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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""> Eli Friedman [mailto:eli.friedman@gmail.com]
<br>
<b>Sent:</b> 16 August 2013 18:37<br>
<b>To:</b> Daniel Sanders<br>
<b>Cc:</b> LLVM Developers Mailing List (llvmdev@cs.uiuc.edu)<br>
<b>Subject:</b> Re: [LLVMdev] Definition of the bitcast instruction for vectors<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Fri, Aug 16, 2013 at 7:51 AM, Daniel Sanders <<a href="mailto:Daniel.Sanders@imgtec.com" target="_blank">Daniel.Sanders@imgtec.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">From
<a href="http://llvm.org/docs/LangRef.html#bitcast-to-instruction" target="_blank">
http://llvm.org/docs/LangRef.html#bitcast-to-instruction</a>:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
The ‘bitcast‘ instruction converts value to type ty2. It is always a no-op cast because no bits change with this conversion. The conversion is done as if the value had been stored to memory and read back as type ty2. Pointer (or vector of pointers) types may
only be converted to other pointer (or vector of pointers) types with this instruction if the pointer sizes are equal. To convert pointers to other types, use the inttoptr or ptrtoint instructions first.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">If I understand the definition correctly, then:<o:p></o:p></p>
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<span style="font-family:"Courier New"">%1 = bitcast <8 x i16> %0 to <4 x i32></span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Should be equivalent to:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">
<span style="font-family:"Courier New"">store <8 x i16> %0, <8 x i16>* %ptr</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%1 = load <4 x i32>* %ptr</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Which the definition above asserts is a no-op cast. However, the store/load sequence is not a no-op cast for vector types when the target does not treat the whole vector as either
little or big-endian. MIPS MSA stores vector elements in big or little endian order according to a configuration bit, but irrespective of this bit the lanes are stored in ascending order. For example, <4 x i32> <i32 1, i32 2, i32 3, i32 4> would be stored
as this in big-endian mode:<o:p></o:p></p>
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<span style="font-family:"Courier New"">%ptr : 00 00 00 01</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%ptr+4 : 00 00 00 02</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%ptr+8 : 00 00 00 03</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%ptr+12: 00 00 00 04</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">And this in little-endian mode:<o:p></o:p></p>
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<span style="font-family:"Courier New"">%ptr : 01 00 00 00</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%ptr+4 : 02 00 00 00</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Courier New"">%ptr+8 : 03 00 00 00</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">%ptr+12: 04 00 00 00</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">How should I resolve this conflicting definition?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:justify">
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;text-align:justify">
<o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It's a "no-op" cast in the sense that there isn't any math involved; it's not a statement about the number of necessary machine instructions.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-Eli<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Thanks for the clarification.<o:p></o:p></span></p>
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