[Bug inline-asm/68095] New: "cc" clobber with Flag Output Operands

[Bug inline-asm/68095] New: "cc" clobber with Flag Output Operands

[gccbugzilla at limegreensocks dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68095

            Bug ID: 68095
           Summary: "cc" clobber with Flag Output Operands
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugzilla at limegreensocks dot com
  Target Milestone: ---

Normally when trying to use a register and clobber it at the same time, the
compiler kicks out an error (operand has impossible constraints).  However that
doesn't happen when using Flag Output Operands and the "cc" clobber:

  asm ("cmp %2, %1" : "=@ccc"(r) : "m"(*p), "ri"(2): "cc");

Should this be a conflict?

On the other hand, the asm is changing the flags.  So is the "cc" *required*? 
But it doesn't seem to be:

  asm ("cmp %2, %1" : "=@ccc"(r) : "m"(*p), "ri"(2));

Now that asm flags are a real thing, we should pick one and enforce it.

[Bug inline-asm/68095] "cc" clobber with Flag Output Operands

[gccbugzilla at limegreensocks dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68095

--- Comment #1 from David <gccbugzilla at limegreensocks dot com> ---
On further reflection, perhaps the best solution is even simpler.

It is my understanding that the "cc" clobber is redundant.  Internally, the
flags are clobbered whether you set this or not.  And I can't see how this
behavior can ever change now that it has been like this for so long.

As a result, perhaps the solution is to just change the docs for the "cc"
clobber to say that it is meaningless.  Maybe something like:

"cc" This clobber is only kept for historical reasons.  It no longer has any
effect on the generation of code.

[Bug inline-asm/68095] "cc" clobber with Flag Output Operands

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68095

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |segher at gcc dot gnu.org

--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
"=@ccc"(r) does not output to the "cc" register, it outputs to a general
register.  It clobbers the "cc" register.  Mentioning that more than
once ("=@ccc", the clobber in the asm, and it is default for the x86
target) is no problem and has clear semantics.

The "cc" clobber is not deprecated, not on other targets anyway.
Even on x86 it still serves to mark those asms that really clobber
the flags register, which can be helpful to the reader.