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<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif">Thanks Tim and Craig. I tried both of your suggestions, and both generate the correct machine code for the test examples I have written. Looking at the documentation for SUBREG_TO_REG:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"> /// SUBREG_TO_REG - This instruction is similar to INSERT_SUBREG except that<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"> /// the first operand is an immediate integer constant. This constant is<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"> /// often zero, because it is commonly used to assert that the instruction<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"> /// defining the register implicitly clears the high bits.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif">So I think I agree with Craig that the more natural solution is to stick with INSERT_SUBREG.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif">Thanks!!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif">Scott<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Intel Clear",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><b>From:</b> Craig Topper <craig.topper@gmail.com> <br>
<b>Sent:</b> Wednesday, January 20, 2021 9:14 AM<br>
<b>To:</b> Tim Northover <t.p.northover@gmail.com><br>
<b>Cc:</b> Constable, Scott D <scott.d.constable@intel.com>; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] [X86] Is it possible to implicitly promote a virtual subregister to a super?<o:p></o:p></p>
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<p class="MsoNormal">If there's no guarantee of a value for the upper bits don't you want INSERT_SUBREG? SUBREG_TO_REG says the upper bits are 0(or whatever the immediate says). Not that I've ever found a place that checks it.<o:p></o:p></p>
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<p class="MsoNormal">~Craig<o:p></o:p></p>
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<p class="MsoNormal">On Wed, Jan 20, 2021 at 9:09 AM Tim Northover via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Wed, 20 Jan 2021 at 16:30, Constable, Scott D<br>
<<a href="mailto:scott.d.constable@intel.com" target="_blank">scott.d.constable@intel.com</a>> wrote:<br>
> The actual implementation I'm doing is a bit more complex than the example I provided for illustrative purposes. In my case, I don't need to care about the upper bits at all.<br>
<br>
OK, in that case I think SUBREG_TO_REG is what you want. Something like<br>
<br>
BuildMI(MBB, InsertPt, Loc, TII->get(X86::SUBREG_TO_REG), ExtendedReg)<br>
.addImm(0) // Never been entirely sure what this was here for.<br>
It's always 0.<br>
.addReg(NarrowReg)<br>
.addImm(X86::sub_16bit);<br>
<br>
then use ExtendedReg (a GR32 vreg) in the compare.<br>
<br>
Cheers.<br>
<br>
Tim.<br>
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