better glibc i_modulo bug handling

pp-i-modulo code currently detects a glibc bug at runtime, at the 1st
exec of each I_MODULO op.  This is suboptimal; the bug should be
detectable early, and PL_ppaddr[I_MODULO] updated just once, before
any optrees are built.

Then, because we avoid the need to fixup I_MODULO ops in already built
optrees, we can drop the !PERL_DEBUG_READONLY_OPS limitation on the
alternative/workaround I_MODULO implementation that avoids the bug.

perl.c:

bug detection code is copied from PP(i_modulo),
into S_fixup_platform_bugs(), and called from perl_construct().
It patches Perl_pp_i_modulo_1() into PL_ppaddr[I_MODULO] when needed.

pp.c:

PP(i_modulo_0), the original implementation, is renamed to PP(i_modulo)

PP(i_modulo_1), the bug-fix workaround, is renamed _glibc_bugfix
                it is #ifdefd as before, but dropping !PERL_DEBUG_READONLY_OPS

PP(i_modulo) - the 1st-exec switcher code, is dropped

ocode.pl:

Two i_modulo entries are added to @raw_alias.
- 1st alias:  Perl_pp_i_modulo		    => 'i_modulo'
- 2nd alt:    Perl_pp_i_modulo_glibc_bugfix => 'i_modulo'

1st is a restatement of the default alias/mapping that would be
created without the line.  2nd line is then seen as alternative to the
explicit mapping set by 1st.

Alternative functions are written to pp_proto.h after the standard
Perl_pp_* list, and include #if-cond, #endif wrappings, as was
specified by 2nd @raw_alias addition.

Changes tested by inserting '1 ||' into the 3 ifdefs and bug-detection code.

TODO:

In pp_proto.h generation, the #ifdef wrapping code which handles the
alternative functions looks like it should also be used for the
non-alternate functions.  In particular, there are a handful of
pp-function prototypes that should be wrapped with #ifdef HAS_SOCKET.
That said, there have been no problem reports, so I left it alone.

TonyC: make S_fixup_platform_bugs static, porting/libperl.t was failing.

1 files changed, 21 insertions, 4 deletions

diff --git a/regen/opcode.pl b/regen/opcode.pl
index 82454bbb48..edb9f4d7c1 100755
--- a/regen/opcode.pl
+++ b/regen/opcode.pl
@@ -71,9 +71,9 @@ while (<OPS>) {
     $args{$key} = $args;
 }
 
-# Set up aliases
+# Set up aliases, and alternative funcs
 
-my %alias;
+my (%alias, %alts);
 
 # Format is "this function" => "does these op names"
 my @raw_alias = (
@@ -139,16 +139,25 @@ my @raw_alias = (
 		 Perl_pp_shostent => [qw(snetent sprotoent sservent)],
 		 Perl_pp_aelemfast => ['aelemfast_lex'],
 		 Perl_pp_grepstart => ['mapstart'],
+
+		 # 2 i_modulo mappings: 2nd is alt, needs 1st (explicit default) to not override the default
+		 Perl_pp_i_modulo  => ['i_modulo'],
+		 Perl_pp_i_modulo_glibc_bugfix => {
+                     'i_modulo' =>
+                         '#if defined(__GLIBC__) && IVSIZE == 8 '.
+                         ' && ( __GLIBC__ < 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ < 8))' },
 		);
 
 while (my ($func, $names) = splice @raw_alias, 0, 2) {
     if (ref $names eq 'ARRAY') {
 	foreach (@$names) {
-	    $alias{$_} = [$func, ''];
+            defined $alias{$_}
+            ? $alts{$_} : $alias{$_} = [$func, ''];
 	}
     } else {
 	while (my ($opname, $cond) = each %$names) {
-	    $alias{$opname} = [$func, $cond];
+            defined $alias{$opname}
+            ? $alts{$opname} : $alias{$opname} = [$func, $cond];
 	}
     }
 }
@@ -1251,6 +1260,14 @@ my $pp = open_new('pp_proto.h', '>',
 	++$funcs{$name};
     }
     print $pp "PERL_CALLCONV OP *$_(pTHX);\n" foreach sort keys %funcs;
+
+    print $pp "\n/* alternative functions */\n" if keys %alts;
+    for my $fn (sort keys %alts) {
+        my ($x, $cond) = @{$alts{$fn}};
+        print $pp "$cond\n" if $cond;
+        print $pp "PERL_CALLCONV OP *$x(pTHX);\n";
+        print $pp "#endif\n" if $cond;
+    }
 }
 
 print $oc "\n\n";