[Bug jit/100380] New: Segfault when using inline asm

[Bug jit/100380] New: Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

            Bug ID: 100380
           Summary: Segfault when using inline asm
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: jit
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: bouanto at zoho dot com
  Target Milestone: ---

Created attachment 50729
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50729&action=edit
Reproducer for the bug

Hi.
The attached example produce a segfault when trying to compile code using
inline assembly.
Thanks.

[Bug jit/100380] Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

Antoni <bouanto at zoho dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #50729|0                           |1
        is obsolete|                            |

--- Comment #1 from Antoni <bouanto at zoho dot com> ---
Created attachment 50730
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50730&action=edit
Fixed reproducer

[Bug jit/100380] Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

--- Comment #2 from Antoni <bouanto at zoho dot com> ---
Created attachment 50731
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50731&action=edit
Working code

So, the segfault seems to happen when creating the variable after creating the
extended asm expression.
Here's a working version of the code.

[Bug jit/100380] Segfault when using inline asm

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org
     Ever confirmed|0                           |1
           Assignee|dmalcolm at gcc dot gnu.org        |marxin at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-05-11

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
I'll take a look.

[Bug jit/100380] Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

--- Comment #4 from Antoni <bouanto at zoho dot com> ---
I just had a similar issue when developing a new feature for libgccjit and it
might be the same problem. If it is (I haven't checked in this case), here's
what's happening:

 * The asm is replayed.
 * The asm tries to access the replayed variable (which wasn't replayed yet
because it was created after the asm).
 * Segfault (the rest is not executed, but is shown to explain what's
happening)
 * The variable is replayed (too late because it was NULL when accessed by the
asm).

Again it's to be verified, and I'm not sure what should be the solution to this
problem because the mementos are replayed in the order they were created.

[Bug jit/100380] Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

--- Comment #5 from Antoni <bouanto at zoho dot com> ---
I can confirm that the problem is indeed what I described in my previous post.

One solution would be to check if the rvalue was replayed (and if not, replay
it now), but that involves adding this check everywhere, so that seems very
invasive.

Do you think there's a better solution?

[Bug jit/100380] Segfault when using inline asm

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Assignee|marxin at gcc dot gnu.org          |dmalcolm at gcc dot gnu.org

--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
Leaving to David..

[Bug jit/100380] Segfault when using inline asm

[bouanto at zoho dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100380

--- Comment #7 from Antoni <bouanto at zoho dot com> ---
Since then, I found a workaround to fix the similar segfault in my other
feature.
It might work for solving this and goes like this:
instead of trying to access the rvalue when first replaying the asm, create an
intermediate memento that does the work of add_output_operand (and most likely
the other actions like add_input_operand).
It works since this memento will necessarily be created after both the asm and
the variable and thus, both will have been replayed when it's time to replay
the new 'add_output_operand' memento.