[Bug 1768] New: i368 direction flag handling

[bugzilla-daemon at rtems.org]

https://www.rtems.org/bugzilla/show_bug.cgi?id=1768

           Summary: i368 direction flag handling
           Product: RTEMS
           Version: 4.9
          Platform: All
        OS/Version: RTEMS
            Status: NEW
          Severity: normal
          Priority: P3
         Component: cpukit
        AssignedTo: joel.sherrill at oarcorp.com
        ReportedBy: joel.sherrill at oarcorp.com


Created an attachment (id=1136)
 --> (https://www.rtems.org/bugzilla/attachment.cgi?id=1136)
Byte nbench with RTEMS main.c and Makefile

The Byte nbench benchmark has a test which is very heavy on string operations. 
When the stack checker.  This highlighted the fact that we do not perform a
"cld" before calling C code in the ISR.  This was historically not a problem
but gcc 4.3 changed the behavior.  From http://gcc.gnu.org/gcc-4.3/changes.html

"GCC no longer places the cld instruction before string operations. Both i386
and x86-64 ABI documents mandate the direction flag to be clear at the entry of
a function. It is now invalid to set the flag in asm statement without reseting
it afterward."

RTEMS 4.8 uses gcc 4.2.x.  RTEMS 4.9 uses gcc 4.3 so it and newer are impacted.

The nbench program ran fine on pc386 until it was run from a second task (not
Init).  At some point, the memcmp in the stack checker extension would fail. 
Debugging showed this was always in the case of a dispatch at the end of an
ISR.  It failed because the direction flag was set wrong.

The attached program can produce the failure during the "DoStringSort" case. 
The attached patch fixes the problem by ensuring the direction flag is cleared
before calling C code in an ISR.  The "iret" restores eflags and restores
whatever state it was in.

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