linux-kernel - Re: [PATCH] powerpc/vdso32: Add missing _restgpr_31

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Message-ID: <20210315234714.GC16691@gate.crashing.org>
Date:   Mon, 15 Mar 2021 18:47:14 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/vdso32: Add missing _restgpr_31_x to fix build failure

On Mon, Mar 15, 2021 at 05:23:44PM +0100, Rasmus Villemoes wrote:
> On 12/03/2021 03.29, Segher Boessenkool wrote:
> > On Tue, Mar 09, 2021 at 06:19:30AM +0000, Christophe Leroy wrote:
> >> With some defconfig including CONFIG_CC_OPTIMIZE_FOR_SIZE,
> >> (for instance mvme5100_defconfig and ps3_defconfig), gcc 5
> >> generates a call to _restgpr_31_x.
> > 
> >> I don't know if there is a way to tell GCC not to emit that call, because at the end we get more instructions than needed.
> > 
> > The function is required by the ABI, you need to have it.
> > 
> > You get *fewer* insns statically, and that is what -Os is about: reduce
> > the size of the binaries.
> 
> Is there any reason to not just always build the vdso with -O2? It's one
> page/one VMA either way, and the vdso is about making certain system
> calls cheaper, so if unconditional -O2 could save a few cycles compared
> to -Os, why not? (And if, as it seems, there's only one user within the
> DSO of _restgpr_31_x, yes, the overall size of the .text segment
> probably increases slightly).

You can use exactly the same reasoning for using -O2 instead of -Os
anywhere else.

-Os doesn't mean "smaller code, but only where that is reasonable".  It
means "smaller code".


Segher