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<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Wingdings"><span style="mso-list:Ignore">Ø<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>Eh, you're just doing what LLVM would do on your behalf. It will open some optimizations but block others. =/<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Well, if that’s the case, I obviously rather have LLVM do it. Unfortunately, I can’t seem to get that working.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The “workaround” (passing a pointer instead of a struct) that I also tested, works just fine. Needless to say that I’ll probably end up doing that.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Wingdings"><span style="mso-list:Ignore">Ø<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>My tests show that we translate __m128i to <2 x i64>, and that we are ABI compatible with MSVC when passing __m128i values, so this should actually work. Behind the scenes LLVM will pass these values indirectly by pointer.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Let me put the test case in code. As a reference point, I tried the following C++ code.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:gray">#include</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> </span><span style="font-size:10.0pt;font-family:Consolas;color:#A31515"><cstdint></span><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:blue">struct</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> </span><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">myInt128</span><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">{<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">
</span><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">uint64_t</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> v1;<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">
</span><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">uint64_t</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> v2;<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">};<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">myInt128</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> Test(</span><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">myInt128</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> </span><span style="font-size:10.0pt;font-family:Consolas;color:gray">lhs</span><span style="font-size:10.0pt;font-family:Consolas;color:black">) {<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:blue"> return</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> </span><span style="font-size:10.0pt;font-family:Consolas;color:gray">lhs</span><span style="font-size:10.0pt;font-family:Consolas;color:black">;<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">}<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:blue">int</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> main() {<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:blue"> __int128</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> lhs = 1;<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black"> Test(*</span><span style="font-size:10.0pt;font-family:Consolas;color:blue">reinterpret_cast</span><span style="font-size:10.0pt;font-family:Consolas;color:black"><</span><span style="font-size:10.0pt;font-family:Consolas;color:#2B91AF">myInt128</span><span style="font-size:10.0pt;font-family:Consolas;color:black">*>(&lhs));<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:blue"> return</span><span style="font-size:10.0pt;font-family:Consolas;color:black"> 0;<o:p></o:p></span></p>
<p class="MsoNormal" style="background:white"><span style="font-size:10.0pt;font-family:Consolas;color:black">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Compiling it with Clang --emit-llvm will give me the following:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<pre style="background:white"><span style="font-family:Consolas;color:black"> %struct.myInt128 = type { i64, i64 }<o:p></o:p></span></pre>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<pre style="background:white"><span style="font-family:Consolas;color:black"> define void @"\01?Test@@YA?AUmyInt128@@U1@@Z"(%struct.myInt128* noalias nocapture sret %agg.result, %struct.myInt128* nocapture readonly %lhs) #0 {<o:p></o:p></span></pre>
<pre style="background:white"><span style="font-family:Consolas;color:black"><o:p> </o:p></span></pre>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">In other words, Clang seems to convert the ‘void’ to an int128 pointer which basically handles the ABI compatibility. I was actually surprised by this, because I expected LLVM
to do the ABI conversions. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Next, I attempted to produce have LLVM handle the ABI conversions by emitting the following LLVM in my JIT (irrelevant code omitted):<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">%Int128Wrapper = type { i64, i64 }<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">declare void @"Helper::Test"(%Int128Wrapper)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">define i32 @main(i32, i8**) {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">code:<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> %passTemp = alloca i128<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> store i128 128932, i128* %passTemp<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> %6 = bitcast i128* %passTemp to %Int128Wrapper*<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> %7 = load %Int128Wrapper, %Int128Wrapper* %6<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> call void @"Helper::Test"(%Int128Wrapper %7)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:.5in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ret i32 0<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></a></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">When executing this program, the it crashes and on closer inspection, the parameter (%7) that’s passed from LLVM has an address of
128932 in Test. <o:p></o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">So, from these tests I concluded that LLVM doesn’t handle the ABI conversions (but Clang usually does).
<o:p></o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Kind regards,<o:p></o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Stefan.<o:p></o:p></span></span></p>
<p class="MsoNormal"><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></span></p>
<span style="mso-bookmark:_MailEndCompose"></span>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Reid Kleckner [mailto:rnk@google.com]
<br>
<b>Sent:</b> Wednesday, December 21, 2016 10:14 PM<br>
<b>To:</b> Stefan de Bruijn <stefan@nubilosoft.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] Correct way to pass int128 from LLVM to C++ function (MSVC)<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Wed, Dec 21, 2016 at 11:18 AM, Stefan de Bruijn <<a href="mailto:stefan@nubilosoft.com" target="_blank">stefan@nubilosoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Thanks for the quick reply. Yes, passing it as int128* is a workaround that obviously works. Still, that leaves me
with the return values. Or are you suggesting that I rewrite</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">int128 Modify(int128& tmp) { … }
</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">to</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">void Modify(int128& result, int128& tmp) { … }</span><a name="m_6417751081998293108__MailEndCompose"></a><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Obviously that will work, it just feels… dirty and wrong… :-)</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">Eh, you're just doing what LLVM would do on your behalf. It will open some optimizations but block others. =/<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">I’ve also attempted to bit-cast i128’s to <2 x i64> in LLVM. The ABI problems are pretty much the same. At a first
glance, it seems to me like this problem is more general, namely: for all structures larger than 8 bytes.</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">My tests show that we translate __m128i to <2 x i64>, and that we are ABI compatible with MSVC when passing __m128i values, so this should actually work. Behind the scenes LLVM will pass these values indirectly by pointer.<o:p></o:p></p>
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