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<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Hi All,<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><o:p> </o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Today, LLVM IR doesn’t include a description for alignment for specific data members inside structures.<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">The implies that LLVM IR Generators need to do the padding themselves in order to achieve the requested alignment.<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Since the information is not represented explicitly by the LLVM IR, optimization passes might unintentionally break the required alignment and lead to unexpected results when executing
the generated program.<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><o:p> </o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Here is an example “C” code (and OpenCL as well)<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black">struct st1_t {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black"> int i;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black"> float __attribute__((align(64))) f;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black">};<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black">struct st1_t ist1;<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><o:p> </o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Generated IR by CLANG:<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black">%struct.st1_t =
</span><i><span style="font-size:10.0pt;font-family:"Courier New";color:green">type</span></i><span style="font-size:10.0pt;font-family:"Courier New";color:black"> {
</span><i><span style="font-size:10.0pt;font-family:"Courier New";color:green">i32</span></i><span style="font-size:10.0pt;font-family:"Courier New";color:black">, [60 x
</span><i><span style="font-size:10.0pt;font-family:"Courier New";color:green">i8</span></i><span style="font-size:10.0pt;font-family:"Courier New";color:black">],
</span><i><span style="font-size:10.0pt;font-family:"Courier New";color:green">float</span></i><span style="font-size:10.0pt;font-family:"Courier New";color:black">, [60 x
</span><i><span style="font-size:10.0pt;font-family:"Courier New";color:green">i8</span></i><span style="font-size:10.0pt;font-family:"Courier New";color:black">] }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><span style="font-size:10.0pt;font-family:"Courier New";color:black">@ist1 =
</span><b><span style="font-size:10.0pt;font-family:"Courier New";color:blue">common</span></b><span style="font-size:10.0pt;font-family:"Courier New";color:black">
</span><b><span style="font-size:10.0pt;font-family:"Courier New";color:blue">global</span></b><span style="font-size:10.0pt;font-family:"Courier New";color:black"> %struct.st1_t
</span><b><span style="font-size:10.0pt;font-family:"Courier New";color:blue">zeroinitializer</span></b><span style="font-size:10.0pt;font-family:"Courier New";color:black">,
</span><b><span style="font-size:10.0pt;font-family:"Courier New";color:blue">align</span></b><span style="font-size:10.0pt;font-family:"Courier New";color:black"> 64<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><o:p> </o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">What is the reason for disallowing alignment of data members inside structs in LLVM IR? What were the design considerations?<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed"><o:p> </o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="text-align:left;direction:ltr;unicode-bidi:embed">Boaz<o:p></o:p></p>
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