<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="">@Stephen @John: </div><div class="">Interesting to learn about the low-level things, thank you,</div><div class="">but efficient or not. somewhere along the way, </div><div class="">conversions are simply unavoidable, </div><div class="">whether explicit or implicit, </div><div class="">regardless of its performance...</div><div class=""><br class=""></div><div class="">Theoretically, doing:</div><div class="">aDouble = Double(aFloat) </div><div class=""> should have the same performance as</div><div class="">aDouble = aFloat //implicitly</div><div class="">The compiler simply generates the same code in both cases, i assume.</div><div class=""><br class=""></div><div class="">Implicit or explicit? Thinking further, it seems to me that it doesn’t</div><div class="">matter because the programmer should be equally aware of the </div><div class="">operation, whether it is an explicit or an implicit conversion.</div><div class=""><br class=""></div><div class="">(seems to be not so difficult assisted by verbose compiler warnings during editing) </div><div class=""><br class=""></div><div class="">So, thinking along this line, (that is, the programmer has to make </div><div class="">almost the same judging effort in both cases anyway) it seems </div><div class="">logically correct that explicit conversions are in fact superfluous, pointless, </div><div class="">and can thus be removed from the language, with possibly the exception for </div><div class="">explicit conversion functions that have extra parameters to influence</div><div class="">the conversion, like its precision, magnitude, rounding etc.</div><div class="">e.g: </div><div class=""> anInt = Int(aFloat, truncation: .roundingUp) </div><div class=""><br class=""></div><div class="">?</div><div class=""><br class=""></div><div class="">TedvG </div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On 20. Jun 2017, at 00:08, Stephen Canon <<a href="mailto:scanon@apple.com" class="">scanon@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" class=""><div class=""><br class="Apple-interchange-newline">On Jun 19, 2017, at 5:43 PM, David Sweeris <<a href="mailto:davesweeris@mac.com" class="">davesweeris@mac.com</a>> wrote:</div><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class=""><br class="">Sent from my iPhone</div>On Jun 19, 2017, at 13:44, John McCall via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class=""><div class=""><blockquote type="cite" class=""><div class="">On Jun 19, 2017, at 1:58 PM, Stephen Canon via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><blockquote type="cite" class="">On Jun 19, 2017, at 11:46 AM, Ted F.A. van Gaalen via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""></blockquote><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br class="Apple-interchange-newline">var result: Float = 0.0</div><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">result = float * integer * uint8 + double </div><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">// here, all operands should be implicitly promoted to Double before the complete expression evaluation.</div></div></blockquote></div><br class=""><div class="">You would have this produce different results than:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let temp = float * integer * uint8</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>result = temp + double</div><div class=""><br class=""></div><div class="">That would be extremely surprising to many unsuspecting users.</div><div class=""><br class=""></div><div class="">Don’t get me wrong; I *really want* implicit promotions (I proposed one scheme for them way back when Swift was first unveiled publicly).</div></div></div></blockquote><div class=""><br class=""></div></div>I don't! At least not for floating point. It is important for both reliable behavior and performance that programmers understand and minimize the conversions they do between different floating-point types.</div></blockquote><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">How expensive is it?</div></div></blockquote><br class=""></div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">On most contemporary hardware, it’s comparable to a floating-point add or multiply. On current generation Intel, it’s actually a little bit more expensive than that. Not catastrophic, but expensive enough that you are throwing away half or more of your performance if you incur spurious conversions on every operation.</div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">This is really common in C and C++ where a naked floating-point literal like 1.2 is double:</div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>float x;</div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>x *= 1.2;</div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">Instead of a bare multiplication (current generation x86 hardware: 1 µop and 4 cycles latency) this produces a convert-to-double, multiplication, and convert-to-float (5 µops and 14 cycles latency per Agner Fog).</div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Avenir-Medium; font-size: 14.000000953674316px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">–Steve</div></div></blockquote></div><br class=""></body></html>