[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[pietro.gcc at sociotechnical dot xyz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

pietro <pietro.gcc at sociotechnical dot xyz> changed:

           What    |Removed                     |Added
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                 CC|                            |pietro.gcc at sociotechnical dot x
                   |                            |yz

--- Comment #2 from pietro <pietro.gcc at sociotechnical dot xyz> ---
I created a plugin that lets you annotate a whole function as musttail:
https://github.com/pietro/gcc-musttail-plugin/

I think that in order to add an attribute to the return statement like clang
has done the parser for the C frontend needs to be modified.

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

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

           What    |Removed                     |Added
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                 CC|                            |dje at gcc dot gnu.org

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 112558 has been marked as a duplicate of this bug. ***

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

David Edelsohn <dje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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           Priority|P3                          |P2
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-16
             Status|UNCONFIRMED                 |NEW

--- Comment #4 from David Edelsohn <dje at gcc dot gnu.org> ---
https://gcc.gnu.org/pipermail/gcc/2021-April/235882.html

https://blog.reverberate.org/2021/04/21/musttail-efficient-interpreters.html

The lack of this feature is motivating CPython to rely on LLVM for its JIT in
future releases.

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[mikpelinux at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #5 from Mikael Pettersson <mikpelinux at gmail dot com> ---
To get guaranteed tail-calls to work you need to adjust the calling convention
for the caller as well as the callee. Things are trivial as long as parameters
always fit in registers. With parameters on the stack you need, in general, to
shift the responsibility for their deallocation from caller to callee, and this
results in a non-standard calling convention.

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to David Edelsohn from comment #4)
> The lack of this feature is motivating CPython to rely on LLVM for its JIT
> in future releases.

Which is interesting because GCC JIT supports this already. Just not exposed to
C. That was mentioned in the gcc mailing list archive that you pointed to too.

e.g.
https://gcc.gnu.org/onlinedocs/jit/topics/expressions.html#c.gcc_jit_rvalue_set_bool_require_tail_call

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

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

           What    |Removed                     |Added
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           See Also|                            |https://github.com/llvm/llv
                   |                            |m-project/issues/72555

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://github.com/llvm/llvm-project/issues/72555

Hmmm

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #8 from Sam James <sjames at gcc dot gnu.org> ---
(In reply to David Edelsohn from comment #4)
> The lack of this feature is motivating CPython to rely on LLVM for its JIT
> in future releases.

Do you have something I can read up on for this part? It's the first I've heard
it.

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #9 from David Edelsohn <dje at gcc dot gnu.org> ---
Brandt Bucher: A JIT compiler for CPython
https://www.youtube.com/watch?v=HxSHIpEQRjs
https://github.com/brandtbucher/cpython/tree/justin

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to David Edelsohn from comment #9)
> Brandt Bucher: A JIT compiler for CPython
> https://www.youtube.com/watch?v=HxSHIpEQRjs
> https://github.com/brandtbucher/cpython/tree/justin

But must tail is there for the gcc jit interface already. Maybe it is not
exposed to c/c++ attribute but it is there already. So maybe this is just not
noticing that point.

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

--- Comment #11 from David Edelsohn <dje at gcc dot gnu.org> ---
GIMPLE supports must_tail, but it is not exposed at the sources level /
attributes in GCC.

CPython is not adding the LLVM JIT at runtime.  The proposal is to utilize LLVM
at build time to generate code templates that can be copied into the CPython
binary and patched with relocations.

CALL_EXPR_MUST_TAIL_CALL and libgccjit are not sufficient for the CPython plan.
 The plan does not propose connecting CPython to an existing JIT in LLVM (or
GCC).

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arsen at gcc dot gnu.org,
                   |                            |dmalcolm at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=110899

--- Comment #12 from Sam James <sjames at gcc dot gnu.org> ---
(In reply to David Edelsohn from comment #11)
> GIMPLE supports must_tail, but it is not exposed at the sources level /
> attributes in GCC.
> 
> CPython is not adding the LLVM JIT at runtime.  The proposal is to utilize
> LLVM at build time to generate code templates that can be copied into the
> CPython binary and patched with relocations.

This is now up as https://github.com/python/cpython/pull/113465 and
https://github.com/python/cpython/issues/113464.

See also PR110899 and
https://discourse.llvm.org/t/rfc-exposing-ghccc-calling-convention-as-preserve-none-to-clang/74233
(which is linked from the CPython PR for adding the JIT).

[Bug c/83324] [feature request] Pragma or special syntax for guaranteed tail calls

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83324

Sam James <sjames at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |andi-gcc at firstfloor dot org

--- Comment #13 from Sam James <sjames at gcc dot gnu.org> ---
Patches posted by Andi Kleen:
https://inbox.sourceware.org/gcc-patches/20240124110800.3154093-2-ak@linux.intel.com/.