linux-kernel - Re: [PATCH] powerpc: Force inlining of csum

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Message-ID: <20210512143105.GW10366@gate.crashing.org>
Date:   Wed, 12 May 2021 09:31:05 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Force inlining of csum_add()

On Wed, May 12, 2021 at 02:56:56PM +0200, Christophe Leroy wrote:
> Le 11/05/2021 à 12:51, Segher Boessenkool a écrit :
> >Something seems to have decided this asm is more expensive than it is.
> >That isn't always avoidable -- the compiler cannot look inside asms --
> >but it seems it could be improved here.
> >
> >Do you have (or can make) a self-contained testcase?
> 
> I have not tried, and I fear it might be difficult, because on a kernel 
> build with dozens of calls to csum_add(), only ip6_tunnel.o exhibits such 
> an issue.

Yeah.  Sometimes you can force some of the decisions, but that usually
requires knowing too many GCC internals :-/

> >>And there is even one completely unused instance of csum_add().
> >
> >That is strange, that should never happen.
> 
> It seems that several .o include unused versions of csum_add. After the 
> final link, one remains (in addition to the used one) in vmlinux.

But it is a static function, so it should not end up in any object file
where it isn't used.

> >>In the non-inlined version, the first sum with 0 was performed.
> >>Here it is skipped.
> >
> >That is because of how __builtin_constant_p works, most likely.  As we
> >discussed elsewhere it is evaluated before all forms of loop unrolling.
> 
> But we are not talking about loop unrolling here, are we ?

Oh, right you are, but that doesn't change much.  The
_builtin_constant_p(len) is evaluated long before the compiler sees len
is a constant here.

> It seems that the reason here is that __builtin_constant_p() is evaluated 
> long after GCC decided to not inline that call to csum_add().

Yes, it seems we do not currently do even trivial inlining except very
early in the compiler.

Thanks,


Segher