linux-kernel - Re: sparse -Wptr-subtraction-blows: still needed?

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Date:	Wed, 2 May 2007 01:26:43 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Josh Triplett <josh@...edesktop.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: sparse -Wptr-subtraction-blows: still needed?

On Tue, May 01, 2007 at 04:59:57PM -0700, Josh Triplett wrote:
> Linus Torvalds wrote:
> > 
> > On Tue, 1 May 2007, Josh Triplett wrote:
> >> Does this still apply?  Do current versions of GCC still have this problem?
> >> If not, can the option and warning go away?
> > 
> > Even if current versions of gcc don't triple the build time (and for the 
> > kernel, I suspect it doesn't, because we've tried to clean up our header 
> > files), the generated _code_ will invariably suck.
> 
> "invariably"?
> 
> Do you know whether the current version of GCC generates poor code for pointer
> subtraction?
> 
> If so, does anything in particular make this an unfixable problem?

Just the fact that calculation itself is nasty.  In the best case you
get (on x86) something like
        sarl    $2, %eax
        imull   $-1431655765, %eax, %eax
(that one is with object size equal to 12).  On other targets it can get
considerably uglier - e.g. on alpha with -O2 the same will result in
        sra $17,2,$17
        s4subq $17,$17,$0
        s8subq $0,$0,$0
        s4addq $0,$17,$0
        sll $0,8,$1
        addq $0,$1,$0
        sll $0,16,$2
        addq $0,$2,$0
        sll $0,32,$1
        addq $0,$1,$0
        addq $0,$0,$0
        addl $0,$17,$0
With -Os it's
        ldah $1,$LC0($29)               !gprelhigh
        sra $0,2,$0
        ldq $1,$LC0($1)         !gprellow
        mull $0,$1,$0
with LC0 in rodata:
$LC0:
        .quad   -6148914691236517205

Now imagine the joy of having a bunch of such wonders in a hot path...
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