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<p class="MsoNormal">Very interesting, thanks for the analysis and further reduction! Over-aligning the struct seems like a simple workaround that would have little impact elsewhere.<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> David Zarzycki <dave@znu.io> <br>
<b>Sent:</b> Monday, October 28, 2019 11:23 PM<br>
<b>To:</b> Seth Brenith <Seth.Brenith@microsoft.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> [EXTERNAL] Re: [llvm-dev] unnecessary reload of 8-byte struct on i386<o:p></o:p></p>
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<p class="MsoNormal">This just looks like a temporary stack variable wasn’t properly eliminated because the compiler modeled `Operand` internally as an “i64” which i386 doesn’t natively support. A further reduction, with notes about changes that sidestep the
bug:<o:p></o:p></p>
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<p class="MsoNormal"><a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2FzcCguv&data=02%7C01%7CSeth.Brenith%40microsoft.com%7C9a7bff8d4a864a3bd7a908d75c3880fb%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637079270882127747&sdata=Mlqw0Jmr9wsh4JYWLm0kX%2B4pj%2FjFR8Ef8YJSCYZOpDg%3D&reserved=0">https://godbolt.org/z/zcCguv</a><o:p></o:p></p>
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<p class="MsoNormal">On Oct 26, 2019, at 12:28 AM, Seth Brenith 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">Hello folks,<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’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<span class="apple-converted-space"> </span><a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2FDsu1WI&data=02%7C01%7CSeth.Brenith%40microsoft.com%7C9a7bff8d4a864a3bd7a908d75c3880fb%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637079270882137742&sdata=OeSk0j2ISpza%2F%2FVUWDsmUcGUP8MX3WRwkAtyIyO3YxQ%3D&reserved=0"><span style="color:#954F72">https://godbolt.org/z/Dsu1WI</span></a><span class="apple-converted-space"> </span>.
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>
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<p class="MsoNormal"><span style="color:blue">movsd</span><span class="apple-converted-space"> </span><span style="color:#4864AA">xmm0</span>,<span class="apple-converted-space"> </span><span style="color:teal">qword</span><span class="apple-converted-space"> </span><span style="color:teal">ptr</span><span class="apple-converted-space"> </span>[<span style="color:#4864AA">ecx</span>]<span class="apple-converted-space"> </span><span style="color:green">#
xmm0 = mem[0],zero</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="color:blue">mov</span><span class="apple-converted-space"> </span><span style="color:teal">dword</span><span class="apple-converted-space"> </span><span style="color:teal">ptr</span><span class="apple-converted-space"> </span>[<span style="color:#4864AA">esp</span><span class="apple-converted-space"> </span>+<span class="apple-converted-space"> </span><span style="color:#09885A">24</span>],<span class="apple-converted-space"> </span><span style="color:#4864AA">edx</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="color:blue">movsd</span><span class="apple-converted-space"> </span><span style="color:teal">qword</span><span class="apple-converted-space"> </span><span style="color:teal">ptr</span><span class="apple-converted-space"> </span>[<span style="color:#4864AA">esp</span><span class="apple-converted-space"> </span>+<span class="apple-converted-space"> </span><span style="color:#09885A">40</span>],<span class="apple-converted-space"> </span><span style="color:#4864AA">xmm0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="color:blue">movsd</span><span class="apple-converted-space"> </span><span style="color:#4864AA">xmm0</span>,<span class="apple-converted-space"> </span><span style="color:teal">qword</span><span class="apple-converted-space"> </span><span style="color:teal">ptr</span><span class="apple-converted-space"> </span>[<span style="color:#4864AA">esp</span><span class="apple-converted-space"> </span>+<span class="apple-converted-space"> </span><span style="color:#09885A">40</span>]<span class="apple-converted-space"> </span><span style="color:green">#
xmm0 = mem[0],zero</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="color:blue">movsd</span><span class="apple-converted-space"> </span><span style="color:teal">qword</span><span class="apple-converted-space"> </span><span style="color:teal">ptr</span><span class="apple-converted-space"> </span>[<span style="color:#4864AA">esp</span><span class="apple-converted-space"> </span>+<span class="apple-converted-space"> </span><span style="color:#09885A">8</span>],<span class="apple-converted-space"> </span><span style="color:#4864AA">xmm0</span><o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<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:<span class="apple-converted-space"> </span><a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.chromium.org%2Fp%2Fv8%2Fissues%2Fdetail%3Fid%3D9774&data=02%7C01%7CSeth.Brenith%40microsoft.com%7C9a7bff8d4a864a3bd7a908d75c3880fb%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637079270882137742&sdata=ujiic5Tb0%2F%2BsHM7sH6JXCJlpsQSKD6txl5g3HtEYOuo%3D&reserved=0"><span style="color:#954F72">https://bugs.chromium.org/p/v8/issues/detail?id=9774</span></a><span class="apple-converted-space"> </span>.<o:p></o:p></p>
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<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>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Seth<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica",sans-serif">_______________________________________________<br>
LLVM Developers mailing list<br>
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