Re: [PATCH 1/1] aacraid: big endian issues

[Grant Grundler]

On Jan 8, 2008 1:37 PM, Salyzyn, Mark <[EMAIL PROTECTED]> wrote:
> I've always assumed that byte swapping of constants would be optimized where 
> a variable would not :-)

definitely...I understood that. It just seemed odd usage.

>
> I have confirmed in assembler output of the compiler that constant
> merely become byte reversed constants optimized or no in at least
> one architectural case.

Thanks!
And apologies, I didn't mean to cause you additional work since I
was just curious.

> I have *not* confirmed that a variable byte reversal requires processing 
> overhead, as one could possibly expect the compiler to instead optimize by 
> byte reversing the constant when comparing. However, I will guarantee you 
> that if optimization is turned off in the compiler that such an optimization 
> will not take place...

:)

>
> I do not think this turns into a readability issue in either case
> and view this as a simple cosmetic coding precaution much
> like likely()/unlikely() offers hints to the compiler on code intent...

Agreed - it's not a readability issue.

thanks,
grant

>
> Sincerely -- Mark Salyzyn
>
>
> > -----Original Message-----
> > From: Grant Grundler [EMAIL PROTECTED]
> > Sent: Tuesday, January 08, 2008 4:17 PM
> > To: Salyzyn, Mark
> > Cc: linux-scsi@vger.kernel.org
> > Subject: Re: [PATCH 1/1] aacraid: big endian issues
> >
> > On Jan 8, 2008 12:48 PM, Salyzyn, Mark
> > <[EMAIL PROTECTED]> wrote:
> > > Big endian systems issues discovered in the aacraid driver.
> >
> > ...
> > --- a/drivers/scsi/aacraid/comminit.c   2008-01-08
> > 15:32:28.329810853 -0500
> > +++ b/drivers/scsi/aacraid/comminit.c   2008-01-08
> > 15:37:35.633163607 -0500
> > @@ -301,10 +301,10 @@
> >         if ((!aac_adapter_sync_cmd(dev, GET_ADAPTER_PROPERTIES,
> >                 0, 0, 0, 0, 0, 0, status+0, status+1,
> > status+2, NULL, NULL)) &&
> >                         (status[0] == 0x00000001)) {
> > -               if (status[1] & AAC_OPT_NEW_COMM_64)
> > +               if (status[1] & le32_to_cpu(AAC_OPT_NEW_COMM_64))
> > ...
> >
> > Why apply le32_to_cpu() to the constant instead of the variable?
> > On systems were le32_to_cpu() is doing something, can gcc or
> > preprocessor optimize the constant?
> > I've always assumed it could not but that might be wrong.
> >
> > thanks,
> > grant
>
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