[Bug rtl-optimization/24414] New: Old-style asms don't clobber memory

[Bug rtl-optimization/24414] New: Old-style asms don't clobber memory

[rth at gcc dot gnu dot org]

#include <signal.h>
#include <unistd.h>

int test;

void handler (int sig)
{
  test = 1;
}

int main()
{
  signal (SIGALRM, handler);
  alarm (1);
  while (test == 0)
    asm("");
  return 0;
}

Results in an infinite loop with -O2.  The load of test in main should not
be hoisted out of the loop.  A more x86-specific example (but without relying
on unix syscalls might be

int test;
int main()
{
  int x = test;
  asm("movl $1,test");
  if (x + test != 1)
    __builtin_trap ();
  return 0;
}

It also wouldn't surprise me if the two tests have different culprits...


-- 
           Summary: Old-style asms don't clobber memory
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-10-17 20:32 -------
I thought this was decided this was not a bug.  I have to find the discussion.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2005-10-17 20:56 -------
An asm instruction without any operands or clobbers (an "old style" asm) will
be treated identically to a volatile asm instruction.

Or what is in the newer doc:
                        3933: An @code{asm} instruction without any output
operands will be treated
                        3934: identically to a volatile @code{asm} instruction.

and in PR 19341 we decided that volatile asm does not clobber memory.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[rth at gcc dot gnu dot org]

------- Comment #3 from rth at gcc dot gnu dot org  2005-10-17 21:02 -------
If we decided this was not-a-bug, then there's a bug in __sync_synchronize,
as that uses asm("") if there's no target fallback.

But personally I think this is a documentation bug.  We should consider
memory to be clobbered in old-style asms.  Your pointer to the old PR is 
irrelevant; that was a new-style asm.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2005-10-17 21:43 -------
The flow code was added by Jeff Law back in 1999:
http://gcc.gnu.org/ml/gcc-patches/1999-03/msg00763.html

Jeff, was there a reason why you mentioned old-style asm as special even though
GCC never handled or documented there were special before that date?


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[law at redhat dot com]

------- Comment #5 from law at redhat dot com  2005-10-17 21:55 -------
Subject: Re:  Old-style asms don't clobber
        memory

On Mon, 2005-10-17 at 21:43 +0000, pinskia at gcc dot gnu dot org wrote:
> 
> ------- Comment #4 from pinskia at gcc dot gnu dot org  2005-10-17 21:43 -------
> The flow code was added by Jeff Law back in 1999:
> http://gcc.gnu.org/ml/gcc-patches/1999-03/msg00763.html
> 
> Jeff, was there a reason why you mentioned old-style asm as special even though
> GCC never handled or documented there were special before that date?
An old-style asm must be assumed to read, write, clobber, well, just
about anything.  Documented or not, that's the only thing that makes
sense.

WHy?  Think about it for a little while, an old-style asm provides us
with zero information on what values it uses, what values it sets and
what values it clobbers.



jeff


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2005-10-17 22:25 -------
(In reply to comment #5)
> WHy?  Think about it for a little while, an old-style asm provides us
> with zero information on what values it uses, what values it sets and
> what values it clobbers.

Because you changed the behavior from older GCC's without updating the
documention and the documention has not changed since 1996 and most likely even
older than that.  So nobody had depended on it at all, why should we fix
something which is never been true except for what you added? which was not
even really discussed that much at all.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[rth at gcc dot gnu dot org]

------- Comment #7 from rth at gcc dot gnu dot org  2005-10-17 23:11 -------
You seem to be under the incorrect impression that gcc invented old-style asms.
I'm confirming the bug.


-- 

rth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-10-17 23:11:58
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2005-10-17 23:14 -------
(In reply to comment #7)
> You seem to be under the incorrect impression that gcc invented old-style asms.
> I'm confirming the bug.

Considering that gcc-1.41 had no special handing old-style asm, I don't see why
you think this is a bug.  Though that did not have clobbers.  They were added
later.

Isn't this like adding aliasing and other things like overflow is undefined? 
If so I don't see why this is a bug.  People just need to update their code.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2005-10-17 23:17 -------
This has been a bug since at least 2.95.3 anyways:
.L9:
#APP

#NO_APP
        testl %eax,%eax
        je .L9

So it seems like what you are claiming is that GCC never got this "correct"
even though it documented different than what you want.  I still don't see the
issue.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |2.95.3 3.3.1 3.0.4 3.4.0
                   |                            |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[rth at gcc dot gnu dot org]

------- Comment #10 from rth at gcc dot gnu dot org  2005-10-17 23:23 -------
Are you being intentionally obtuse, Andrew?

