(HASATTRIBUTE_VISIBILITY) && !defined(_WIN32)
+/* On Windows, instead of this, we use __declspec(dllexport) and a .def file */
+#  define __attribute__visibility__(x) __attribute__((visibility(x)))
+#endif
 
 /* If we haven't defined the attributes yet, define them to blank. */
 #ifndef __attribute__deprecated__
@@ -426,6 +433,9 @@ Now a no-op.
 #ifndef __attribute__always_inline__
 #  define __attribute__always_inline__
 #endif
+#ifndef __attribute__visibility__
+#  define __attribute__visibility__(x)
+#endif
 
 /* Some OS warn on NULL format to printf */
 #ifdef PRINTF_FORMAT_NULL_OK

plan9/config_sh.sample

@@ -124,6 +124,7 @@ d_attribute_nonnull='undef'
 d_attribute_noreturn='undef'
 d_attribute_pure='undef'
 d_attribute_unused='undef'
+d_attribute_visibility='undef'
 d_attribute_warn_unused_result='undef'
 d_backtrace='undef'
 d_bsd='undef'

pod/perldelta.pod

@@ -366,6 +366,16 @@ C<PL_use_safe_putenv> has been removed and C<PERL_USE_SAFE_PUTENV> is always
 defined. This means XS modules can now call C<setenv> and C<putenv> without
 causing segfaults. [L<perl #19399|https://github.com/Perl/perl5/issues/19399>]
 
+=item *
+
+Internal C API functions are now hidden with C<__attribute__((hidden))> on the
+platforms that support it. This means they are no longer callable from XS
+modules on those platforms.
+
+It should be noted that those functions have always been hidden on Windows. This
+change merely brings that to the other platforms.
+[L<perl #19655|https://github.com/Perl/perl5/pull/19655>]
+
 =back
 
 =head1 Selected Bug Fixes