linux-kernel - Re: [PATCH] powerpc: inline current_stack

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Message-ID: <20160524222156.GA7963@fergus.ozlabs.ibm.com>
Date:	Wed, 25 May 2016 08:21:56 +1000
From:	Paul Mackerras <paulus@...abs.org>
To:	Gabriel Paubert <paubert@...m.es>
Cc:	Christophe Leroy <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: inline current_stack_pointer()

On Mon, May 23, 2016 at 07:17:38PM +0200, Gabriel Paubert wrote:
> On Mon, May 23, 2016 at 10:46:02AM +0200, Christophe Leroy wrote:
> > current_stack_pointeur() is a single instruction function. it
> > It is not worth breaking the execution flow with a bl/blr for a
> > single instruction
> 
> Are you sure that the result is always the same? 
> 
> Calling an external function prevents the compiler from considering the
> caller of of current_stack_pointer a leaf function, which certainly 
> does not help the compiler, but in a leaf function the compiler is free 
> not to establish a new frame.
> 
> If the compiler decides to establishes a new frame (typically with 
> "stwu r1,-frame_size(r1)"), *r1 is the previous stack pointer, or
> the caller's stack pointer, or the current function frame pointer if
> I remember correctly the ABI definitions. 
> 
> However, if the compiler decides that it can get away without a frame
> for the function, *r1 is the stack pointer of the caller's caller.
> 
> Depending on the application, this may or may not be important.

Right.  I think I wrote the original current_stack_pointer()
implementation, and that I deliberately didn't make it an inline
so that the caller would have to establish its own stack frame,
and thus its stack pointer value would be a well-defined thing.

Paul.