<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="">Is there any guarantee that these two loops have the exact same runtime performance?</div><div class=""><div class=""><br class=""></div><blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><font face="Menlo" class="">for (i, j) in zip(10.stride(to: 0, by: -1), 20.stride(to: 0, by: -2)) {</font></div><div class=""><font face="Menlo" class=""> if i % 2 == 0 { continue }</font></div><div class=""><font face="Menlo" class=""> print(i, j)</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">for var i = 10, j = 20; i > 0 && j > 0; i -= 1, j -= 2 {</font></div><div class=""><font face="Menlo" class=""> if i % 2 == 0 { continue }</font></div><span class="" style="font-family: Menlo;"> print(i, j)</span><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><blockquote type="cite" class=""></blockquote><blockquote type="cite" class=""></blockquote></div></div></div></div></blockquote></div><div class=""><br class=""></div><div class="">In a quick and dirty test, the second is approximately 34% slower.</div><div class=""><br class=""></div><div class="">I’d say that’s more than acceptable for the readability gain. If you’re in that rare stretch of critical code where the extra 34% actually matters, write it using a while loop instead.</div><div class=""><br class=""></div><div class="">P</div><div class=""><br class=""></div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 10, 2015, at 4:07 PM, David Owens II via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 10, 2015, at 1:57 PM, thorsten--- via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Yes, performance is one thing neglected by the discussions and the proposal.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""></div></blockquote></div><br class=""><div class="">This is my primary objection to to this proposal; it assumes (or neglects?) that all of the types used can magically be inlined to nothing but the imperative code. This isn’t magical, someone has to implement the optimizations to do this.</div><div class=""><br class=""></div><div class="">Is there any guarantee that these two loops have the exact same runtime performance?</div><div class=""><div class=""><br class=""></div><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><font face="Menlo" class="">for (i, j) in zip(10.stride(to: 0, by: -1), 20.stride(to: 0, by: -2)) {</font></div><div class=""><font face="Menlo" class=""> if i % 2 == 0 { continue }</font></div><div class=""><font face="Menlo" class=""> print(i, j)</font></div><div class=""><font face="Menlo" class="">}</font></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class=""><font face="Menlo" class="">for var i = 10, j = 20; i > 0 && j > 0; i -= 1, j -= 2 {</font></div><div class=""><font face="Menlo" class=""> if i % 2 == 0 { continue }</font></div><span style="font-family: Menlo;" class=""> print(i, j)</span><div class=""><font face="Menlo" class="">}</font></div></blockquote><div class=""><blockquote type="cite" class=""></blockquote><blockquote type="cite" class=""></blockquote></div><div class=""><br class=""></div><div class="">And can you guarantee that performance is relatively the same across debug and release builds? Because historically, Swift has suffered greatly in this regard with respects to the performance of optimized versus non-optimized builds.</div></div><div class=""><br class=""></div><div class="">These types of optimizer issues are real-world things I’ve had to deal with (and have written up many blog posts about). I get the desire to simplify the constructs, but we need an escape hatch to write performant code when the optimizer isn’t up to the job.</div><div class=""><br class=""></div><div class="">-David</div>
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