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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D">I guess it’s designed for language portability. You can use this type across different platforms. Nevertheless, I’m not a FE expert, so I cannot think out other intentions.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D">The _Float16 is a primitive type in the latest x86 ABI, but there’s no X86 target that supports it yet. So you cannot use it on X86 by now. I think that’s the difference from __fp16 and why should
use it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D">We also have some discussion here.
<a href="https://reviews.llvm.org/D97318">https://reviews.llvm.org/D97318</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="color:#1F497D">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Pengfei<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b>From:</b> Sjoerd Meijer <Sjoerd.Meijer@arm.com> <br>
<b>Sent:</b> Friday, March 5, 2021 5:49 PM<br>
<b>To:</b> Jason Hafer <jhafer@mathworks.com>; Wang, Pengfei <pengfei.wang@intel.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: Is it legal to pass a half by value on x86_64?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D;background:white">__fp16 is a pure storage format. You cannot pass it by value, because only<span class="apple-converted-space"> </span></span><span style="font-size:12.0pt;color:black"><a href="https://gitlab.com/x86-psABIs/x86-64-ABI" target="_blank">ABI</a></span><span class="apple-converted-space"><span style="font-size:12.0pt;color:#1F497D;background:white"> </span></span><span style="font-size:12.0pt;color:#1F497D;background:white">permissive
types can be passed by value while __fp16 is not one of them.</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black;background:white">Yep. Any specific reason to use a pure storage format? The native type is _Float16 and would give some benefits, but this is not yet supported on x86, see also:</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D;background:white"><br>
<br>
</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D;background:white"><a href="https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point">https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point</a><br>
<br>
</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Cheers,<br>
Sjoerd.<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org">llvm-dev-bounces@lists.llvm.org</a>> on behalf of Wang, Pengfei via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>Sent:</b> 05 March 2021 06:28<br>
<b>To:</b> Jason Hafer <<a href="mailto:jhafer@mathworks.com">jhafer@mathworks.com</a>><br>
<b>Cc:</b> llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>Subject:</b> Re: [llvm-dev] Is it legal to pass a half by value on x86_64?</span>
<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D">Hi Jason,</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D">__fp16 is a pure storage format. You cannot pass it by value, because only
<a href="https://gitlab.com/x86-psABIs/x86-64-ABI">ABI</a> permissive types can be passed by value while __fp16 is not one of them.</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D"> </span><o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="xmsolistparagraph" style="color:#1F497D;margin-left:0in;mso-list:l0 level1 lfo1">
<span style="font-size:12.0pt;color:black">if "<span style="background:white">define void @foo(i8, i8, i8, i8, half) " is even legal to use</span></span><o:p></o:p></li></ul>
<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D">half as a target independent type is legal for LLVM. It’s not legal for unsupported target like X86. The behavior depends on how we lowering it. But I don’t know why there’s differences between
Linux and Windows. Maybe because “__gnu_f2h_ieee” is a Linux only function?</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D"> </span><o:p></o:p></p>
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<p class="xmsonormal"><span style="color:#1F497D">Thanks</span><o:p></o:p></p>
<p class="xmsonormal"><span style="color:#1F497D">Pengfei</span><o:p></o:p></p>
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<p class="xmsonormal"><span style="font-size:12.0pt;color:#1F497D"> </span><o:p></o:p></p>
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<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="xmsonormal"><b>From:</b> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org">llvm-dev-bounces@lists.llvm.org</a>>
<b>On Behalf Of </b>Jason Hafer via llvm-dev<br>
<b>Sent:</b> Friday, March 5, 2021 10:46 AM<br>
<b>To:</b> <a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Cc:</b> Jason Hafer <<a href="mailto:jhafer@mathworks.com">jhafer@mathworks.com</a>><br>
<b>Subject:</b> [llvm-dev] Is it legal to pass a half by value on x86_64?<o:p></o:p></p>
</div>
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<p class="xmsonormal"> <o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Hello,</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">I am attempting to understand an anomaly I am seeing when dealing with half on Windows and could use some help.</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Using LLVM 8 or 10, if I have IR of the flavor below:<br>
define void @foo(i8, i8, i8, i8, half) { </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> %6 = alloca half</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> store half %4, half* %6, align 1</span><o:p></o:p></p>
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<div>
<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> ...</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> ret void</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">}</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Using x86_64-pc-linux, we convert the float passed in with __gnu_f2h_ieee.</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Using x86_64-pc-windows I do not get the conversion, so we end up with incorrect math operations.</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<div>
<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">While investigating I noticed clang gave me the error below:</span><o:p></o:p></p>
<div>
<p class="xmsonormal" style="background:white"><span style="font-size:10.5pt;font-family:"Segoe UI",sans-serif;color:black">error: parameters cannot have __fp16 type; did you forget * ?<br>
void foo(int dc1, int dc2,int dc3,int dc4, __fp16 in)</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">So, this got me wondering if "<span style="background:white">define void @foo(i8, i8, i8, i8, half) " is even legal to use or if I should rather pass by ref? I have yet
to find documentation to convince me one way or the other. Thus, I was hoping someone here might be able to shed some light on the issue.</span></span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Thank you in advance!</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Cheers,</span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">JP</span><o:p></o:p></p>
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