> -----Original Message-----
> From: Andrew Pinski (QUIC) <quic_apin...@quicinc.com>
> Sent: Monday, June 23, 2025 8:01 AM
> To: gcc-patches@gcc.gnu.org
> Cc: Andrew Pinski (QUIC) <quic_apin...@quicinc.com>
> Subject: [PATCH] expand: Allow sibcalling for return structures
> in some cases [PR71761]
>
> In the case of tailing call with a return of a structure, currently
> all large structures are rejected. We can allow the case were
> the return of the "tail call" function is setting the return value
> of the current function. This allows for the musttail that is
> located in pr71761-1.c.
>
> This should be safe as sibcalls here is RSO is always set for this
> case so the current function no longer owns the space.
>
> Bootstrapped and tested on x86_64-linux-gnu with no
> regressions.
Looks like there is some issues with aarch64 which I will need to look into to
see what is going wrong. Bootstrap fails on aarch64; looks like some wrong code
happening while compiling gcc itself.
So right for now, this patch is withdrawn while I figure out what is going
wrong.
Thanks,
Andrew
>
> PR middle-end/71761
>
> gcc/ChangeLog:
>
> * calls.cc (can_implement_as_sibling_call_p): Don't reject
> "structured" returns if the addr is the same as the current
> result decl's memory.
>
> gcc/testsuite/ChangeLog:
>
> * c-c++-common/pr71761-1.c: New test.
>
> Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
> ---
> gcc/calls.cc | 23 +++++++++++++++++------
> gcc/testsuite/c-c++-common/pr71761-1.c | 17
> +++++++++++++++++
> 2 files changed, 34 insertions(+), 6 deletions(-) create mode
> 100644 gcc/testsuite/c-c++-common/pr71761-1.c
>
> diff --git a/gcc/calls.cc b/gcc/calls.cc index
> 1dcda20a06e..d1f3e17591f 100644
> --- a/gcc/calls.cc
> +++ b/gcc/calls.cc
> @@ -2575,14 +2575,25 @@ can_implement_as_sibling_call_p
> (tree exp,
> return false;
> }
>
> - /* Doing sibling call optimization needs some work, since
> - structure_value_addr can be allocated on the stack.
> - It does not seem worth the effort since few optimizable
> - sibling calls will return a structure. */
> + /* For sibling call optimization with a structure return,
> + the return value needs to be the same as the current
> function,
> + otherwise the structure return will be allocated in the
> stack. */
> if (structure_value_addr != NULL_RTX)
> {
> - maybe_complain_about_tail_call (exp, _("callee returns a
> structure"));
> - return false;
> + tree res = DECL_RESULT (current_function_decl);
> + bool ok = false;
> + if (res && DECL_RTL_SET_P (res))
> + {
> + rtx res_rtx = DECL_RTL (res);
> + if (MEM_P (res_rtx)
> + && XEXP (res_rtx, 0) == structure_value_addr)
> + ok = true;
> + }
> + if (!ok)
> + {
> + maybe_complain_about_tail_call (exp, _("callee returns a
> structure"));
> + return false;
> + }
> }
>
> /* Check whether the target is able to optimize the call diff --
> git a/gcc/testsuite/c-c++-common/pr71761-1.c
> b/gcc/testsuite/c-c++-common/pr71761-1.c
> new file mode 100644
> index 00000000000..41f4f834fc7
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/pr71761-1.c
> @@ -0,0 +1,17 @@
> +/* PR middle-end/71761 */
> +/* { dg-do compile { target musttail } } */
> +/* { dg-options "-O2" } */
> +
> +typedef struct Foo {
> + int o[16];
> +}Foo;
> +
> +Foo moo();
> +
> +Foo goo()
> +{
> + [[gnu::musttail]]
> + return moo();
> +}
> +
> +/* { dg-final { scan-assembler "jmp moo" { target { { i?86-*-*
> x86_64-*-* } && { ! ia32 } } } } } */
> --
> 2.34.1
