When you are preparing *.md files for gcc, you may get following message.

*.md:iii: operand 0 missing output reload

It means missing = (or +) for constraint (no output specified).
gcc-4.2.1/gcc/genrecog.c:

721     else if (constraints0 != '=' && constraints0 != '+')
722       {
723         message_with_line (pattern_lineno,
724                            "operand %d missing output reload",
725                            XINT (pattern, 0));
726         error_count++;

I have googled but no clue was found (yet). So this the memo for it.