<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="">Keep in mind we had a real GLSL conformance test fail because of exactly this sort of “optimization” (using float operations to perform non-destructive operations on values that are actually integers).<div class=""><br class=""></div><div class="">—escha</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Apr 12, 2016, at 9:18 AM, Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com" class="">Daniel.Sanders@imgtec.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; 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;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif;" class="">Good point. The same argument seems to apply to copy() too so I suppose it depends how strict we want to be about it.<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></span></div><div style="border-style: none none none solid; border-left-color: blue; border-left-width: 1.5pt; padding: 0cm 0cm 0cm 4pt;" class=""><div class=""><div style="border-style: solid none none; border-top-color: rgb(181, 196, 223); border-top-width: 1pt; padding: 3pt 0cm 0cm;" class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><b class=""><span lang="EN-US" style="font-size: 10pt; font-family: Tahoma, sans-serif;" class="">From:</span></b><span lang="EN-US" style="font-size: 10pt; font-family: Tahoma, sans-serif;" class=""><span class="Apple-converted-space"> </span><a href="mailto:fglaser@apple.com" class="">fglaser@apple.com</a> [<a href="mailto:fglaser@apple.com" class="">mailto:fglaser@apple.com</a>]<span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b><a href="mailto:escha@apple.com" class="">escha@apple.com</a><br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>11 April 2016 20:55<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Daniel Sanders<br class=""><b class="">Cc:</b><span class="Apple-converted-space"> </span>Alex Rosenberg; llvm-dev@lists.llvm.org; Carlos Liam<br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [llvm-dev] Implementing a proposed InstCombine optimization<o:p class=""></o:p></span></div></div></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div><div class=""><blockquote style="margin-top: 5pt; margin-bottom: 5pt;" class=""><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">On Apr 11, 2016, at 4:23 AM, Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com" style="color: purple; text-decoration: underline;" class="">Daniel.Sanders@imgtec.com</a>> wrote:<o:p class=""></o:p></div></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div><div class=""><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">> I am not entirely sure this is safe. Transforming this to an fsub could change the value stored on platforms that implement negates using arithmetic instead of with bitmath (such as ours)<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""> <o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">I think it's probably safe for IEEE754-2008 conformant platforms because negation was clarified to be a non-arithmetic bit flip that cannot cause exceptions in that specification.<o:p class=""></o:p></div></div></div></blockquote></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">I did some digging into IEEE-754 and it seems like this is actually not even safe on fully conformant IEEE-754-2008 platforms.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class="">5.5.1 Sign bit operations<o:p class=""></o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class="">5.5.1.0 Implementations shall provide the following homogeneous quiet-computational sign bit operations for all supported arithmetic formats; they only affect the sign bit. The operations treat floating-point numbers and NaNs alike, and signal no exception. These operations may propagate non-canonical encodings.<o:p class=""></o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class=""><o:p class=""> </o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class="">copy(x) copies a floating-point operand x to a destination in the same format, with no change to the sign bit. <o:p class=""></o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class="">negate(x) copies a floating-point operand x to a destination in the same format, reversing the sign bit. negate(x) is not the same as subtraction(0, x) (see 6.3).<o:p class=""></o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><span style="font-family: Times, serif;" class=""><o:p class=""> </o:p></span></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">Note the MAY. fneg is required to flip the top bit even if the input is a NaN. But fneg is not required to maintain the other bits. If the input is a non-canonical NaN, the fneg MAY canonicalize it. In fact, even the ‘copy’ MAY canonicalize it. (it also MAY choose to not canonicalize it)<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">Thus, if the integer being fneg’d is a non-canonical NaN, fneg MAY modify bits other than the top bit.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman', serif;" class="">—escha</div></div></div></div></div></div></blockquote></div><br class=""></div></body></html>