[Bug target/69151] New: missing docs for H8/3000 monitor/OS_Task attributes and related breakage in -mexr/-mno-exr options

[sandra at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69151

            Bug ID: 69151
           Summary: missing docs for H8/3000 monitor/OS_Task attributes
                    and related breakage in -mexr/-mno-exr options
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sandra at gcc dot gnu.org
  Target Milestone: ---

In reviewing the current state of PR1078, I found that there is no
documentation for the H8/3000 function attributes "monitor" and "OS_Task".

The "monitor" attribute is mentioned in invoke.texi, though:

@item -mexr
@opindex mexr
Extended registers are stored on stack before execution of function
with monitor attribute. Default option is @option{-mexr}.
This option is valid only for H8S targets.

@item -mno-exr
@opindex mno-exr
Extended registers are not stored on stack before execution of function 
with monitor attribute. Default option is @option{-mno-exr}. 
This option is valid only for H8S targets.

Besides needing markup fixes and copy-editing, this text is totally confusing: 
*both* of these behaviors cannot be the default.  Trying to figure this out
from the code, in "monitor_prologue" in h8300.md I see:

...
  else if (TARGET_H8300S && TARGET_NEXR )
    return
"mov.l\\ter0,@-er7\;stc\tccr,r0l\;mov.b\tr0l,@(4,er7)\;mov.l\\t@er7+,er0\;orc\t#128,ccr"; 
  else if (TARGET_H8300S && TARGET_NEXR && TARGET_NORMAL_MODE)
    return
"subs\\t#2,er7\;mov.l\\ter0,@-er7\;stc\tccr,r0l\;mov.b\tr0l,@(4,er7)\;mov.l\\t@er7+,er0\;orc\t#128,ccr";
...

The second clause is clearly dead code.  I think there is a mistake in logic
here, and that a target expert needs to figure out what the intended behavior
is and correct both the implementation and documentation.