Re: PATCH: Disable hint in B unit for Montecito

["H. J. Lu" <hjl at lucon dot org>]

On Thu, Feb 17, 2005 at 01:05:44PM -0800, James E Wilson wrote:
> On Thu, 2005-02-17 at 10:47, H. J. Lu wrote:
> > On Wed, Feb 16, 2005 at 08:43:11PM -0800, H. J. Lu wrote:
> > > Unfortunately, it is trickier than I thought. With 2.6 kernel, my
> > > old change works OK. My new change doesn't work. Jim, do you have
> > > any suggestions?
> 
> An option here is to make it a warning by default, which I mentioned in
> my earlier message, and which is what we have done with almost all of
> the other new diagnostics.
> 
> > I think it is OK to change unit when manual bundling is off there is no
> > user template. I am testing 2.4 and 2.6 kernel build now.
> 
> Yes, this is true.  However, I see other problems here.
> 
> Forcing the hint to an I unit doesn't do what it appears to do.  Since B
> slots are always at the end or followed by another B slot, the
> instruction will not fit into the current bundle.  So we will fill the
> current bundle with nops, and then come back to this loop again, at
> which point we will have a different template, and the hint could very
> well end up in a M or F slot, even though we tried to force it into an I
> slot earlier.  This is potentially confusing.  There should at least be
> a comment here saying that the purpose of this is to prevent the
> instruction from going into the current slot.  It will go into a later
> slot, at which point we may choose a different unit for it.

I enclosed my current patch. I will check it in shortly.

> 
> The asm source specifies a MIB template, but there is no MIB template in
> the output.  I believe this is an assembler bug.  The problem arises
> because the bundle before the MIB template is incomplete.  We should pad
> it with nops, but instead we steal instructions from the MIB template,
> and that causes us to lose track of the MIB template which is attached
> to the nop.m instruction.  The result is a BBB template.  This is a
> problem, because BBB templates use different less efficient branch
> prediction.  That is presumably why the MIB template was specified in
> the first place.  This needs to be fixed.
> 
> With that problem fixed, this problem goes away, as we are no longer
> trying to force the hint instruction into a B slot.
> 
> Anyways, your patch is OK with the comment added that explains that we
> are using insn_unit to force the hint insn to the next slot, instead of
> forcing it into an I slot.
> -- 
> Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
> 

> This is an untested patch to fix the problem I pointed out with bundle
> handling.  This works for your testcase.  We get the user requested MIB
> template, and the hint ends up in an M slot.
> 

This patch makes dv-srlz.o much larger and fails

./gas/testsuite/gas.log:FAIL: gas/ia64/dv-srlz


H.J.

Attachment: gas-ia64-hint-12.patch
Description: Text document