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<p class="MsoNormal">Hello folks,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’ve recently been looking at the generated code for a few functions in Chromium while investigating crashes, and I came across a curious pattern. A smallish repro case is available at
<a href="https://godbolt.org/z/Dsu1WI">https://godbolt.org/z/Dsu1WI</a> . In that case, the function Assembler::emit_arith receives a struct (Operand) by value and passes it by value to another function. That struct is 8 bytes long, so the -O3 generated code
uses movsd to copy it up the stack. However, we end up with some loads that aren’t needed, as in the following chunk:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="color:blue">movsd</span><span style="color:black">
</span><span style="color:#4864AA">xmm0</span><span style="color:black">, </span>
<span style="color:teal">qword</span><span style="color:black"> </span><span style="color:teal">ptr</span><span style="color:black"> [</span><span style="color:#4864AA">ecx</span><span style="color:black">]
</span><span style="color:green"># xmm0 = mem[0],zero</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:blue">mov</span><span style="color:black">
</span><span style="color:teal">dword</span><span style="color:black"> </span><span style="color:teal">ptr</span><span style="color:black"> [</span><span style="color:#4864AA">esp</span><span style="color:black"> +
</span><span style="color:#09885A">24</span><span style="color:black">], </span><span style="color:#4864AA">edx</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:blue">movsd</span><span style="color:black">
</span><span style="color:teal">qword</span><span style="color:black"> </span><span style="color:teal">ptr</span><span style="color:black"> [</span><span style="color:#4864AA">esp</span><span style="color:black"> +
</span><span style="color:#09885A">40</span><span style="color:black">], </span><span style="color:#4864AA">xmm0</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:blue">movsd</span><span style="color:black">
</span><span style="color:#4864AA">xmm0</span><span style="color:black">, </span>
<span style="color:teal">qword</span><span style="color:black"> </span><span style="color:teal">ptr</span><span style="color:black"> [</span><span style="color:#4864AA">esp</span><span style="color:black"> +
</span><span style="color:#09885A">40</span><span style="color:black">] </span><span style="color:green"># xmm0 = mem[0],zero</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:blue">movsd</span><span style="color:black">
</span><span style="color:teal">qword</span><span style="color:black"> </span><span style="color:teal">ptr</span><span style="color:black"> [</span><span style="color:#4864AA">esp</span><span style="color:black"> +
</span><span style="color:#09885A">8</span><span style="color:black">], </span><span style="color:#4864AA">xmm0</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As far as I can tell, the fourth line has no effect. On its own, that seems like a small missed opportunity for optimization. However, this sequence of instructions also appears to trigger a hardware bug on a small fraction of devices which
sometimes end up storing zero at esp+8. A more in-depth discussion of that issue can be found here:
<a href="https://bugs.chromium.org/p/v8/issues/detail?id=9774">https://bugs.chromium.org/p/v8/issues/detail?id=9774</a> .<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m hoping that getting rid of the second load in the sequence above would appease these misbehaving machines (though of course I don’t know that it would), as well as making the code a little smaller for everybody else. Does that sound
like a reasonable idea? Would LLVM be interested in a patch related to eliminating reloads like this? Does anybody have advice about where I should start looking, or any reasons it would be very hard to achieve the result I’m hoping for?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Seth<o:p></o:p></p>
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