How many different ways do I have to say that GCC did not invent the asm?
It exists and has existed in *many* C compilers, many of which pre-date GCC.
THERE IS A TRADITIONAL INTERPRETATION of "asm ( character-string-literal ) ;".


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2005-10-17 23:31 -------
2.7.2.3 gets the same code too:
.L5:
#APP

#NO_APP
        testl %eax,%eax
        je .L5

So this really have never worked.

if you consider this is a bug, please don't apply it while we are in regression
only mode since this is obviously not a regression as we have never got it
correct.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2005-10-17 23:43 -------
2.0 gives what you wantted:
.L5:
/APP

/NO_APP
        cmpl $0,test
        je .L5

But that is 13 years ago.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #13 from pinskia at gcc dot gnu dot org  2005-10-17 23:51 -------
2.5.8 gives what you wantted:
.L5:
/APP

/NO_APP
        cmpl $0,test
        je .L5


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[law at redhat dot com]

------- Comment #14 from law at redhat dot com  2005-10-17 23:54 -------
Subject: Re:  Old-style asms don't clobber
        memory

On Mon, 2005-10-17 at 22:25 +0000, pinskia at gcc dot gnu dot org wrote:
> 
> ------- Comment #6 from pinskia at gcc dot gnu dot org  2005-10-17 22:25 -------
> Because you changed the behavior from older GCC's without updating the
> documention and the documention has not changed since 1996 and most likely even
> older than that.  So nobody had depended on it at all, why should we fix
> something which is never been true except for what you added? which was not
> even really discussed that much at all.
The old behavior was wrong and painfully obvious once we had to go
look at the code for volatile asms.  The new behavior is conservatively
correct.

You can argue there should be a doc update to go along with that and
I'd tentatively agree with that.

jeff


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #15 from pinskia at gcc dot gnu dot org  2005-10-17 23:57 -------
2.6.3 gives what you wants:
.L11:
/APP

/NO_APP
        cmpl $0,test
        je .L11


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug rtl-optimization/24414] Old-style asms don't clobber memory

[law at redhat dot com]

------- Comment #16 from law at redhat dot com  2005-10-18 00:01 -------
Subject: Re:  Old-style asms don't clobber
        memory

On Mon, 2005-10-17 at 23:14 +0000, pinskia at gcc dot gnu dot org wrote:
> 
> ------- Comment #8 from pinskia at gcc dot gnu dot org  2005-10-17 23:14 -------
> (In reply to comment #7)
> > You seem to be under the incorrect impression that gcc invented old-style asms.
> > I'm confirming the bug.
> 
> Considering that gcc-1.41 had no special handing old-style asm, I don't see why
> you think this is a bug.  Though that did not have clobbers.  They were added
> later.
Err, old-style asms predate GCC by a long long long time.  Fundamentally
they're a blob of assembly code that can do literally anything.  Thus 
making any assumption about what they do is wrong.

jeff


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug c/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu dot org]

------- Comment #17 from pinskia at gcc dot gnu dot org  2005-10-18 01:04 -------
The easy way to fix this is in the front-ends.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |c


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug c/24414] Old-style asms don't clobber memory

[angad dot dongare at gmail dot com]

------- Comment #18 from angad dot dongare at gmail dot com  2009-10-10 10:40 -------
iteruwioturei jkldkldj f fidoifuio rwepooruiou roipruowi weruweoiruowei


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug c/24414] Old-style asms don't clobber memory

[angad dot dongare at gmail dot com]

------- Comment #19 from angad dot dongare at gmail dot com  2009-10-10 10:48 -------
iteruwioturei jkldkldj f fidoifuio rwepooruiou roipruowi weruweoiruowei
hfkjhdfkjk lksdjljdl fkdl;fkl;kdsfa fjkljkjdfnjkjklfj jklfjklsd
klfjkweioj jkljkdfkjsklaf


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

[Bug c/24414] Old-style asms don't clobber memory

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24414

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED
      Known to fail|                            |
   Target Milestone|---                         |7.0

--- Comment #21 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/pipermail/gcc-patches/2016-May/447597.html

So fixed (r7-1269-g93671519e202e3).