On Mon, Sep 14, 2020 at 9:46 AM Eric Botcazou <botca...@adacore.com> wrote:
>
> Hi,
>
> when a thunk cannot be emitted in assembly directly, cgraph_node::expand_thunk
> generates regular GIMPLE code but unconditionally forces a tail call to the
> target of the thunk. That's theoretically OK because the thunk essentially
> forwards its parameters to the target, but in practice the RTL expander can
> spill parameters passed by reference on the stack, see assign_parm_setup_reg:
>
> /* If we were passed a pointer but the actual value can safely live
> in a register, retrieve it and use it directly. */
> if (data->arg.pass_by_reference && TYPE_MODE (TREE_TYPE (parm)) != BLKmode)
> {
> /* We can't use nominal_mode, because it will have been set to
> Pmode above. We must use the actual mode of the parm. */
> if (use_register_for_decl (parm))
> {
> parmreg = gen_reg_rtx (TYPE_MODE (TREE_TYPE (parm)));
> mark_user_reg (parmreg);
> }
> else
> {
> int align = STACK_SLOT_ALIGNMENT (TREE_TYPE (parm),
> TYPE_MODE (TREE_TYPE (parm)),
> TYPE_ALIGN (TREE_TYPE (parm)));
> parmreg
> = assign_stack_local (TYPE_MODE (TREE_TYPE (parm)),
> GET_MODE_SIZE (TYPE_MODE (TREE_TYPE (parm))),
> align);
> set_mem_attributes (parmreg, parm, 1);
> }
>
> use_register_for_decl always return false at -O0 so, in this case, the thunk
> will pass an address within its frame to its target, so it cannot use a tail
> call to invoke it.
>
> Tested on x86_64-suse-linux, OK for the mainline?
ISTR the tailcall flag is only a hint and RTL expansion can decide to
not tailcall based on targets. So to me it looks like a missed disqualification
on the RTL expansion side. Or do we, besides from this very single spot,
simply never tailcall at -O0 and thus never hit this latent issue?
How does this change the debug experience at -O0 when GIMPLE thunks
are used?
Thanks,
Richard.
>
> 2020-09-14 Eric Botcazou <ebotca...@adacore.com>
>
> * cgraphunit.c (cgraph_node::expand_thunk): Force a tail call only
> when optimizing.
>
>
> 2020-09-14 Eric Botcazou <ebotca...@adacore.com>
>
> * gnat.dg/thunk1.adb: New test.
> * gnat.dg/thunk1_pkg1.ads: New helper.
> * gnat.dg/thunk1_pkg2.ads: Likewise.
> * gnat.dg/thunk1_pkg2.adb: Likewise.
>
> --
> Eric Botcazou
