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

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Date:   Mon, 15 Mar 2021 16:38:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Rasmus Villemoes' <rasmus.villemoes@...vas.dk>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
CC:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "Paul Mackerras" <paulus@...ba.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] powerpc/vdso32: Add missing _restgpr_31_x to fix build
 failure

From: Rasmus Villemoes
> Sent: 15 March 2021 16:24
> 
> On 12/03/2021 03.29, Segher Boessenkool wrote:
> > Hi!
> >
> > 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).

Sometimes -Os generates such horrid code you really never want to use it.
A classic is on x86 where it replaces 'load register with byte constant'
with 'push byte' 'pop register'.
The code is actually smaller but the execution time is horrid.

There are also cases where -O2 actually generates smaller code.

Although you may need to disable loop unrolling (often dubious at best)
and either force or disable some function inlining.

	David

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