linux-kernel - Re: [PATCH] x86/platform/uv: make const pointer dots a static const array

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Date:   Wed, 1 Dec 2021 15:39:57 -0600
From:   Steve Wahl <steve.wahl@....com>
To:     Joe Perches <joe@...ches.com>
Cc:     Steve Wahl <steve.wahl@....com>,
        Colin Ian King <colin.i.king@...glemail.com>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        platform-driver-x86@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/platform/uv: make const pointer dots a static const
 array

On Tue, Nov 30, 2021 at 04:26:39PM -0800, Joe Perches wrote:
> On Tue, 2021-11-30 at 13:34 -0600, Steve Wahl wrote:
> > On Sat, Nov 27, 2021 at 05:03:20PM +0000, Colin Ian King wrote:
> > > Don't populate the const array dots on the stack
> []
> > Examination of the disassembly shows that the compiler actually
> > eliminates the creation of the pointer "dots" on the stack and just
> > passes the address of the string constant to the printk function.
> > 
> > So this change should not have any actual effect (I don't know where
> > you got the "shrinks object code" from), and in my humble opinion
> > makes the code less clear.
> 
> Probably shrinks an allmodconfig where the symbols are referenced.
> It probably doesn't do anything to a defconfig.

OK, I looked. Under allmodconfig, the new code is one byte smaller.

Defconfig doesn't include CONFIG_X86_UV and this file doesn't get
compiled.

Using defconfig plus CONFIG_X86_UV and prerequisites, the new code is
24 bytes larger, probably because of alignment added.

allmodconfig:

   text	   data	    bss	    dec	    hex	filename
  30827	  18358	   1472	  50657	   c5e1	uv_nmi.o
  30828	  18358	   1472	  50658	   c5e2	uv_nmi.orig.o

default config + CONFIG_X86_UV:

   text	   data	    bss	    dec	    hex	filename
   9918	    216	    160	  10294	   2836	uv_nmi.o
   9894	    216	    160	  10270	   281e	uv_nmi.orig.o

So I still don't think this patch makes sense.

--> Steve Wahl

> > As such, unless there's something here I don't understand, I vote to
> > reject this patch.
> []
> > > but make it static
> > > const and make the pointer an array to remove a dereference. Shrinks
> > > object code a few bytes too.
> []
> > > diff --git a/arch/x86/platform/uv/uv_nmi.c b/arch/x86/platform/uv/uv_nmi.c
> []
> > > @@ -725,7 +725,7 @@ static void uv_nmi_dump_cpu_ip(int cpu, struct pt_regs *regs)
> > >   */
> > >  static void uv_nmi_dump_state_cpu(int cpu, struct pt_regs *regs)
> > >  {
> > > -	const char *dots = " ................................. ";
> > > +	static const char dots[] = " ................................. ";
> 
> 

-- 
Steve Wahl, Hewlett Packard Enterprise