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From: "iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102528] Unused out-of-line functions emitted for trivial
 coroutines
Date: Mon, 04 Oct 2021 19:31:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102528

--- Comment #3 from Iain Sandoe <iains at gcc dot gnu.org> ---
 (In reply to Mathias Stearn from comment #0)
 > https://godbolt.org/z/aoab9W4xG
 >=20
 > This should all compile away, and test() should just be a single ret
 > instruction. That is not what happens now, even with -O3.

Hi Mathias,

Yes, ideally, that would be the case - so I agree it's a missed optimisatio=
n.

On the other hand, to say that the emitted code is unused is not quite righ=
t;

We inline the ramp but fail to inline the 'actor' (i.e. the resume).
GCC does not have 'halo' coroutine frame elision, so that we create a frame=
 in
the inlined ramp, and that holds references to the resumer and destroyer.

The net result is that all the code is "used" in the sense that it is refer=
red
to by the emitted test ().

I have an idea (old not new) about altering the internal interfaces in the
generated code to avoid a devirtualisation being necessary to see the inline
opportunity - although I cannot be sure that this is the effect in play, it
seems plausible.

-- so the bottom line is that this is a failure to inline rather than a fai=
lure
to elide unused code.=