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<div class=Section1>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi Eric,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal>IMO you should specify the minimum cpu you want to run on
via<o:p></o:p></p>
<p class=MsoNormal>-march and the one you want to tune for via -mtune (not that
we really support<o:p></o:p></p>
<p class=MsoNormal>doing this in the llvm backend anyhow) and that -mcpu should
basically<o:p></o:p></p>
<p class=MsoNormal>die in a fire. Historically though the cpu is automagically
set from the architecture<o:p></o:p></p>
<p class=MsoNormal>targeted and I think we should continue that and use default
tuning to<o:p></o:p></p>
<p class=MsoNormal>some processor of that sort.<o:p></o:p></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>OK, I give in, I’ll find a way of getting the defaults generated
from ARM.td (perhaps by #including ARMGenSubtarget.inc?) instead of deleting it
outright.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I may delay this though until after the conference in case
people have different ideas about the direction of the driver.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Cheers,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>James<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>
<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> Eric Christopher
[mailto:echristo@apple.com] <br>
<b>Sent:</b> 15 October 2011 02:38<br>
<b>To:</b> James Molloy<br>
<b>Cc:</b> cfe-dev@cs.uiuc.edu<br>
<b>Subject:</b> Re: [cfe-dev] Driver: Default CPUs<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<div>
<p class=MsoNormal>On Oct 12, 2011, at 2:40 AM, James Molloy wrote:<o:p></o:p></p>
</div>
<p class=MsoNormal><br>
<br>
<o:p></o:p></p>
<p class=MsoNormal><span class=apple-style-span><span style='font-size:13.5pt;
font-family:"Helvetica","sans-serif"'>The arguments I've heard so far are:</span></span><span
style='font-size:13.5pt;font-family:"Helvetica","sans-serif"'><br>
<span class=apple-style-span> * Mine, that there should be no
default CPU selected if the user doesn't</span><br>
<span class=apple-style-span>select it himself. I feel it adds a hidden option
that the user is unaware</span><br>
<span class=apple-style-span>of, and depending on what that default is may
cause inferior performance on</span><br>
<span class=apple-style-span>the CPU the user actually runs it on.</span><br>
<span class=apple-style-span> * Jim's (? I forget who responded ?), that
there *should* be a default CPU</span><br>
<span class=apple-style-span>all the time, but perhaps some pseudo-CPUs could
be added that exhibit a</span><br>
<span class=apple-style-span>blended scheduling. For example for ARM, a
"v7" pseudo-cpu could be added</span><br>
<span class=apple-style-span>that has an itinerary that performs well on all v7
cores. By the way, I</span><br>
<span class=apple-style-span>think ARM is the only high-visibility target with
such a difference between</span><br>
<span class=apple-style-span>cores that this matters. I assume MIPS and PowerPC
are similar though?</span><br>
<br>
</span><o:p></o:p></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'>
<p class=MsoNormal>What do people think? Do people care?<o:p></o:p></p>
</blockquote>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>I think I hate the arm notion of "cpu" in the
output file. Only arch and tune should<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>matter ;)<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>MIPS and (IIRC) powerpc don't work this way. For MIPS -march
is the minimum<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>ISA you're compiling for and if it's a specific processor
then by default will<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>tune for that unless you want something else. There's no
real idea of a "generic"<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>cpu for tuning or scheduling itinerary, you'd usually tune
for whatever processor<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>is the most important.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>So it looks like this decision is mostly ARM related. For
ease of compatibility<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>I'd suggest doing whatever gcc does, but I don't really
think it matters that<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>much. <o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>IMO you should specify the minimum cpu you want to run on
via<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>-march and the one you want to tune for via -mtune (not that
we really support<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>doing this in the llvm backend anyhow) and that -mcpu should
basically<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>die in a fire. Historically though the cpu is automagically
set from the architecture<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>targeted and I think we should continue that and use default
tuning to<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal>some processor of that sort.<o:p></o:p></p>
</div>
<div>
<p class=MsoNormal><o:p> </o:p></p>
</div>
<div>
<p class=MsoNormal>-eric<o:p></o:p></p>
</div>
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