[Bug c/39375] asm with a "=X" output overwrites the output

[Bug c/39375] asm with a "=X" output overwrites the output

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-01-02 04:16:52 UTC ---
__asm__ ("xxx" : "+X" (sum));

Is most likely what you want to use.  This says the sum might be clobbered but
it might not be as the same value is used if it was not.

[Bug c/39375] asm with a "=X" output overwrites the output

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2009-03-16 17:02 -------
(In reply to comment #4)
> Reopening because
> int params; __asm__ ("xxx" : "=X" (params));
> and
> int params[1]; __asm__ ("xxx" : "=X" (params[0]));
> still produce different output in a way that is undocumented.

How so?  "=X" (params[0]) says it can be in memory which means params is
addressable.  This is documented as "=X" really means "=rfm" (plus extra
constraints which don't correspond to r, f, or m).


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375

[Bug c/39375] asm with a "=X" output overwrites the output

[balrogg at gmail dot com]

------- Comment #4 from balrogg at gmail dot com  2009-03-16 16:53 -------
Reopening because
int params; __asm__ ("xxx" : "=X" (params));
and
int params[1]; __asm__ ("xxx" : "=X" (params[0]));
still produce different output in a way that is undocumented.


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balrogg at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375

[Bug c/39375] asm with a "=X" output overwrites the output

[balrogg at gmail dot com]

------- Comment #3 from balrogg at gmail dot com  2009-03-06 10:34 -------
(In reply to comment #2)
> You need to use a "memory" clobber instead.  "=X" (params[1]) says to GCC
> that the asm operand 0 should be stored to params[1], which it does
> (it allocates %eax to it).

Note that "=r" doesn't produce the useless move.  Neither does "=X" (params) if
params is an integer.

Note also that the documentation uses "=X" (sum) as an example of adding a
dependency on the asm statement.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375

[Bug c/39375] asm with a "=X" output overwrites the output

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-03-05 10:42 -------
You need to use a "memory" clobber instead.  "=X" (params[1]) says to GCC
that the asm operand 0 should be stored to params[1], which it does
(it allocates %eax to it).  Note that plain use of %eax and %dx is a
bad idea as well, instead you should use proper inputs and outputs with
register constraints.


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rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375

[Bug c/39375] asm with a "=X" output overwrites the output

[balrogg at gmail dot com]

------- Comment #1 from balrogg at gmail dot com  2009-03-05 02:55 -------
(In reply to comment #0)
> Similarly for "=X" but not "=m" or "=r".

Rather, similarly for "=g".


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39375