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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">I'm just talking about the IEEE754-2008 case in my previous email. You mentioned that your target uses arithmetic negation which isn't permitted by IEEE754-2008. For arithmetic
negation, the optimization is definitely unsafe.<o:p></o:p></span></p>
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<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""> fglaser@apple.com [mailto:fglaser@apple.com]
<b>On Behalf Of </b>escha@apple.com<br>
<b>Sent:</b> 12 April 2016 17:24<br>
<b>To:</b> Daniel Sanders<br>
<b>Cc:</b> Alex Rosenberg; llvm-dev@lists.llvm.org; Carlos Liam<br>
<b>Subject:</b> Re: [llvm-dev] Implementing a proposed InstCombine optimization<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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).<o:p></o:p></p>
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<p class="MsoNormal">—escha<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Apr 12, 2016, at 9:18 AM, Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com">Daniel.Sanders@imgtec.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Good point. The same argument seems to apply to copy() too so I suppose it depends how strict we want to be about it.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span class="apple-converted-space"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> </span></span><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""><a href="mailto:fglaser@apple.com">fglaser@apple.com</a>
[<a href="mailto:fglaser@apple.com">mailto:fglaser@apple.com</a>]<span class="apple-converted-space"> </span><b>On Behalf Of<span class="apple-converted-space"> </span></b><a href="mailto:escha@apple.com">escha@apple.com</a><br>
<b>Sent:</b><span class="apple-converted-space"> </span>11 April 2016 20:55<br>
<b>To:</b><span class="apple-converted-space"> </span>Daniel Sanders<br>
<b>Cc:</b><span class="apple-converted-space"> </span>Alex Rosenberg; llvm-dev@lists.llvm.org; Carlos Liam<br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: [llvm-dev] Implementing a proposed InstCombine optimization</span><o:p></o:p></p>
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<p class="MsoNormal">On Apr 11, 2016, at 4:23 AM, Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com"><span style="color:purple">Daniel.Sanders@imgtec.com</span></a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">> 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></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">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></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">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></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif"">5.5.1 Sign bit operations</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif"">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif""> </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif"">copy(x) copies a floating-point operand x to a destination in the same format, with no change to the sign bit. </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif"">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).</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Times","serif""> </span><o:p></o:p></p>
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<p class="MsoNormal">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></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Thus, if the integer being fneg’d is a non-canonical NaN, fneg MAY modify bits other than the top bit.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">—escha<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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