<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Oct 12, 2011, at 2:40 AM, James Molloy wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; font-size: medium; ">The arguments I've heard so far are:<br> * Mine, that there should be no default CPU selected if the user doesn't<br>select it himself. I feel it adds a hidden option that the user is unaware<br>of, and depending on what that default is may cause inferior performance on<br>the CPU the user actually runs it on.<br> * Jim's (? I forget who responded ?), that there *should* be a default CPU<br>all the time, but perhaps some pseudo-CPUs could be added that exhibit a<br>blended scheduling. For example for ARM, a "v7" pseudo-cpu could be added<br>that has an itinerary that performs well on all v7 cores. By the way, I<br>think ARM is the only high-visibility target with such a difference between<br>cores that this matters. I assume MIPS and PowerPC are similar though?<br></span></blockquote></div><br><div><blockquote type="cite">What do people think? Do people care?<br></blockquote><br></div><div>I think I hate the arm notion of "cpu" in the output file. Only arch and tune should</div><div>matter ;)</div><div><br></div><div>MIPS and (IIRC) powerpc don't work this way. For MIPS -march is the minimum</div><div>ISA you're compiling for and if it's a specific processor then by default will</div><div>tune for that unless you want something else. There's no real idea of a "generic"</div><div>cpu for tuning or scheduling itinerary, you'd usually tune for whatever processor</div><div>is the most important.</div><div><br></div><div>So it looks like this decision is mostly ARM related. For ease of compatibility</div><div>I'd suggest doing whatever gcc does, but I don't really think it matters that</div><div>much. </div><div><br></div><div>IMO you should specify the minimum cpu you want to run on via</div><div>-march and the one you want to tune for via -mtune (not that we really support</div><div>doing this in the llvm backend anyhow) and that -mcpu should basically</div><div>die in a fire. Historically though the cpu is automagically set from the architecture</div><div>targeted and I think we should continue that and use default tuning to</div><div>some processor of that sort.</div><div><br></div><div>-eric</div></body></html>