GCC 10 issues

[Jerry James]

Hi Camm,

The Fedora project is currently rebuilding all packages with what will
soon be GCC 10.  I have encountered two issues so far and thought I
should give you a heads up.

GCC has long defaulted to -fcommon, but GCC 10 flips the default to
-fno-common.  As a result, the link step fails with complaints about
lots of multiply-defined variables.  A quick workaround is to add
-fcommon to the build flags.  Otherwise, the code would have to be
altered to ensure that there is exactly one compilation unit
containing a definition for each variable.

After adding -fcommon, the build still fails like this:

COMPILER>
Error: ERROR "Caught fatal error [memory may be damaged]"
Signalled by EQUAL-TAIL-RECURSION-CHECK.
ERROR "Caught fatal error [memory may be damaged]"

Broken at SYSTEM::DO-BREAK-LEVEL.  Type :H for Help.
    1  Return to top level.
COMPILER>>2148155904 heap words available
NIL
COMPILER>>
NIL
COMPILER>>make[1]: *** [makefile:98: saved_pre_gcl] Error 255

Here is a disassembly of cclear_stack on an x86_64 machine:

0000000000005370 <cclear_stack>:
    5370:       f3 0f 1e fa             endbr64
    5374:       31 c0                   xor    %eax,%eax
    5376:       c3                      retq

GCC 10 has concluded that it can optimize cclear_stack away, as that
simply returns 0.  GCC may feel it is justified in doing so because of
this warning:

main.c: In function 'cclear_stack':
main.c:454:10: warning: function returns address of local variable
[ 
]8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wreturn-local-addr
-Wreturn-local-addr ]8;; ]
  454 |   return v;
      |          ^
In file included from /usr/include/stdlib.h:568,
                 from main.c:27:
main.c:452:11: note: declared here
  452 |   void *v=alloca(size);
      |           ^~~~~~

The same thing has happened with gcl_strncpy_chk; it just returns 0.
What are these functions supposed to do?  EQUAL-TAIL-RECURSION-CHECK
in particular, even if cclear_stack is not optimized away, touches
memory it isn't allowed to touch, memory belonging to a stack frame
that has been popped.  A conforming compiler is allowed to generate
code that segfaults (precisely what GCC 10 is doing).

I should also mention this output from the configure script:

checking finding default linker script... gawk: cmd. line:1: warning:
regexp escape sequence `\=' is not a known regexp operator
got it

Perhaps all of the "\=" should be "\\="?

Regards,
-- 
Jerry James
http://www.jamezone.org/