Slight recoding of util.c:repeatcpy() to circumnavigate

a Digital C compiler optimizer bug that broke the 'x'
operator under certain circumstances.  See t/op/repeat.t
test #20 for graphic details.  Reported in

From: Gisle Aas <gisle@aas.no>
To: Mark Martinec <Mark.Martinec@nsc.ijs.si>
Cc: ach@xray.mpe.mpg.de, cpan-testers@perl.org, perl5-porters@perl.org
Subject: Re: Digest-MD5-2.00 test fails on DEC Alpha - a patch
Date: 18 Dec 1998 14:27:40 +0100
Message-ID: <m37lvpa8c3.fsf@furu.g.aas.no>

and discussed further in the thread

From: Jarkko Hietaniemi <jhi@iki.fi>
To: Gisle Aas <gisle@aas.no>
Cc: Mark Martinec <Mark.Martinec@nsc.ijs.si>, ach@xray.mpe.mpg.de,
        cpan-testers@perl.org, perl5-porters@perl.org
Subject: x operator broken in DEC Alpha for 8-bit characters (Re: Digest-MD5-2.00 test fails on DEC Alpha - a patch)
Date: Fri, 18 Dec 1998 16:18:37 +0200 (EET)
Message-ID: <13946.25661.193449.138023@alpha.hut.fi>

p4raw-id: //depot/cfgperl@2498

1 files changed, 52 insertions, 1 deletions

diff --git a/t/op/repeat.t b/t/op/repeat.t
index 54fa590836..f935bf106f 100755
--- a/t/op/repeat.t
+++ b/t/op/repeat.t
@@ -2,7 +2,7 @@
 
 # $RCSfile: repeat.t,v $$Revision: 4.1 $$Date: 92/08/07 18:28:21 $
 
-print "1..19\n";
+print "1..20\n";
 
 # compile time
 
@@ -40,3 +40,54 @@ print join(':', () x 4) eq '' ? "ok 16\n" : "not ok 16\n";
 print join(':', (9) x 4) eq '9:9:9:9' ? "ok 17\n" : "not ok 17\n";
 print join(':', (9,9) x 4) eq '9:9:9:9:9:9:9:9' ? "ok 18\n" : "not ok 18\n";
 print join('', (split(//,"123")) x 2) eq '123123' ? "ok 19\n" : "not ok 19\n";
+
+#
+# The test #20 is actually testing for Digital C compiler optimizer bug.
+#
+# Dec C versions 5.* and 6.0 (used in Digital UNIX and VMS) used
+# to produce (as of December 1998) broken code for util.c:repeatcpy()
+# (a utility function for the 'x' operator) in the case *all* these
+# four conditions held:
+#
+# (1) len == 1
+# (2) "from" had the 8th bit on in its single character
+# (3) count > 7 (the 'x' count > 16)
+# (4) the highest optimization level was used in compilation
+#     (which is the default when compiling Perl)
+#
+# The bug looked like this (. being the eight-bit character and ? being \xff):
+#
+# 16 ................
+# 17 .........???????.
+# 18 .........???????..
+# 19 .........???????...
+# 20 .........???????....
+# 21 .........???????.....
+# 22 .........???????......
+# 23 .........???????.......
+# 24 .........???????.???????
+# 25 .........???????.???????.
+#
+# The bug could be (obscurely) avoided by changing "from" to
+# be an unsigned char pointer.
+#
+# The bug was triggered in the "if (len == 1)" branch.  The fix
+# was to introduce a new temporary variable.  In diff -u format:
+#
+#     register char *frombase = from;
+# 
+#     if (len == 1) {
+#-       todo = *from;
+#+       register char c = *from;
+#        while (count-- > 0)
+#-           *to++ = todo;
+#+           *to++ = c;
+#        return;
+#     }
+#
+# This obscure bug was not found by the then test suite but instead
+# by Mark.Martinec@nsc.ijs.si while trying to install Digest-MD5-2.00.
+#
+# jhi@iki.fi
+#
+print "\xdd" x 24 eq "\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd" ? "ok 20\n" : "not ok 20\n";